Field of the Invention

The present invention relates generally to ice detection and associated areas of detection technology.

Some aspects of the present invention relate to detection apparatus for measuring liquid water and/or ice crystals in an airstream, and also to an associated detection method.

For example, the airstream may be an airstream flowing past an aircraft or other moving aerodynamic structure such as a blade of a wind turbine. An alternative example of the airstream may be an airstream in an icing research tunnel.

Other aspects of the present invention relate to an ice detection method and apparatus.

Background of the Invention For an aircraft, the in-flight formation of ice on the external surface of the aircraft is undesirable. The ice destroys the smooth flow of air over the aircraft surface, increases drag and decreases the ability of an aerofoil to perform its intended function.

Also, built-up ice may impede the movement of a movable control surface such as a wing slat or flap. Ice which has built up on an engine air inlet may be suddenly shed in large chunks which are ingested into the engine and cause damage.

It is therefore common for aircraft, and particularly commercial aircraft, to incorporate an ice protection system. A commercial aircraft may use a system which involves bleeding hot air off from the engines, and the hot air is then ducted to the airframe components such as the leading edges of the wing and the tail which are prone to ice formation. More recently, electrically powered systems have been proposed, such as in EP-A-1, 757,519 (GKN Aerospace) which discloses a wing slat having a nose skin which incorporates an electro- thermal heater blanket or mat. The heater mat is bonded to the rear surface of a metallic erosion shield which comprises the forwardly-facing external surface of the nose skin.

The heater mat is of the "Spraymat" (trade mark) type and is a laminated product comprising dielectric layers made of preimpregnated glass fibre cloth and a heater element formed by flame spraying a metal layer onto one of the dielectric layers. The "Spraymat" has a long history from its original development in the 1950s by D. Napier & Sons Limited (see their GB-833,675 relating to electrical de-icing or anti-icing apparatus for an aircraft) through to its subsequent use by GKN Aerospace.

Modem designs of heater mat are disclosed in GB-A-2,477,336 and GB-A-2,477,337 and GB-A-2,477,338 and GB-A-2,477,339 and GB-A-2,477,340 which are all in the name of GKN Aerospace. In order to know when to initiate operation of a heater mat, an aircraft is currently provided with an ice-detection sensor which may be incorporated into a stub-shaped probe which is mounted on the fuselage adjacent to the cockpit and which projects into the airstream.

The ice-detection sensor may be an optical sensor such as the forward-looking sensor which is described in WO-2004/110865 and which has a fibre optic light emitter at the centre of an array of fibre optic sensor elements (for example, a linear array of six sensor elements). Accreted ice causes the emitted light to be diffusely scattered and reflected back into the sensor elements, and the characteristics of the output signals from the sensor elements enable control electronics to determine the thickness of the accreted ice and the type of the accreted ice (e.g. glaze ice, rime ice, or mixed ice comprising glaze ice and rime ice).

The result of the ice detection is provided as advice or information to the pilot in the cockpit using a visual and/or audible annunciator, and the pilot makes a decision as to whether to activate the heater mats of the ice-protection system.

First problem with the current technology When the pilot is responding to the detection of accreted ice, and the pilot initiates operation of the ice-protection system, the ice-protection system is performing a de-icing function by removing ice which has already formed. An ice-protection system may also be used before ice has formed, in order to prevent the formation of accreted ice on an airframe component. This is termed an anti-icing function.

Measurement of liquid water content (LWC) of an airstream would be desirable when operating an aircraft in order to determine the risk of ice accretion. For example, by knowing the LWC in the airstream, a decision could be made as to whether to prevent the likely accretion of ice by operating the aircraft's ice-protection system in an anti-icing mode.

It would also be desirable to be able to determine the presence and preferably also the size of ice crystal particles in an airstream in order to be aware of the risk of ice crystals being ingested into measurement probes, air intakes and engines of an aircraft.

Second problem with the current technology

Determining the amount of accreted ice involves the use of an algorithm, and it is difficult to determine the presence or location of the accreted ice along a length of a leading edge. Accordingly, there is a desire to provide an alternative detection technique for detecting ice accretion on aircraft or aero-frame components.

Third problem with the current technology

The activation of an ice protection system is critical such that it is necessary to verify the response of an ice detection sensor. Accordingly, there is a desire to provide a detection technique that provides the verification for such ice protection systems. Summary of the Invention

First group of aspects of the invention According to a first aspect of the present invention, there is provided a detection apparatus for measuring liquid water content and/or ice crystal particle concentration in an airstream, the detection apparatus comprising:

a light source arranged to emit a beam of light along an emission path, wherein the emission path is located in a detection volume which is arranged to receive the airstream; a first optical sensor arranged to receive scattered light along a first viewing path from the detection volume and to produce a first detection output; and

processing means arranged to analyse the first detection output of the first optical sensor and to produce a status output indicative of measured liquid water content and/or ice crystal particle concentration;

wherein the first viewing path is at a first oblique angle to the emission path.

If there are no water droplets or ice crystals in the airstream, there will be less scattering of light compared with when droplets or ice crystals are present in the airstream, and thus the processing means can use the intensity of the first detection output to discriminate between absence and presence of water droplets/ice crystals in the airstream.

In a preferred embodiment, the light source is a laser. For example, the laser may be a 522 nm wavelength laser having an output power of 3 to 50 microwatts. Instead of using a laser, an alternative polarised or un-polarised light source could be used.

In a preferred embodiment, the detection volume is configured to receive the airstream along an airstream path; and the emission path of the beam of light is transverse to the airstream path. For example, the emission path may be substantially perpendicular to the airstream path.

In a preferred embodiment, the first oblique angle is such that the first optical sensor is positioned to detect scattered light from water droplets in the detection volume; and the first detection output relates to detected water droplets. For example, the first oblique angle may be in the range of from about 10° to about 15°.

In a preferred embodiment, the first oblique angle is such that the first optical sensor is positioned to detect scattered light from ice crystals in the detection volume; and the first detection output relates to detected ice crystals. For example, the first oblique angle may be in the range of from about 25° to about 30°.

It may be desirable to measure both LWC and ice crystals. Thus, in a preferred embodiment, the detection apparatus further comprises a second optical sensor arranged to receive scattered light along a second viewing path from the detection volume and to produce a second detection output; the second viewing path is at a second oblique angle to the emission path; and the processing means is arranged to analyse the first and second detection outputs of the first and second optical sensors and the status output of the processing means is indicative of measured liquid water content and ice crystal particle concentration. For example, the first oblique angle may be in the range of from about 10° to about 15°. For example, the second oblique angle may be in the range of from about 25° to about 30°. More generally, the second oblique angle is different to the first oblique angle, and preferably the difference between the first and second angles is at least 5° or at least 10°.

In a preferred embodiment, the or each optical sensor comprises an array (e.g. a linear array) of sensor elements. Optics such as optical fibres may be positioned in front of the sensor elements to guide the light received by the optical sensor to the sensor elements. For example, a linear array of the second optical sensor may be generally aligned with a linear array of the first optical sensor.

In a preferred embodiment, the or each array of sensor elements is an array of photodiodes. In a preferred embodiment, the or each an-ay of sensor elements is arranged (preferably along an arc) so as to lie in a plane containing the emission path and the viewing path associated with the array of sensor elements.

If first and second sensor element arrays are provided, they may be arranged to lie along a common arc.

In a preferred embodiment, the light source is arranged to emit along the emission path a beam comprising light having a first wavelength and light having a second wavelength different to the first wavelength. For example, the first wavelength may be suited to detecting water droplets, and the second wavelength may be better suited to detecting ice crystals.

In a preferred embodiment, the light source is arranged to scan at least a portion of the detection volume with the beam of light. This may help to obtain an average measured value for a "cloud" of the airstream in the detection volume at multiple points in the cloud.

In a preferred embodiment, the or each optical sensor includes optics which are positioned in front of sensor element(s) and which are adjustable to adjust the angle of the viewing path and/or the focussing point of the optical sensor.

For example, the optics may be fibre optics. In an embodiment, optical fibres in front of a sensor could be scanned across the detection volume. This may help to obtain an average measured value for a "cloud" of the airstream in the detection volume at multiple points in the cloud.

In some of our current experimental embodiments, the light source is at a first side of the detection volume; and the optical sensor(s) are at a second, opposite side of the detection volume. This configuration may be suitable if the detection apparatus is, for example, to be incorporated into a dedicated probe which may be positioned on the airframe of an aircraft. If the detection volume is, for example, the nacelle of an engine inlet, the first side may be one side of the engine inlet, and the second side may be the other side of the engine inlet. In this way, the detection apparatus may monitor the characteristics of the airstream entering the engine.

In other versions of our current experimental embodiments, the light source is at a first side of the detection volume; the detection apparatus includes a mirror which is at a second, opposite side of the detection volume and which is arranged to reflect the emission path back towards the first side of the detection volume; and the optical sensor(s) are at the first side of the detection vo lume .

By folding the emission path back on itself, when the detection apparatus is incorporated into a probe, a more-compact probe is produced, because the light source and the optical sensor(s) can be located in the base of the probe. A free end of the probe remote from the probe base contains the mirror, which means that the free end of the probe may be substantially smaller than the base of the probe, which offers aerodynamic advantages for a probe which is projecting into the air stream around an aircraft.

The beam of light from the light source passes twice through the detection volume. There is a first leg before the light beam reaches the mirror, and there is a second leg after the light beam has been reflected by the mirror. For a cloud of water droplets and/or ice crystals in the detection volume, the sensor(s) may detect back- scattering from the first leg, and may detect forward-scattering from the second leg.

Detecting both back-scattering and forward-scattering permits analysis of the sensor output(s) which is better at discriminating between water droplets and ice crystals, and may also permit more parameter(s) of the water droplets/ice crystals to be inferred from the characteristics of the sensor output(s). For example, Mie scattering is mainly forward scattering. Rayleigh scattering is more uniformly a combination of forward scattering and backward scattering.

In a preferred embodiment, the detection apparatus further comprises a probe housing; the probe housing has first and second side portions; the detection volume is located between the first and second side portions of the probe housing.

