A method for warning of engine icing conditions and application of runup procedures for a jet engine

BACKGROUND OF THE INVENTION

The invention relates to a method for warning of the risk of engine icing and the need for initiating run-up procedures to avoid damage to aircraft jet engines when operating in meteorological conditions known to catalyze the building of ice.

It is well known from accident investigations and reports that the fan blade of a jet engine may build up ice when moving through atmospheric air containing supercooled precipitation (freezing fog, freezing drizzle or freezing rain) or snow. This is called "engine icing" and is a problem in particular on the ground where the humidity is higher and jet engines are running in idle or in a low power setting during taxiing. Build up ice may at some point break off at high velocity and damage the interior parts, in particular the fan blades of a jet engine. Therefore pilots are cautioned to use specific run-up procedures whereby the engine is powered up to a predetermined level to shed the ice on the ground before the ice layer becomes heavy enough to cause damage to the jet engine or have the engine inspected from the outside and potentially de-iced.

Incident reports cover a wide range of damages from a compressor stall during take off to mechanical damage to fan blades and other interior parts.

Damage to jet engines has been reported in snow as well as supercooled precipitation conditions such as freezing drizzle, which is assumed to be the most critical conditions for building up of engine ice.

The manufacturers of jet engines may provide information on the critical conditions for a safe operation of a jet engine such as idle time in specific meteorological conditions including the intensity of the precipitation, run-up procedures and time intervals between applications of the run-ups. It may also be the appropriate flight authority or the airline itself setting the recommended limits for a safe operation of jet engines in icing conditions.

All these safety measures are reasonably well-defined relative to the atmospheric conditions and the concentration of freezing precipitation. However, the problem is that the measurement measures available for ascertaining the concentration of freezing precipitation have so far been inadequate.

It is the object of the invention to provide a reliable method and an apparatus for improving the known measurement methods.

SUMMARY OF THE INVENTION

The invention is based on the findings that by combining measurements from known meteorological sensors capable of measuring and providing information such as the ambient temperature and the type of precipitation with the measurement of a very specific apparatus which is known per se and combining those measurements in an algorithm containing e.g. a database with critical engine icing parameters for known jet engine types and models it is possible to accomplish a far more accurate measurement and determination of the risk of engine icing and thus also the appropriate warning and initiation of proper run up procedures and/or inspection followed by external de-icing. Measuring of dew point and humidity known per se may also form part of the meteorological measurements and inputs for the determination of risk of engine icing.

According to the invention the measurement is based on the amount of ice deposited on a measurement sensor which is moved through the air. As it is, the problem of measuring freezing precipitation is that the super-cooled water droplets are so tiny that they stay floating in the air and do not deposit on other types of measurement apparatuses.

Below, a number of features of the method and the apparatus according to the invention will be given.

A method for initiating a run-up procedure for a jet engine to avoid engine icing, the method comprising the following steps: providing at least one surface element that is made of a material suitable for ice in atmospheric air to freeze on, said element having a predetermined surface area; moving said at least one surface element through the atmospheric air at a predetermined velocity for a predetermined period of time to enable ice to freeze thereon; measuring a thickness or mass of adhered ice to said at least one surface element by means of a measurement device configured therefore after said predetermined period of time, said measurement device being arranged to calculate the concentration of supercooled precipitation and snow in the atmospheric air; and comparing the calculated concentration of freezing precipitation with the specifications of the jet engine.

A method wherein the supercooled precipitation comprises freezing drizzle.

A method wherein the surface element is rotated.

A method wherein the velocity of the surface element is comparable with the velocity of an idling jet engine's rotor blades.

A method wherein the velocity of the surface element is controlled within the interval from zero to the velocity of an idling jet engine's rotor blades.

A method wherein the ice adhered to the surface element is, following measurement of said mass or thickness thereof, removed from the at least one surface element, whereupon a renewed measurement process can be performed.

A method wherein the ice adhered to the surface element is removed by heating of said at least one surface element.

A method wherein a cover is provided that, in a first position, extends at least across said at least one surface element and covers and shields the at least one surface element; and said cover being removed from the at least one surface element at least for the predetermined period of time during which the at least one surface element is moved through the atmospheric air at a predetermined rate.

A method wherein the at least one surface element is caused to move for a predetermined period of time after the cover has reverted to its first position following a measurement procedure, whereupon the thickness or mass of the ice adhered to the at least one surface element is measured.