The first and second side portions may have forward ends which provide an "open mouth" to the probe for unimpeded admission of the airstream into the detection volume between the first and second side portions.

The light source and/or the optical sensors may be externally fixed to the first and second side portions.

However, in preferred embodiments, such as for use on an aircraft, the light source and the optical sensor(s) are housed within the side portions of the probe housing. Internally mounting the components helps to make the probe housing more aerodynamic. A probe housing may be provided with window(s) and the light source and/or the optical sensors may be positioned behind respective windows. The or each window may be made aero-conformal with the adjacent part of the probe housing. In a preferred embodiment, the optical sensor(s) are positioned forward of the light source. If a sensor is positioned upstream of the light source, relative to the direction of the incoming airstream, the sensor looks back into any cloud which the airstream brings into the detection volume. In a preferred embodiment, a side portion of a probe housing has a portion which is concavely curved and which faces back into the detection volume and contains the optical sensor(s). This side portion may be a base of the probe.

In a preferred embodiment, a probe housing has a connecting strut which connects together first and second side portions and which is positioned at the rear of the detection volume.

The connecting strut may be the only structural component which connects together the first and second side portions. The detection volume may be open at its front, top and bottom, and may be closed at its sides (by side portions of a probe housing) and at its rear (by a rear strut connecting the side portions).

The connecting strut may be aerodynamic in shape. The connecting strut may include a heater to prevent ice accretion on the strut, for example on a leading edge of the strut.

In a preferred embodiment, the strut has a leading edge and the emission path is generally parallel to and positioned in front of the leading edge. For example, the strut, the light source and the optical sensor(s) may lie in a substantially common plane.

We envisage that a major use of the detection apparatus will be on an aircraft. In a preferred embodiment, the detection apparatus is configured as a probe projecting from an airframe component of an aircraft.

In a preferred embodiment, the detection apparatus is positioned adjacent to a leading edge of an airframe component (e.g. the leading edge of an engine nacelle, or the leading edge of a (standard) aircraft measurement probe). Alternatively, the detection apparatus, itself in the form of a probe, could be positioned on the fuselage of an aircraft, such as adjacent to the cockpit.

In a preferred embodiment, the status output of the processing means is arranged to be displayed in a cockpit of the aircraft. Thus, the cockpit crew may be informed, for example, of outside icing conditions in the form of LWC and/or ice particles in the external air stream that the aircraft is currently passing through. The crew may then make an informed decision as to whether to activate an ice-protection system in an anti-icing mode of operation.

In a preferred embodiment, an aircraft includes an ice-protection system comprising de-icing and/or anti-icing apparatus and a control system arranged to control operation of the de-icing and/or anti-icing apparatus; and the control system of the ice-protection system is arranged to be responsive to the status output of the processing means. For example, the control system may respond to an output of an (optical) ice-detection sensor (e.g. an ice-detection sensor of the general type disclosed in WO-2004/110865) as well as to the status output of the processing means of the detection apparatus. The status output of the processing means may act as a secondary (independent or back-up) confirmation of the need to activate the ice- protection system (whether in de-icing or anti-icing mode) such that automatic operation of the ice-protection system is possible (without needing involvement of the cockpit crew).

According to a second aspect of the present invention, there is provided a method of detecting liquid water and/or ice particles in an airstream, comprising the steps of:

emitting a beam of light along an emission path into a detection volume through which the airstream passes; and

detecting, along a first viewing path, light scattered by liquid water and/or ice particles in the detection volume; wherein the first viewing path is at an oblique angle to the emission path.

In a preferred embodiment, the detection along the first viewing path is of light scattered by liquid water; the method further comprises the step of detecting, along a second viewing path, light scattered by ice particles in the detection volume; and the second viewing path is at a second oblique angle to the emission path, and the second oblique angle is different to the first oblique viewing angle.

In a preferred embodiment, the airstream is an airstream adjacent to an airframe component of an aircraft.

In a preferred embodiment, in response to the detected scattered light indicating that the liquid water in the airstream has a parameter in excess of a predetermined threshold value or indicating that the ice particles in the airstream have a parameter in excess of a predetermined threshold value, an ice-protection system of the aircraft is operated.

In a preferred embodiment, operation of the ice-protection system is automatic in response to the parameter exceeding the threshold value. Preferred features of the first aspect of the present invention are applicable, mutatis mutandis, to the second aspect of the present invention, and vice versa.

Second group of aspects of the invention According to a first aspect of the invention there is provided an apparatus for detecting ice accretion on a surface. The apparatus comprising an electromagnetic radiation source configured in use to be positioned adjacent the surface and a detector spaced apart from the electromagnetic radiation source, wherein the electromagnetic radiation source is operable to project electromagnetic radiation across a portion of the surface and on to the detector such that ice accretion on the surface attenuates (produces a shadow in) the projected electromagnetic radiation impinging on the detector.

In accordance with the first aspect of the invention it is possible to detect the accretion of ice on a surface, such as the leading edge of the aero-frame component, by detecting a change in the intensity or the intensity of the electromagnetic radiation detected. The apparatus is more straightforward than currently available probes and may be used without algorithms to determine if ice accretion has formed on the leading edge of the aero-frame component. It will be appreciated that the surface may be on a part or component on an aircraft, e.g., a wing or tail section, but may also be used for other moving aerodynamic structures such as a blade of a wind turbine.

In accordance with some embodiments the electromagnetic radiation source comprises a plurality of individual electromagnetic radiation sources arranged in a line configured to extend from the surface. By providing multiple sources of electromagnetic radiation it may be possible to more easily detect the thickness of ice accretion on the surface (e.g., a leading edge of the aero-frame component). This is because the electromagnetic sources span a distance extending from the surface, so that if the thickness of the ice on the leading edge increases the electromagnetic radiation from one or more of the sources will be attenuated.

In accordance with some embodiments each individual electromagnetic radiation source comprises a laser, a laser diode and a collimating lens, a lamp and a parabolic reflector or a collimated electromagnetic radiation source coupled to an optical fibre. The type of source will depend on the specific arrangement, but by using a collimated source the accuracy of the detection may be increased, since the electromagnetic radiation from neighbouring sources may not interfere with one another. In accordance with some embodiments the electromagnetic radiation source is configured to project an elongate pattern to extend from the surface. The elongate pattern can be used to detect the thickness of accreted ice, since more or less electromagnetic radiation will reach the detector from the source dependent on the thickness of the ice.

In accordance with some embodiments the detector is a linear-array detector arranged to extend from the surface. The linear-array detector can be used to detect the thickness of accreted ice, since the electromagnetic radiation can be detected at different points along a line extending from the leading edge of the aero-frame component.

In accordance with some embodiments the detector comprises a plurality of detectors arranged to extend from the surface. The plurality of detectors can be used to detect the thickness of accreted ice, since the electromagnetic radiation can be detected at different points along a line extending from the surface (e.g. leading edge of the aero-frame component). By using individual detectors it may be possible to alter the accuracy of the detection of the thickness of the accreted ice, which may be otherwise limited by a linear array detector of fixed length.

In accordance with some embodiments the detector comprises a plurality of optical fibres each having a distal end arranged to receive electromagnetic radiation from the electromagnetic radiation source and a proximal end coupled to a detector array. Thus it is possible to locate the electromagnetic radiation source at remote locations from the surface (e.g., a leading edge of the aero-frame component) to avoid damaging the source and also to allow the control electronics and sources to be arranged in a single location. In accordance with some embodiments the distal ends of the plurality of optical fibres are arranged in a line to extend from the surface.

In accordance with some embodiments the apparatus is configured as a probe projecting from an aero-frame component of an aircraft. Thus the apparatus can be applied to a suitable part of the aircraft, preferably near an aero-frame component of the aircraft on which ice accretion is known to occur, for example the leading edge of a wing. Furthermore, the apparatus can be placed adjacent blades of a wind turbine to detect ice accretion on the blades or on moving parts which may prevent the rotation of the turbine.

In accordance with some embodiments the apparatus is positioned adjacent to a leading edge of an aero-frame component. In accordance with some embodiments the apparatus comprises an elongate aerofoil section, wherein the electromagnetic radiation source and the detector are positioned at opposing ends of the elongate aerofoil section, and wherein the electromagnetic radiation source is configured to project electromagnetic radiation across a portion of a leading edge of the aerofoil section. Therefore, a stand-alone probe can be formed which may be more freely attached to a suitable location on an aircraft, or turbine, for example.

In accordance with some embodiments the surface is a leading edge of an aero-frame component. In accordance with some embodiments the aero-frame component is a wing, engine nacelle, or tail section of an aircraft. Therefore, it is possible to detect the ice accretion directly on an aero-frame component of an aircraft or turbine blade, for example, rather than at a remote location to obtain a more accurate measurement of the ice accreted on the component surface.

In accordance with some embodiments the surface is elongate, and the electromagnetic radiation source and the detector are arranged such that electromagnetic radiation is projected from the source along the length of the elongate surface to the detector.

In accordance with some embodiments the surface is elongate, and the electromagnetic radiation source and the detector are arranged such that electromagnetic radiation is projected from the source to the detector along a path that is angularly offset with respect to the elongate surface. That is to say that a line going from the source to the detector is rotated with respect to a line along the length of the elongate surface of an aero-frame component for example. Thus it may be possible to determine the presence and thickness of ice accretion at a point on the elongate surface of an aero-frame component where the path going from the source to the detector crosses the line along the length of the elongate surface of the aero- frame component.

In accordance with some embodiments the detector is operable to provide an output signal indicative of an amount of electromagnetic radiation received from the electromagnetic radiation source. The output signal may be fed to control circuitry that controls any de-icing apparatus (e.g., a heater mat) on an aircraft, turbine or other surface that may experience unwanted ice or may be fed to a display in the cockpit of the aircraft for a pilot to monitor.

In accordance with some embodiments the apparatus comprises a comparator operable to compare the output signal to a predetermined threshold to determine if there is ice accretion on the surface of the aero-frame component. Other environmental factors may alter the detection of the electromagnetic radiation, such that the output signal may be compared to a predetermined threshold to better determine if the signal from the detector represents ice accretion or some other environmental factor. Furthermore, the electromagnetic radiation may only be attenuated (partially shadowed) and not completed shadowed by any ice accretion so a threshold can be used to determine if a measured intensity is low enough to be determined to be due to ice accretion.

In accordance with some embodiments the apparatus comprises a processor operable to determine a thickness of accreted ice on the surface based on the output signal from the detector. The intensity of the signal received from the detector can be used to determine the thickness of any accreted ice, since as the accreted ice becomes thicker, the intensity of the received electromagnetic radiation will be reduced as the shadow on the detector increases.

In accordance with some embodiments the processor is operable to periodically determine the thickness of accreted ice on the surface and to determine a rate of growth of the accreted ice. It may be useful to know the rate at which the ice accretion is occurring to determine a point in the future to activate de-icing apparatus. For example, the thickness of the ice may not be enough to the warrant activation of the de-icing system, but the de-icing system could be programed, for example, to active in a set time in the future.

In a preferred embodiment, the source and the detector extend forwards from the surface, for example from respective opposite ends of the surface.

In a preferred embodiment, the surface is elongate. The surface may be a leading edge, for example of an elongate aerofoil portion.

In a preferred embodiment, the source and the detector are arranged opposite one another in front of the (elongate) surface. In a preferred embodiment, the electromagnetic radiation comprises an electromagnetic radiation beam pattern having a proximal edge adjacent to (and in front of) the (elongate) surface and a distal edge spaced apart from the (elongate) surface. This cross-sectional envelope of the beam pattern is, in a preferred embodiment, generally elliptical.

According to a second aspect of the invention there is provided an aircraft comprising the apparatus according any one or more of the aspects and embodiments of the invention.

According to a third aspect of the invention there is provided a kit of parts for detecting ice accretion on a surface. The kit of parts comprising an electromagnetic radiation source and a detector, wherein the electromagnetic radiation source is configured to be positioned adjacent the surface and the detector is configured to be spaced apart from the electromagnetic radiation source, wherein the electromagnetic radiation source is operable to project electromagnetic radiation across a portion of the surface and on to the detector such that ice accretion on the surface attenuates the projected electromagnetic radiation impinging on the detector. According to a fourth aspect of the invention there is provided a method for detecting ice accretion on a surface. The method comprising the steps of projecting electromagnetic radiation across a portion of the surface and detecting the intensity of the electromagnetic radiation at a position spaced apart from the electromagnetic radiation source such that ice accretion on the surface attenuates the intensity of the projected electromagnetic radiation.

In accordance with some embodiments the method comprises the steps of projecting electromagnetic radiation across a portion of the surface and detecting the intensity of the electromagnetic radiation at a position spaced apart from the electromagnetic radiation source such that ice accretion on the surface attenuates the intensity of the projected electromagnetic radiation.

In accordance with some embodiments the method comprises the step of projecting an elongate electromagnetic radiation pattern extending from surface.

In accordance with some embodiments the method comprises the step of projecting a plurality of electromagnetic radiation beams in a line extending from the surface. In accordance with some embodiments the method comprises the step of detecting the projected electromagnetic radiation at discrete positions along a line extending from surface.

In accordance with some embodiments the surface is a leading edge of an aerofoil section, and the method comprises the step of projecting the electromagnetic radiation across a portion of the leading edge of the aerofoil section. In accordance with some embodiments the surface is elongate, and the method comprises the step of projecting the electromagnetic radiation along the length of the elongate surface to the detector.

In accordance with some embodiments the surface is elongate, and the method comprises the step of projecting the electromagnetic radiation along a path that is angularly offset with respect to the elongate surface.

In accordance with some embodiments the surface is a leading edge of an aero-frame component. In accordance with some embodiments the aero-frame component is a wing, engine nacelle, or tail section of an aircraft.

In accordance with some embodiments the method comprises the step of comparing the intensity of detected electromagnetic radiation with a predetermined threshold to determine if there is ice accretion on the surface.

In accordance with some embodiments the method comprises the step of determining a thickness of accreted ice on the surface based on the detected intensity of the electromagnetic radiation.

In accordance with some embodiments the method comprises the step of periodically determining the thickness of accreted ice on the surface and determining a rate of growth of the accreted ice.

Third group of aspects of the invention

According to a first aspect of the invention there is provided an apparatus for detecting ice accretion. The apparatus comprising: a first ice accretion detector having a surface arranged in use such that an airflow passes over said surface; and a second ice accretion detector arranged to project a beam of electromagnetic radiation to intersect the surface of the first detector.

Accordingly it is possible to detect ice accretion on a leading edge of an aerofoil or aero- frame component using at least two different sensing techniques such that it is possible to verify the detection of ice on a surface. Therefore, two different ice detection techniques are able to detect ice accretion at the same point, such that the detection may be more accurate. It will be appreciated that the detector may be arranged on a surface of a part or component on an aircraft, e.g., a wing or tail section, but may also be used for other moving aerodynamic structures such as a blade of a wind turbine.

In accordance with some embodiments the first detector is operable to output a first signal indicative of the presence of ice accreted on the surface of the first detector and the second detector is operable to output a second signal indicative of the presence of ice accreted on the surface of the first detector. In accordance with some embodiments the apparatus comprises a processor operable to determine the presence of ice accreted on the surface of the first detector based on the first signal and to verify the determination of the presence of ice accreted on the surface of the first detector based on the second signal. In accordance with some embodiment the first and second signals are indicative of a thickness of accreted ice, and wherein the processor is operable to determine the thickness of ice accreted on the surface of the first detector based on the first and second signals.

Thus it is possible to determine the thickness of the ice accreted on a surface, e.g., a leading edge of an aerofoil or aerofoil component (e.g., wing, engine nacelle, or tail section of an aircraft) to allow a determination to be made regarding the activation of de-icing apparatus. Furthermore, the two different techniques allows for verification of the thickness of the accreted ice to be obtained.

In accordance with some embodiments the first detector comprises an electromagnetic radiation source and the second detector comprises an electromagnetic radiation source having a different wavelength than the electromagnetic radiation source of the first detector.

This prevents interference or cross-talk between the different detection techniques used.

In accordance with some embodiments the apparatus comprises a third detector operable to determine the liquid water content and/or ice content in a volume ahead of the surface of the first detector. A third detection technique is used to further verify the ice accretion detection of the other two detection techniques.

In accordance with some embodiments the apparatus comprises an elongate surface having the first detector arranged thereon, and the second ice accretion detector is arranged such that electromagnetic radiation is projected along the length of the elongate surface to the first detector.

In accordance with some embodiments the apparatus comprises an elongate surface having the first detector arranged thereon, and the second ice accretion detector is arranged such that electromagnetic radiation is projected along a path that is angularly offset with respect to the elongate surface. That is to say that a line going from a source to a detector of the second detector may be rotated with respect to a line along the length of the elongate surface of an aero-frame component for example. Thus it may be possible to determine the presence and thickness of ice accretion at a point on the elongate surface of an aero-frame component where the path going from the source to the detector crosses the line along the length of the elongate surface of the aero-frame component, where the intersection point corresponds with the location of the surface or foot-print of the first detector.

In accordance with some embodiments the elongate surface is a leading edge of an aero-frame component. In accordance with some embodiments the aero-frame component is a wing, engine nacelle, or tail section of an aircraft. Therefore, it is possible to detect the ice accretion directly on an aero-frame component of an aircraft or turbine blade, for example, rather than at a remote location to obtain a more accurate measurement of the ice accreted on the component surface.

In accordance with some embodiment the apparatus comprises a heater mat positioned on the surface of the first detector. Once the ice has been cleared from parts of the aircraft (e.g., the wings or tail section) or turbine using de-icing apparatus, the heater mat allows the same to be achieved for the detector apparatus, so that the ice accretion on the apparatus can commence from the same starting point as that found of the aircraft or turbine. That is to say that the apparatus can be cleared of accreted ice at the same time as the aircraft or turbine.

In accordance with some embodiments the apparatus comprising an elongate aerofoil section having the first detector arranged thereon. Therefore, a stand-alone probe can be formed which may be more freely attached to a suitable location on an aircraft, or turbine, for example.

In accordance with some embodiments the apparatus is configured as a probe for projecting from an aero-frame component of an aircraft. In accordance with some embodiments the apparatus is positioned adjacent to a leading edge of an aero-frame component.

Thus the apparatus can be applied to a suitable part of the aircraft, preferably near an aero- frame component of the aircraft on which ice accretion is known to occur, for example the leading edge of a wing. Furthermore, the apparatus can be placed adjacent blades of a wind turbine to detect ice accretion on the blades or on moving parts which may prevent the rotation of the turbine.

In accordance with some embodiments the apparatus is configured to be attached to an aircraft.

According to a second aspect of the invention there is provided an aircraft comprising the apparatus according to aspects and embodiments of the invention.

According to a third aspect of the invention there is provided a method for detecting ice accretion. The method comprising the steps of: providing a first ice accretion detector having a surface arranged in use such that an airflow passes over said surface; providing a second ice accretion detector arranged to project a beam of electromagnetic radiation to intersect the surface of the first detector; and detecting the presence of ice accretion on the surface using the first and second detectors.

In accordance with some embodiments the method comprises the steps of determining the presence of ice accreted on the surface of the first detector and verifying the determination of the presence of ice accreted on the surface of the first detector based on the second detector.

In accordance with some embodiments the method comprises the step of determining the thickness of ice accreted on the surface of the first detector and verifying the determined thickness of ice accreted on the surface of the first detector based on the second detector.

In accordance with some embodiments wherein the first detector comprises an electromagnetic radiation source and the second detector comprises an electromagnetic radiation source having a different wavelength than the electromagnetic radiation source of the first detector.

In accordance with some embodiments the method comprises the step of determining the liquid water content and/or ice content in a volume ahead of the surface of the first detector.

In accordance with some embodiments the method comprises the steps of providing an elongate surface and arranging the first detector thereon, and projecting electromagnetic radiation along the length of the elongate surface to the detector. In accordance with some embodiments the method comprises the steps of providing an elongate surface and arranging the first detector thereon, and projecting electromagnetic radiation along a path that is angularly offset with respect to the elongate surface.

In accordance with some embodiments the elongate surface is a leading edge of an aero-frame component.

In accordance with some embodiments the aero-frame component is a wing, engine nacelle, or tail section of an aircraft.

In accordance with some embodiments the method comprises the step of providing an elongate aerofoil section and arranging the first detector thereon. In accordance with some embodiments the method comprises the step of positioning the apparatus adjacent to a leading edge of an aero-frame component.

In accordance with some embodiments the method comprises the step of positioning a heater mat on the surface of the first detector.

Features of the above groups of aspects of the invention

The above groups are laid out for purely for the reading convenience of the reader. Features from one group may be added to or may substitute for features of another group. The above groups of aspects may be read together as a continuous summary of the invention.

Brief description of the Drawings

Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: -

Fig. 1 is a diagrammatic perspective view of an aircraft incorporating a plurality of detection apparatuses in accordance with the present invention.

Fig. 2 is a schematic diagram of a detection apparatus in accordance with the present invention and shows the basic principle of LWC or ice crystal particle detection. Fig. 3 is a graph showing LWC (horizontal axis) versus optical intensity (vertical axis) of forward (Mie scattered) light as measured in an icing research tunnel. Fig. 4 is a schematic diagram of a detection apparatus in accordance with the present invention and shows how detection using a range of Mie scattering angles may be used to detect LWC and ice crystal particles at the same time.

Fig. 5 is a schematic diagram of a detection apparatus in accordance with the present invention and shows how the light source and the sensors may be mounted adjacent to each other by using a mirror to fold the light beam back on itself, thereby producing a detection apparatus which is more compact.

Fig. 6 is a diagram containing three views which illustrate the general characteristics of Rayleigh scattering (top view) and Mie scattering (bottom two views).

Fig. 7 is a diagrammatic plan view of a detection apparatus in accordance with the present invention. Fig. 8 is a diagrammatic plan view of the detection apparatus of Fig. 7 and shows an icing cloud in the detection volume.

Fig. 9 is a diagrammatic perspective view of the detection apparatus of Fig. 7. Figure 10 illustrates schematically an ice detector apparatus according to an embodiment of the invention.

Figure 11 illustrates schematically the ice detector apparatus illustrated in Figure 10 viewed from above.

Figure 12 illustrates schematically a leading edge of the ice detector apparatus illustrated in Figure 11 with ice accretion.

Figure 13 illustrates an example shadow graph for the two accreted ice layers illustrated in Figure 12. Figure 14 illustrates schematically the optical set-up used to obtain an elliptical beam pattern.

Figure 15 illustrates schematically an ice detector apparatus viewed from above according to an embodiment of the invention.

Figure 16 illustrates schematically an ice detector apparatus viewed from above according to an embodiment of the invention.

Figure 17 illustrates schematically an aircraft having fitted thereon an ice detector apparatus according to an embodiment of the invention.

Figure 18 illustrates schematically an alternative an-angement of the ice detector illustrated in Figure 10 according to an embodiment of the invention. Figure 19 illustrates schematically a perspective view of detector according to an embodiment of the invention.

Figure 20 illustrates schematically viewed from above the detector illustrated in Figure 19.

Figure 21 is a diagrammatic perspective view of an aircraft incorporating a plurality of detection apparatuses in accordance with an embodiment of the invention.

While the invention is susceptible to various modifications and alternative forms, embodiments are shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the drawings and detailed description of the embodiments are not intended to limit the invention to the particular forms disclosed. On the contrary, the invention covers all modifications, equivalents and alternatives falling within the spirit and the scope of the present invention as defined by the appended claims.

Description of Embodiments Section 1

Fig. 1 is a diagrammatic perspective view of an aircraft 1 having wings 11, engine nacelles 12, a tail plane 13 and a tail fin 14 which incorporate heater mats 15 of an ice-protection system in their leading edges. An LWC/ice crystal detection apparatus in the form of a probe 2 is mounted on the fuselage 16 adjacent to the cockpit 17.

A simple type of detection apparatus in accordance with the present invention (which could be incorporated in the probe 2) is shown in Fig. 2 in diagrammatic form. A light source 3 produces a polarized or un-polarized light beam 31 which is emitted along an emission path or axis 32. In our preferred experimental embodiments, the light source 3 is a point light source such as a laser. For example, the laser 3 may have a 522 rrm wavelength output with an output power of 3 to 50 micro watts.

An airstream 4 introduces an icing cloud 41 into a detection volume 42 along an airstream path or axis 43 which is substantially perpendicular to the emission axis 32. The detection volume 42 exists in the vicinity of where the airstream axis 43 intersects the emission axis 32. Because the cloud 41 has a significant LWC and/or contains ice crystal particles, the light beam 31 undergoes scattering in the cloud 41.

Forward (Mie) scattering occurs as a result of interaction of the light beam 31 with the water droplets and/or ice crystals in the cloud 41. An optical sensor 5 is provided which is "off axis" relative to the emission path 32. The optical sensor 5 comprises a linear array of sensor elements 51 such as photodiodes which function as a photodetector. Optics in the form of a linear array of optical fibres 52 are positioned in front of photodetector 51 and gather and focus scattered light from the cloud 41 onto the photodetector 51.

The optical sensor 5 views the cloud 41 along a viewing path or axis 53 which is at an acute angle Θ to the emission axis 32.

Fig. 6 diagrammatically illustrates the type of scattering which occurs in the cloud 41. In the top view in Fig. 6, the light beam 31R undergoes Rayleigh scattering, which is approximately equally forward scattering and back scattering. In the middle view in Fig. 6, the light beam 31 Ml undergoes Mie scattering when it hits a small particle. The scattering has an angular distribution which is mainly in the forward direction. In other words, the scattering is mainly forward scattering. In the bottom view in Fig. 6, the light beam 31M2 undergoes Mie scattering when it hits a large particle. The scattering has an angular distribution which is still mainly in the forward direction but (compared with the light beam 31 Ml) the scattering has a more pronounced bias to being in the forward direction. In Fig. 2, the optical sensor 5 may be positioned such that the angle Θ is 10° to 15° when it is desired to detect water droplets and thus measure LWC.

The optical sensor 5 may be positioned such that the angle Θ is 25° to 30° when it is desired to detect ice crystal particles.

The refractive index of ice is different to the refractive index of water, and thus an optical sensor at different angular positions can detect different components (ice or water) of the airstream. When detecting LWC, the quantity as well as the presence of LWC may be measured from the overall intensity of the scattered light, which in the embodiment of Fig. 2 is forward scattered light primarily due to Mie scattering. In the embodiment of Fig. 5 (which is discussed later) the overall intensity of scattered light which is measured also includes rearward scattered light due to Mie and Rayleigh scattering.

The embodiment of Fig. 2 has been calibrated or tested in an icing research tunnel, and Fig. 3 presents the measurement results. In Fig. 3, the vertical axis plots the intensity detected by the optical sensor 5 with the detected optical power being measured in microwatts. The vertical axis spans from -0.02 in increments of 0.02 through zero up to 0.26.

The horizontal axis in Fig. 3 plots the liquid water content (LWC) which is measured in g/m ³ and the horizontal axis spans from -0.2 in increments of 0.2 through zero up to 2.2. It may be seen that the measured relationship between detected optical power and LWC is substantially linear.

In the test, the angle Θ was 25°.

When measuring the concentration of ice particles, the angle Θ can be set at a different value to that used when measuring LWC. The optical sensor 5 can be calibrated for measuring ice crystal particles, bearing in mind that the scattering from ice crystals will have a different characteristic to the scattering from water droplets, for example because ice crystals produce intense reflections when scattering light.

Different wavelengths (λ _1; λ ₂ , λ ₃ etc.) for the light beam 31 could be used when detecting ice particles and when detecting LWC, with the wavelength being optimised to suit detection of ice particles or LWC.

The profile of the light beam 31, its polarization and its wavelength may be adjusted. The focussing point of the optics 52 and the viewing angle Θ of the viewing axis 53 may also be adjusted. Thus, the sensor 5 may measure LWC/ice particle concentration in a particular (small) volume of the cloud 41, or in substantially all of the illuminated portion of the cloud 41. This enables an average value for LWC or ice particle concentration to be produced by a processor 6 which receives a detection output 54 in the form of output signals from the photodiodes of the sensor 5.

The processor 6 has been calibrated with the characteristics of the output signals from the sensor 5 in order to be able to detect the presence of LWC and the quantity of LWC (in g/m ³ ) in the airstream 4, and the processor produces an output 61 which is indicative of the LWC status of the airstream 4.

Alternatively, if ice crystal particles and not LWC are being measured, the processor has been calibrated with the characteristics of the output signals from the sensor 5 in order to be able to detect the presence and the concentration and preferably also the size of ice crystal particles in the airstream 4, and the processor produces an output 61 which is indicative of the ice particle status of the airstream 4. The status output 61 from the processor 6 may be used in a variety of ways. It may be displayed in the cockpit 17 on a cockpit display 171 in order to advise the aircrew on the risk of ice accretion (based on measured LWC) or on the risk of ice ingestion from the airstream (based on the presence, concentration and preferably also size of the ice crystals in the airstream).

If the aircrew judges that the risk of ice accretion on the leading edges of the aircraft 1 looks to be high, the aircrew may decide to activate the heater mats 15 of the ice-protection system in order to perform an anti-icing function. The aircrew may select a standard sequence of operation of the heater mats 15.

The status output 61 may be supplied to a controller 7 of the ice-protection system (IPS) of the aircraft in order to trigger activation of the heater mats 15 on an automatic basis (without aircrew involvement) when, for example, the quantity of LWC exceeds a predetermined threshold value. For example, in Fig. 3, the threshold value 62 may be an LWC of 0.5 g/m ³ . Based on just the status output 61, the controller 7 may automatically trigger anti-icing operation of the heater mats 15. Based on two outputs which are consistent with one another, the controller 7 may automatically trigger de-icing operation of the heater mats 15.

For example, the aircraft 1 may include an ice-detection sensor 8 for detecting the presence of accreted ice, and preferably also the thickness and type of the accreted ice. A suitable optical ice-detection sensor 8 is disclosed in WO-2004/110865. The sensor 8 may be positioned in a window in one of the heater mats 15 in order to detect ice accreting directly on the heater mat. Alternatively, the sensor 8 could be positioned on the fuselage 16 such as adjacent to the cockpit 17, and accretion of ice on the leading edges (on the heater mats 15) could be inferred from detecting accretion of ice on the sensor 8.

An output signal 81 from the sensor 8 may be supplied to the IPS controller 7. Thus, the controller 7 may be responsive to the output signal 81 and to the status output 61. If the output signal 81 indicates the presence of accreted ice (for example, accreted ice which exceeds a predetermined thickness) and if the status output 61 is consistent with the output signal 81, because the status output 61 indicates a high LWC in the airstream (for example, in excess of a predetermined threshold value), then the controller 7 has confirmation from the status output 61 that the controller 7 should respond to the output signal 81 by initiating operation of the heater mats 15 to perform a de-icing function.

A more complex type of detection apparatus in accordance with the present invention is shown in Fig. 4 in diagrammatic form. The same reference numerals as in Fig. 2 are used in Fig. 4 for similar components. The main improvement of the detection apparatus of Fig. 4 is that it is able to measure LWC and ice crystal particles at the same time. The differences relative to Fig. 2 are discussed below.

In Fig. 4, the light source (with integral optics) includes a polarizer 33 in the path of the light beam 31.

First, second and third optical sensors 5 A, 5B and 5C are provided. Each optical sensor comprises a respective photodetector 51A, 51B and 51C. The optics 52 gather and focus scattered light from the cloud 41 along a viewing axis 53A onto the photodetector 51 A. The optics 52 also gather and focus scattered light from the cloud 41 along a viewing axis 53B onto the photodetector 5 IB. Furthermore, the optics 52 also gather and focus scattered light from the cloud 41 along a viewing axis 53C onto the photodetector 51C.

The viewing axis 53A is at an acute angle Q to the emission axis 32. The viewing axis 53B is at an acute angle θ ₂ to the emission axis 32, and the angle θ ₂ is larger than the angle θ ₁ . The viewing axis 53C is at an acute angle θ ₃ to the emission axis 32, and the angle 0 ₃ is larger than the angle θ ₂ .

Polarizers and optical filters 55A, 55B and 55C may be placed in front of the photodetectors 51 A, 51 B and 51 C respectively.

The detection outputs 54A, 54B and 54C of the individual photodetectors 51 A, 5 IB and 51C are fed, as an overall detection output 54, to the processor 6. The photodetectors 51A-C cover a range of viewing angles Q\ to θ ₃ and form a linear detector array. The scattered light detected by the photodetectors 51A-C enables the processor 6 to perform analysis on the outputs 54A-C to measure both LWC and ice crystal particles at the same time.

It is possible for the processor 6 to obtain information on the size of the ice crystal particles because the characteristic of the angular distribution of the Mie scattering caused by the ice particles changes with the size of the particles. This characteristic angular distribution also enables the processor 6 to discriminate between LWC droplets and ice crystal particles, because of the range of viewing angles Qi to 0 ₃ of the optical sensors 5A-C.

A further detection apparatus in accordance with the present invention is shown in Fig. 5 in diagrammatic form. The same reference numerals as in Fig. 4 are used in Fig. 5 for similar components. The main improvement of the detection apparatus of Fig. 5 is that it is more compact, and is thus better suited to being incorporated into a probe. The differences relative to Fig. 4 are discussed below.

In Fig. 5, the light source 3 and the optical sensors 5A-5C are positioned adjacent to one another at a first side of the detection volume 42, and a mirror 34 is provided at the opposite, second side of the detection volume 42 in order to reflect the light beam 31 back towards the sensors 5A-5C.

A first portion or leg 31A of the light beam extends between the light source 3 and the mirror 34, and a second portion or leg 3 IB of the light beam extends between the mirror 34 and the sensors 5A-5C. The mirror 34 is a few degrees away from being perpendicular to the first leg 31 A of the light beam, so that the second leg 3 IB is not superimposed on the first leg 31 A. Thus, the sensors 5A-5C may be positioned adjacent to the light source 3.

The emission axis of the light beam 31 A, 3 IB therefore has a first portion 32A and a second portion 32B corresponding respectively to the light beam legs 31 A, 3 IB. The viewing angles Gi to 0 ₃ of the photodetectors 51A to 51C are measured relative to the second portion 32B of the emission axis of the light beam. The scattered light which is received by the photodetectors 51 A to 51C is a combination of back-scattered light (Rayleigh and Mie) from the first light beam leg 31 A as it passes through the cloud 41 and forward-scattered light (mainly Mie) from the second light beam leg 3 IB as it passes through the cloud 41.

This provides extra information to the photodetectors 51 A to 51C. We envisage that the optics 52 and the software in the processor 6 may be optimised to extract and use this extra information in order to improve the discrimination between LWC water droplets and ice crystal particles, and also to assist with determining the size of the detected water droplets and ice crystals. For the photodetectors 51 A to 51C, a large area CCD may be used.

Figs. 7 to 9 are diagrammatic views which show how a detection apparatus in accordance with the present invention may be incorporated into a probe, such as the probe 2 of Fig. 1. Fig. 1 merely shows an example of the location of the probe 2 on the aircraft 1, and Fig. 1 does not show the shape of the probe.

The shape of the probe may be seen in Figs. 7 to 9. The probe 2 has a housing 21 having first and second side portions 22, 23 and a rear transverse strut 24 which provides the only connection between the side portions 22, 23. The side portion 22 forms a free end of the probe, and the side portion 23 forms a base of the probe which is connected to the fuselage 16 of the aircraft 1.

The strut 24 has a heater along its leading edge 241 in order to prevent ice from accreting on the strut 24.

The probe 2 includes within the housing 21 a detection apparatus of the type shown in Fig. 4. The light source 3 is located in the side portion 22, and the photodetectors 51A-51C are located in the side portion 23.

The detection volume 42 is located between the side portions 22, 23 and the upstream entrance of the detection volume is open to the airstream approaching along the airstream axis 43. The photodetectors 51A-51C are forward of (or upstream of) the light source 3 along the airstream axis 43. A side face 231 of the side portion 23 faces into the detection volume 42 and has a forward portion 2310 which faces backwards into the detection volume, and it is this forward portion 2310 which contains the photodetectors 51A-51C. This forward portion 2310 may be concavely curved.

A rearward portion 2311 of the side face 231 is generally parallel to an opposite inwardly- facing side face 221 of the side portion 22.

The emission axis 32 of the light source 3 is aligned to be generally parallel to the leading edge 241 of the rear strut 24 and to be generally perpendicular to the airstream axis 43 of the airstream. Fig. 7 does not show the cloud 41, but the cloud 41 is shown in Fig. 8.

Fig. 9 is a perspective view of the probe 2 and therefore more clearly shows its shape in three dimensions. Whilst a detection apparatus of the present invention may be incorporated in a probe (as shown in Figs. 7 to 9) the detection apparatus may be mounted on the aircraft in alternative ways.

For example, as shown in Fig. 1, each of the engine nacelles 12 may incorporate a detection apparatus of the present invention. Each of the engine nacelles has, on one side of its air inlet, a light source 3. On the opposite side of the air inlet (although not visible in Fig. 1 because of the viewing angle of the perspective view) there would be positioned optical sensor(s) 5. Thus, the detection apparatus monitors the airstream entering the air inlet of the engine nacelle. If a risk of ice accretion is detected, the controller 7 may activate the heater mat 15 of the particular engine nacelle.

Section 2 Figure 10 illustrates schematically an ice detector 2002 according to an embodiment of the invention. The ice detector 2002 illustrated in the figure is a stand-alone probe. The probe is stub-like and is orientated to face into airflow. The probe 2002 comprises a central portion 2006 having an aerofoil cross section. The aerofoil portion 2006 is generally elongate and extends between two end portions 2008, 2010. The first end portion 2008 (illustrated on the left hand side of the figure) is attached at a proximal end of the elongate aerofoil section 2006, and includes one or more fixing points (not shown) to allow the probe to be attached to an aircraft. For example, the probe 2002 may be attached to a fuselage of an aircraft near one of the wings or a tail section. The second end portion 2010 (illustrated on the right hand side of the figure) is attached at a distal end of the elongate aerofoil section 2006. When installed the probe 2002 is orientated such that in normal use (i.e., attached to an aircraft moving forward) airflow will be in the direction of arrow 2004 illustrated in the figure. The central aerofoil portion 2006 will mimic a wing or tail portion and air will flow to and round a leading edge 2020 and provides a surface on which ice accretes and forms a surface or base from which a measurement may be performed.

Each of the first and second end portions 2008, 2010 and the central portion 2006 is manufactured from a composite material in a similar manner to other aircraft parts. However, it will be appreciated that the parts of the probe 2002 may be manufactured from other suitable materials, e.g. metal, or plastics. Each of the first and second end portions 2008, 2010 and the central portion 2006 will be generally hollow to allow for the passage of cables, or optical fibres or the location of control electronics. Further, as is illustrated, the first and second end portions 2008, 2010 generally extend beyond the leading edge 2020 of the central portion 2006.

The first proximal end portion 2008 in this example includes a detector 2012. In this example the detector 2012 is a linear array detector. For example, a Tarry Series Linear CCD Array detector could be used having a detector length in the region of between 20 and 30 mm and having a resolution in the region of 2000 to 3000 pixels. The selection of the size and resolution of the detector 2012 will determine the thickness of ice accreted on the leading edge 2020 that can be detected and the accuracy of the detector. The detector 2012 is arranged to extend forwards from the leading edge 2020 of the central portion 2006 of the probe 2002. That is to say that the detector extends generally perpendicularly from the surface of the leading edge 2020. The detector 2012 in this example is a linear array detector and may be used as a continuous array. Alternatively, the sensor array may be divided into a number of discrete areas comprising a predetermined number of pixels, such that the signals from all the pixels in each discrete area are summed to generate an intensity signal for each discrete area of the detector array. For example, if the detector array has 3000 pixels, the detector may be divided into 5 discrete areas each having 600 pixels as is illustrated in the figure.

The second distal end portion 2010 in this example includes a source of electromagnetic radiation 2014. The source 2014 projects electromagnetic radiation along the length of the leading edge 2020 of the aerofoil section 2006. The source 2014 in this example projects an elliptical shaped beam 2018, as illustrated by the dash elliptical shape in the figure. The elongate (i.e., elliptical shaped) beam 2018 extends outwardly from the leading edge 2020. That is to say that the elongate beam is projected to follow the leading edge 2020 such that the leading edge 2020 and the beam 2018 are generally parallel or in-line with one another and further a proximal edge of the beam 2018 is adjacent or immediately adjacent the surface of the leading edge 2020 and extends outwardly from the leading edge 2020 in a direction generally perpendicular to the leading edge 2020, such that a distal edge of the beam 2018 is spaced apart from the leading edge 2020.

The detector 2012 and the source 2014 are arranged opposite one another so that electromagnetic radiation is projected to the detector 2012 along the length of the leading edge or surface 2020, or width of the elongate portion 2006, such that the electromagnetic radiation impinges on the detector 2012. The direction of the electromagnetic radiation from the source 2014 to the detector 2012 is in the direction of the arrow 2016 illustrated in the figure. That is to say that the beam of electromagnetic radiation 2018 is directed from the source 2014 toward the detector 2012 alongside and in-line with the leading edge 2020. The electromagnetic radiation then impacts, strikes or contacts the detector 2012. Each of the source 2014 and detector 2012 may be protected with a form of shield that allows the electromagnetic radiation to pass without attenuation or limited attenuation. For example, the detector 2012 and/ or source 2014 may be positioned behind a glass window positioned in the respective end portion of the probe. Figure 11 illustrates schematically the ice detector apparatus illustrated in Figure 10 viewed from above. The same reference numerals are used to identify the same features as those illustrated in Figure 10. The probe 2002 further includes a processor 2022 that is coupled to the detector 2012 and the source 2014. The processor 2022 controls the source 2014 and receives signals indicative of detected intensity of the electromagnetic radiation from the detector 2012. The processor 2022 typically includes a central processing unit or CPU and memory. The processor 2022 is coupled to the aircraft on which the probe 2002 is installed via the data bus 2024. It will be appreciated that the processor 2022 may be a general purpose computer or may form part of the control system of the aircraft. The processor 2022 may include a comparator to compare signals indicative of the detected electromagnetic radiation impinged on the detector 2012 from the source with a predetermined value stored in the memory of the processor 2022. It will be appreciated that the communication between the processor 2022 and the detector 2012 is shown as a single line, but may be a serial data bus or a parallel data bus. Furthermore, the control line to the source 2014 is a power connection in this example which provides power to the source 2014.

Figure 12 illustrates schematically a leading edge or surface 2020 of the ice detector 2002 illustrated in Figure 10 with ice accretion on the leading edge 2020 of the central portion 2006. The same reference numerals are used to identify the same features as those illustrated in Figure 10. When the probe 2002 is in operation and the leading edge 2020 is free of ice accretion and other debris, the electromagnetic radiation from the source 2014 is projected in the direction 2016 of the detector 2012 and impinges on the detector 2012. In this example the source emits an elliptical shaped beam having a near "top hat" profile across the width of the elliptical beam such that each pixel or discrete pixel group of the detector 2012 will output a signal (e.g., intensity) having a similar value/level indicative of the electromagnetic radiation impinged on the detector 2012. Accordingly, the intensity from each pixel may be plotted on a graph or shadow graph as intensity in arbitrary units against distance from the leading edge 2020. This can be done since each pixel will be positioned a fixed known distance from the leading edge 2020 dependent on the position of the pixel in the array. When ice 2026 accretes on the leading edge 2020 the detector will be shadowed and the ice may block or attenuate the electromagnetic radiation such that less or no electromagnetic radiation will be received by a portion of the detector 2012. By continuously plotting the intensity of the electromagnetic radiation for each pixel or discrete block of pixels, the intensity detected and plotted from the pixels closer to the leading edge 2020 will be reduced, such the thickness of the accreted ice can be deduced. The thickness of the ice may be deduced by determining the point on the plotted values of intensity against distance from the leading edge 2020 at which the intensity falls below a predetermined threshold. Alternatively, if the processor 2022 includes a comparator, the processor 2022 may simply return a low or high signal indicative of electromagnetic radiation received by or impinged on the detector 2012. Therefore, the plotted intensity would either have a zero value or an arbitrary non-zero value for each pixel in the array. A threshold may be used because the ice accretion may only attenuate the electromagnetic radiation and may not completely block it.

In the figure, two examples of ice accretion are shown. A first ice accretion layer 2026 shadows a region of approximately 5 mm of the array detector 2012 such that the detected ice thickness will be approximately 5 mm. The shadow is illustrated in the figure with a horizontal dashed line 2028. It will be appreciated that the thickest portion of the accreted ice will shadow the detector 2012. That is to say that the thickest portion of the accreted ice on the leading edge 2020 will attenuate or block the electromagnetic radiation impinging on the detector 2012. A second ice accretion layer represented by dashed line 2030 shadows a region of approximately 20 mm of the array detector 2012, such that the detected ice thickness will be approximately 20 mm. The shadow is illustrated in the figure with a horizontal dashed line 2032.

Figure 13 illustrates an example shadow graph for the two accreted ice layers 2026, 2030 illustrated in Figure 12. The shadow graph in the figure is a plot of intensity in arbitrary units against distance from the leading edge 2020 of the aerofoil portion 2006 of the probe 2002. The intensity of the detected electromagnetic radiation for the first accreted ice layer 2026 is illustrated with a solid line 2034 and the intensity of the detected electromagnetic radiation for the second accreted ice layer 2030 is illustrated with a dash-dot-dash line 2036. The graph also includes threshold intensity value shown by a horizontal dotted line 2038. The graph is used to determine the thickness of the accreted ice by determining when the intensity of the detected electromagnetic radiation crosses the threshold value. For the first accreted layer 2026, the intensity 2034 of the detector 2012 crosses the threshold value line 2038 at approximately 5 mm. For the second, thicker, accreted layer 2030, the intensity 2036 of the detector 2012 crosses the threshold value line 2038 at approximately 20 mm, as is illustrated in Figure 13.

It will be appreciated that the graph may be presented on a display within a cockpit for a user to interpret. Alternatively, the processor 2022 may compare the intensity at each of the pixels of the array detector with a predetermined threshold value to determine a location on the array that coincides with the shadow created by the accreted ice. For example the intensity detected for each pixel could be compared with the predetermined threshold value until the value of the detected intensity value falls below the threshold (or above the threshold depending on the starting point of the comparison). The location of the pixel is translated to a distance from the leading edge 2020. Furthermore, the processor 2022 may periodically detect the thickness of the ice accreted and use this to determine an average rate of growth of the accreted ice.

Figure 14 illustrates schematically an optical set-up which may be used to obtain an elliptical beam pattern for the source 2014. In Figure 14, an optical fibre 2500 transmits laser light to a small focal length convex lens 2501 via a fibre optic ferrule 2502. The diverging laser light beam 2503 then passes through a cylindrical lens 2504. The two lenses 2501, 2504 shape the light beam so that the outputted laser beam 2505 has a profile (see 2506) which is generally elliptical in shape, and the laser beam 2505 is a generally parallel beam.

The source 2014 uses a 650 nm wavelength laser with a power in the range of 3 to 10 micro watts. However, it will be appreciated that other laser sources may be used, both inside the visible range and outside the visible range. Figure 15 illustrates schematically an ice detector 2002 viewed from above according to an embodiment of the invention. The same reference numerals are used to identify the same features as those illustrated in Figures 10 and 11. The ice detector or probe 2002 illustrated in the figure includes a different source 2014 and detector 2012. The location of the emitted electromagnetic radiation is the same as that illustrated in Figures 10 and 11, but the source of the electromagnetic radiation is moved to a remote location from the end portion 2010. Nevertheless, the electromagnetic radiation is emitted from the same location as indicated in the figure by reference numeral 2014. In the example the source of electromagnetic radiation is provided by one or more laser diodes coupled to a respective optical fibre. That is to say that a proximal end of each optical fibre is coupled to a laser diode and a distal end of the optical fibre is fed from the laser diode to the source location 2014. It will be appreciated that known methods of coupling the optical fibre to the laser diode will be used, for example, pig- tailing or a lens is used to focus the electromagnetic radiation in to the optical fibre. In the example shown, five individual laser diodes 2042 are used, each being coupled to a respective optical fibre that is grouped in a fibre bundle 2040 and fed to the source location 2014 of the end portion 2010. The laser diodes may be individual diodes that are electrically connected to the processor 2022 to be controlled, or may be a diode array, that may be formed within a single enclosure with the processor 2022. In the figure the fibre bundle 2040 is fanned-out at the source location 2014 such that the individual optical fibres are arranged in a line extending from the leading edge 2020 of the central aerofoil portion 2006. Thus an elongate pattern of electromagnetic radiation is emitted from the optical fibres. The individual optical fibres may be arranged immediately adjacent one another or may be spaced apart. Furthermore, the distal ends of the optical fibres may be protected by a glass window at the source location 2014. Furthermore, the distal ends of the optical fibres may be provided with a collimating lens. It will be appreciated that more or fewer optical fibres may be used to create a longer or shorter elongate pattern.

The detector 2012 in this example is provided with one or more optical fibres coupled to a linear array detector. In this example, a fibre bundle 2044 is provided containing 5 optical fibres, the distal end of each optical fibre being located at the detector location, which corresponds to the location of detector 2012 as is illustrated in Figure 10. The distal ends of the optical fibres may be protected using a glass window that is at least transparent to the wavelength of electromagnetic radiation used, for example, and is located in the end (or mounting) portion 2008. The distal ends of the optical fibres in bundle 2044 are arranged in a line extending from the leading edge 2020 in this example to create a linear array detection area. The proximal end of each optical fibre is coupled to a linear detector array 2046 similar to the array 2012. It will be appreciated that a focusing lens may be used to couple the electromagnetic radiation from the optical fibres to the detector 2046 or the optical fibres may be abutted with a sensing surface of the detector 2046. The detector 2046 is coupled to the processor 2022 such that the processor receives the signals output from the detector indicative of the intensity detected by the detector 2046. The detector 2046 may be an integral part of the processor 2022. It will be appreciated that, if optical fibres are used as in this example, the resolution of the detection is typically reduced, because the surface area of the end face of the optical fibre may be larger than a single pixel of the array. However, the number of optical fibres can be increased to increase the resolution of the detection. It will be further appreciated that the detector is operated as a number of discrete detectors, for example one or more silicon photo diodes, (one for each optical fibre), where each discrete detector is formed of one or more pixels dependent on the area of the detector array, the surface area of the end face of the optical fibre and the number of optical fibres used.

The operation of the probe 2002 shown in Figure 15 is similar to that described for the previous embodiment of probe 2002. It will be appreciated that, in the example shown in Figure 15, an array detector (e.g., X*Y) and not a linear array (e.g., 1*X) may be used. If an array detector is used, the fibre bundle will not be fanned out as for a linear array, rather the optical fibres will be kept in a bundle and abutted with the detector surface, for example. The location of each optical fibre is recorded such that the output signal from the array detector can be translated to a shadow graph.

Also, the source of electromagnetic radiation in this example may be provided by one or more laser diodes located at source location 2014, and without the use of the optical fibres. It will be appreciated that a collimation lens may be used if the electromagnetic radiation from the laser diodes is divergent. The optical fibres described above may be glass or plastic optical fibres and have a diameter ranging from 100 μηι to 500 μηι. However, the examples are not limited to only this type of optical fibre.

Figure 16 illustrates schematically an ice detector apparatus viewed from above according to an embodiment of the invention. The same reference numerals are used to identify the same features as those illustrated in Figure 15. In this example, the source is the same as the source described in the example illustrated in Figure 15, and the detector is taken from the example shown in Figures 10 and 11. Alternatively, the detector 2012 may be provided with individual photo detectors. It will be appreciated that more or fewer photo detectors may be used, which will determine the resolution of the detection. It will be appreciated that any of the various examples of source and detector configurations illustrated and described may be combined.

Figure 17 illustrates schematically an alternative arrangement of the ice detector 2002 illustrated in Figure 10 according to another embodiment of the invention. The same reference numerals are used to identify the same features as those illustrated in Figure 10. In the example shown in the figure, an electromagnetic radiation source 2052 is located in the right hand portion 2010 of the probe 2002. The source 2052 in this example may be any of the sources described in reference to the embodiments of Figures 10 to 16. The source 2052 is located in a higher position than the source of the probe illustrated in Figure 10. In other words the source 2052 in this example is located above a line extending along the leading edge 2020 of the aerofoil 2006. The line extending along the leading edge 2020 of the aerofoil 2006 is illustrated in the figure by a dash-dot-dash line 2056. In the example shown in the figure, a detector 2050 is arranged to detect the electromagnetic radiation projected from source 2052 and is located in the left hand portion 2008 of the probe 2002. The detector 2050 in this example may be any of the detectors described in reference to the embodiments of Figures 10 to 16. The detector 2050 is located in a lower position than the detector of the probe illustrated in Figure 10. In other words the detector 2050 in this example is located below the line 2056 extending along the leading edge 2020 of the aerofoil 2006. It will be appreciated that the source 2052 and detector 2050 will be appropriately angled based on their position relative to the line 2056. By displacing the source 2052 and detector 2050 above and below the line 2056 respectively, the beam of electromagnetic radiation 2058 no longer extends or follows the leading edge 2020 of the aerofoil central portion 2006, rather the electromagnetic radiation beam 2058 is projected along a line that is angularly offset with respect to the line 2056 extending along the leading edge 2020 of the aerofoil 2006. In other words the position of the source 2052 and detector 2050 are rotated counter clockwise as compared to the source and detector of Figure 10. Since the beam 2058 is angularly offset with respect to the line 2056 extending along the leading edge 2020 of the aerofoil 2006, the beam 2058 crosses the line 2056 extending along the leading edge 2020 of the aerofoil 2006 at a single point 2054. Therefore, it is possible to determine the ice accretion at a point along the leading edge 2020 of the aerofoil using the same technique as described above. However, it will be appreciated that only a portion of the ice accreted on the leading edge will be capable of shadowing the detector 2050. In the example shown in Figure 17, only a single detector and source are shown. However, multiple sources and respective detectors could be used, each source-detector pair being located at a different vertical distance from the line 2056 extending along the leading edge 2020 of the aerofoil 2006. Accordingly the thickness of ice accreted at different points along the leading edge 2020 could be measured. It is noted that the multiple sources and detectors are stacked, one above the other and in a line on opposite sides of the aerofoil central portion 2006. If multiple detectors and sources are used, different wavelength sources may be used and respective filters for the detectors to avoid interference or signal contamination. However, since collimated beams may be used and the sources and detectors are all offset such that the beams of electromagnetic radiation are parallel to one another, any interference or signal cross-talk between sources should be avoided.

Figure 18 illustrates schematically an aircraft 2060 according to an embodiment of the invention having fitted thereon an ice detector 2002. The ice detector or probe 2002 is any one of the previously described examples and embodiments and is illustrated as being attached to a fuselage 2062 near or adjacent a wing 2064 or tail portion 2066. Alternatively, the aerofoil or central portion 2006 may be replaced in this example with a portion or length of a section of the wing 2064 or tail section 2066, for example. A section or portion of the aircraft may be referred to as an aero-frame or airframe component. Accordingly, the end portions 2008, 2010 of the probe 2002 illustrated in various examples may be attached directly to the surface of a wing such that ice accretion on the wing 2064 is directly detected, as is illustrated in the figure.

It will be appreciated that the thickness of ice accretion is generally determined herein using a detector having multiple sensor elements (e.g., pixels) in a line on a linear array. However, it will be understood that it is not necessary to determine the thickness of accreted ice in this manner. Indeed, the intensity received from a single photo sensor coupled to optical fibres receiving the electromagnetic radiation from the source may be used to determine the thickness of accreted ice. This is because the electromagnetic source has a near top-hat intensity distribution, such that the total intensity impinging on the detector is reduced as the thickness of the ice increases and shadows the detector. For example, it will be possible to calibrate the detector without any ice accretion present on the surface of the detector. If a plurality of stacked optical fibres are used, it can be determined that each optical fibre will receive a fraction of the electromagnetic radiation from the source. Accordingly, if five optical fibres are used, each optical fibre will receive approximately a fifth of the electromagnetic radiation from the source, such that the intensity of the signal from a signal detector (or array detector where the respective outputs are summed) reduces by a fifth. Thus it may be determined that a thickness of ice approximately equal to the thickness of the optical fibre has accreted on the surface of the detector. It will appreciated that embodiments of the invention have been described as being applied to a surface that may be on a part or component of an aircraft, e.g., a wing or tail section, but may also be used for other moving aerodynamic structures such as a blade of a wind turbine. For example, a probe similar to the one illustrated in Figure 10 may be positioned adjacent a wind turbine blade or point of rotation of the blades of a wind turbine, such that unwanted ice accretion may be detected.

The leading edge or surface (i.e., elongate surface) 2020 of the detector 2002 may be provided with a heater mat of the form described in any one of GB-A-2,477,336, GB-A-2,477,337, GB- A-2,477,338, GB-A-2,477,339 or GB-A-2,477,340. The heater mat may be provided to clear any accreted ice to allow for a new sensing cycle to be initiated. That is to say that, after each sensing cycling, the ice accreted on the leading edge or surface 2020 may be cleared using a heater mat. For example, if it is determined using the detector 2002 that there is sufficient ice accretion on the leading edge or surface 2020 such that a de-icing apparatus of an aircraft on which the detector is attached is activated, the heater mat of the detector 2002 may also be activated to de-ice the leading edge or surface 2020 for subsequent detection cycles.

Section 3

Figure 19 illustrates schematically a perspective view of detector 3000 according to an embodiment of the invention. The ice detector 3000 illustrated in the figure is a stand-alone probe. The probe is stub-like and is orientated to face into airflow 3002. The probe 3000 comprises a central portion or aero-frame component 3004 having an aerofoil cross section. The aerofoil portion 3004 is generally elongate and extends between two end portions 3006, 3008. The first end portion 3006 (illustrated on the left hand side of the figure) is attached at a proximal end of the elongate aerofoil section 3004. The second end portion 3008 (illustrated on the right hand side of the figure) is attached at a distal end of the elongate aerofoil section 3004. When installed the probe 3000 is orientated such that in normal use (i.e., attached to an aircraft moving forward) airflow will be in the direction of arrow 3002 illustrated in the figure. The central aerofoil portion 3004 will mimic a wing or tail portion and air will flow to and round a leading edge 3010 and provides a surface on which ice accretes and forms a surface or base from which a measurement may be performed.

The detector 3000 includes an end portion 3012 arranged at a proximal end of the probe to allow the detector 3000 to be attached to an aircraft using one or more fixing points (not shown). For example, the probe 3000 may be attached to a fuselage of an aircraft near one of the wings or a tail section. Each of the first and second end portions 3006, 3008 and the central portion 3004 is manufactured from a composite material in a similar manner to other aircraft parts. However, it will be appreciated that the housing parts of the detector 3000 may be manufactured from other suitable materials, e.g. metal, or plastics. The detector 3000 includes three separate detectors (sensing systems). A first forward facing detector 3014 is provided along the leading edge or surface 3010 of the aero-frame component 3004. In other words the first detector 3014 is arranged to face the direction of airflow 3002 in normal use (i.e., when the detector 3000 is attached to an aircraft and the aircraft is moving forward). The detector 3014 is a sensor of the type described in WO- 2004/110865.

A second sideways facing detector 3016 is arranged to detect the presence and thickness of ice accretion on the leading edge 3010 of the central aero-foil portion 3004. That is to say that the sideways facing detector detects the presence and thickness of accreted ice based on a detection that crosses the direction of airflow 3002. In general, the first detector 3014 detects ice accretion in a direction generally perpendicular to or crossing the ice accretion detection direction of the second detector 3016. The second detector 3016 includes a source 3018 and respective detector 3020. Furthermore, the first and second detectors are arranged to detect ice accretion at the same point on the leading edge 3010 or portion of the leading edge 3010. For example, the first and second sensors 3014, 3016 may be arranged to detect ice accretion on a portion of the leading edge corresponding to the footprint of the first forward facing sensor 3014. Furthermore, the first forward facing sensor comprises a sensing surface, such that a beam of electromagnetic radiation emitted by the second sensor 3016 is arranged to intersect or cross the sensing surface of the first detector 3014. As illustrated in Figure 19 and described below, this may be achieved by projecting the beam of electromagnetic radiation at an angle to the leading edge 3010.

A third sideways facing detector 3022 is arranged to detect the liquid water content and ice content in a detection volume ahead of the aero-frame component 3004. The end portions 3006, 3008 extend generally from the leading edge 3010 of the aero-foil central portion 3004, such that a volume is generally enclosed by the extensions of the end portions 3006, 3008 and the leading edge 3010. The third sideways facing detector 3022 detects liquid water content and ice content in a volume in a direction generally perpendicular to or crossing the ice accretion detection direction of the first detector 3014. The third detector 3022 includes a source 3024 and respective detector 3026. The leading edge 3010 of the central aero-frame portion 3004 includes a heater mat 3028 of the form described in any one of GB-A-2,477,336, GB-A-2,477,337, GB-A-2,477,338, GB- A-2,477,339 or GB-A-2,477,340. The heater mat 3028 is provided to clear any accreted ice to allow for a new sensing cycle to be initiated. That is to say that, after each sensing cycle, the ice accreted on the leading edge 3010 of the aero-frame component 3004 may be cleared using heater mat 3028. For example, if it is determined using the probe 3000 that there is sufficient ice accretion on the leading edge 3010 such that a de-icing apparatus of an aircraft on which the probe is attached is activated, the heater mat 3028 will also be activated to de- ice the leading edge 3010 for subsequent detection cycles.

Figure 20 illustrates schematically, viewed from above, the detector 3000 according to an embodiment of the invention. The same reference numerals are used in Figure 20 as in Figure 19 to identify the same features. The probe 3000 includes an inner aircraft portion 3030 that is coupled to the outer aircraft portion 3036. The two portions 3030, 3036 of the probe 3000 are coupled together on either side of a panel 3038 of an aircraft. The panel 3038 of the aircraft is illustrated by a dashed line in the figure, since it is not part of the probe 3000. The panel 3038 of the aircraft is provided with through holes to allow for signals to be fed to and from the sensors 3014, 3016, 3022 and the heater mat 3028 of the outer portion 3036 and for fixing the inner and outer portions 3030, 3036 together.

The inner aircraft portion 3030 also houses control electronics 3034 that is coupled to each of the sensors 3014, 3016, 3022 and the heater mat 3028 using one or more connections 3040. The connections 3040 may include parallel or serial type data connections and power connections. The control electronics 3034 may be a general purpose computer and may include a central processing unit or CPU and memory. The control electronics or processor 3034 is coupled to the aircraft via data bus 3042 and connector 3032. It will be appreciated that the communication between the processor 3034 and the connector 3032 is shown as a single line, but may be a serial data bus or a parallel data bus. The processor 3034 may also be provided outside of the probe 3000 (for example, inside the aircraft) or the functionality of the processor 3034 may be provided by a control system of the aircraft to which the probe 3000 is attached.

The second sideways facing sensor 3016 is of the type already described with reference to Figures 10 to 17. The probe 2002 illustrated in Figures 10 to 17 is similar to the probe 3000 illustrated in Figures 19 and 20, except it only includes a single, sideways facing, sensor. The source of the second sideways facing sensor 3016 illustrated in Figures 19 and 20 using reference numeral 3018 corresponds to the source illustrated in Figures 10 to 17 and identified using reference numerals 2014 and 2052. The detector of the second sideways facing sensor 3016 illustrated in Figures 19 and 20 using reference numeral 3020 corresponds to the detector illustrated in Figures 10 to 17 and identified using reference numerals 2012 and 2050. Furthermore the housing portions 3004, 3006, 3008 of the probe and the leading edge 3010 illustrated in Figures 19 and 20 correspond to the housing portions 2006, 2010, 2008 and the leading edge 2020 of the probe 2002 illustrated Figures 10 to 17.

The third sideways facing sensing system 3022 is of the type already described with reference to Figures 2 to 9. The probe 2 illustrated in Figures 2 to 9 is similar to the probe 3000 illustrated in Figures 19 and 20, except it only includes a single, sideways facing, sensor. The source of the sideways facing sensor 3022 illustrated in Figures 19 and 20 using reference numeral 3024 corresponds to the source illustrated in Figures 2 to 9 and identified using reference numeral 3. The detector of the third sideways facing sensor 3022 illustrated in Figures 19 and 20 using reference numeral 3026 corresponds to the detector illustrated in Figures 2 to 9 and identified using reference numeral 51. Furthermore, the housing portions 3004, 3006, 3008 of the probe and the leading edge 3010 illustrated in Figures 19 and 20 correspond to the housing portions 24, 22, 23, and the leading edge 241 of the probe 2 illustrated Figures 2 to 9. Each of the detection techniques (sensing systems) is described individually as having its own individual processor. However, it will be appreciated that all the processing of the detectors may be performed centrally within processor 3034 illustrated in Figure 20.

Figure 21 is a diagrammatic perspective view of an aircraft 1 having wings 11, engine nacelles 12, a tail plane 13 and a tail fin 14 which incorporate heater mats 15 of an ice- protection system in their leading edges. A detector or probe apparatus in the form of a probe 3000 is mounted on the fuselage 16 adjacent to the cockpit 17. If the probe detects the presence of ice on the leading edge 3010 of the probe 3000, the heater mats 15 of an ice- protection system are activated. At the same time, the heater mat 3028 of the probe 3000 is activated to clear the ice for the next measurement cycle. The probe 3000 has been described to include three separate detection techniques (sensing systems). The processor 3034 may receive an output signal from each of the detectors (or the individual signals may be generated within the processor 3034). The signals are used to determine if it is necessary to activate a de-icing apparatus, e.g., the heater mats 15 illustrated in Figure 21. For example, the processor may use two of the three signals to determine if ice is present on the leading edge 3010 of the probe 3000. That is to say that the processor 3034 may take the signal from the forward facing detector 3014 (e.g., primary sensor) and determine that ice is present (or has accreted) on the leading edge 3010 and take the signal from the sideways facing ice accretion detector 3016 (e.g. secondary sensor) and determine that ice is present on the leading edge 3010. Accordingly, if both signals determine that ice is present, the de-icing apparatus 15 is activated. However, if one or other of the two signals indicates that no ice is present on the leading edge, the processor may determine that ice is not present on the leading edge. It will be appreciated that any one of the three sensors can be used as a primary, secondary or tertiary sensor. Alternatively, the processor may take the output signal from the water content and/or ice crystal particle concentration detector 3022 to further verify the presence of ice on the leading edge 3010. For example, if the water content and/or ice crystal particle concentration detector 3022 deems that there is ice/water present ahead of the leading edge 3010 and there is a disparity between the signals of the forward and sideways facing detectors 3014, 3016, the processor 3034 may activate the heater mats 15. It will be appreciated that the probe may only include two different types of sensor. It will be further appreciated that ice detected on the leading edge 3010 of the probe 3000 is indicative of ice formed on the leading edge of the wings 11 of the aircraft 1, for example. Furthermore, the processor 3034 may take the signals from each of the forward and sideways facing detectors 3014, 3016 to determine the thickness of the ice accreted on the leading edge 3010. This may be done by obtaining an estimation of the thickness of ice accreted on the leading edge 3010 of the probe 3000 using the signals output from the forward and sideways facing detectors 3014, 3016 and by taking an average of the estimated thicknesses. The average may be a mean, minimum or maximum or other suitable average, for example, an average taken over a predetermined period of time.

It will be appreciated that the housing of the sensor illustrated in Figures 19 and 20 differs from the housing illustrated in Figure 7 and 8. In the example illustrated in Figures 19 and 20, the plane of the detector is rotated such that the detector 3026 is rotated to be vertical with respect to the example detector 51 illustrated in Figures 7 and 8, where the detector 51 is illustrated in a horizontal plane. It will be appreciated that embodiments of the invention have been described as being applied to a surface that may be on a part or component of an aircraft, e.g., a wing or tail section, but may also be used for other moving aerodynamic structures such as a blade of a wind turbine. For example, a probe similar to the one illustrated in Figure 19 may be positioned adjacent a wind turbine blade or point of rotation of the blades of a wind turbine, such that unwanted ice accretion may be detected.

Section 4 The section headings in the Description of Embodiments are for the reading convenience of the reader, and the Description of Embodiments may be read as a continuous description.

While the invention is described herein by way of example for several embodiments and illustrative drawings, those skilled in the art will recognize that the invention is not limited to the embodiments or drawings described. It should be understood that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed. On the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention. The headings used herein are for organizational purposes only and are not meant to be used to limit the scope of the description. As used throughout this application, the word "may" is used in a permissive sense (i.e. meaning "might") rather than the mandatory sense (i.e., meaning "must"). Similarly, the words "include", "including", and "includes" mean including, but not limited to.