A method wherein the at least one surface element is caused to move through the atmospheric air at a velocity that ensures that atmospheric precipitation not frozen fast thereon is substantially thrown off.

A method wherein at least two surface elements are used that are rotatably arranged on a rotor shaft; and wherein the rotor shaft is rotated to move the two surface elements through the ambient air.

A method wherein an alarm time period is calculated from the calculated concentration of freezing precipitation.

A method wherein an inspection time period is calculated from the calculated concentration of freezing precipitation.

An apparatus comprising at least one surface element made of a material suitable for ice in atmospheric air to freeze on, wherein the at least one surface element has a predetermined surface area, and comprising means for moving the at least one surface element through the atmospheric air at a predetermined rate and for a predetermined period of time, and wherein further means are provided for measuring the thickness or mass of the ice adhered to the at least one surface element after the predetermined period of time, during which the at least one surface element has been moved through the atmospheric air, said apparatus comprising a computer for performing the following steps: calculating from the measured thickness or mass of ice information being representative for the concentration of freezing precipitation in the atmospheric air; comparing said information with the specifications of a jet engine.

An apparatus wherein the computer is arranged for calculating an alarm time period in response to the calculated concentration of freezing precipitation.

An apparatus wherein the computer is arranged for calculating an inspection time period in response to the calculated concentration of freezing precipitation.

An apparatus comprising a weighing device configured for weighing and recording at least the weight of the at least one surface element before and after the at least one surface element has moved through the atmospheric air.

An apparatus comprising means for heating the at least one surface element.

An apparatus wherein the apparatus comprises a rotor element with a rotor shaft, and at least two surface elements that extend from the rotor shaft and protrude therefrom, and wherein means are provided for rotating the rotor about an axis thereof.

An apparatus wherein the velocity of the rotor element is controlled. The velocity may be comparable with the velocity of the rotor blades of an idling jet engine.

An apparatus wherein the apparatus comprises a cover whose inside faces towards the at least one surface element and which is configured for occupying a first position in which it extends across the at least one surface element that is hereby covered upwardly, and a second position in which the cover is removed.

An apparatus wherein the cover is configured such that it forms, in said first position, a closed space around the at least one surface element.

An apparatus wherein means are provided for heating the closed space underneath the cover.

An apparatus wherein the apparatus moves the at least one surface element for a predetermined period of time after the cover has, following a

measurement procedure, reverted to its first position, whereupon the thickness or mass of ice adhered to the surface element can be determined. An apparatus wherein the cover is, in said second position, positioned such that its inside is substantially protected against atmospheric precipitation and consequently remains dry.

An apparatus wherein the each of the at least one surface element consists of a plate having a front and a back oriented opposite thereto, and wherein the plate is configured such that the front of the plate faces in the direction in which the respective surface element is moved through the atmospheric air, and wherein — through the plate — a plurality of passageways extend from the front of the plate to back thereof such that the atmospheric air is allowed to flow through the passageways from the front of the plate to the back of the plate.

An apparatus including a system of surface elements mounted on a rotatable shaft configured for being positioned in an essentially vertical position; and wherein the individual surface elements are configured and arranged such that the individual surface elements abut or overlap other surface elements seen in the direction of said shaft, whereby no space remains between the individual surface elements when the apparatus is viewed from above, and thus that all atmospheric precipitation falling within the expanse of the apparatus, when the rotatable shaft is positioned vertically, essentially hits the surface elements and is thus able to settle in the form of ice.

An apparatus wherein the surface elements are configured and arranged such that the individual surface elements corresponding to their projection on a face parallel with the rotatable shaft abuts on or overlaps other surface elements, whereby there is no space between the individual surface elements, when the apparatus is viewed from the side, and such that the

atmospheric air conveyed across the surface elements in a direction substantially perpendicular to the shaft by a relative movement between the atmospheric air and the surface elements substantially hits a surface element and is thus able to deposit the water contained therein as ice.

An apparatus wherein the surface elements include passageways; and the apparatus comprises means such that air can be conveyed through the passageways.

An apparatus wherein the apparatus comprises means for providing air in the form of either heated air or air essentially at ambient temperature.

An apparatus including a computer or calculating device for recording the measurement results for the thickness or mass of ice deposited on the surface element(s), and producing a visual or audio signal to the pilots in the flight deck — either automatically or through the pilots checking the risk of engine icing through their flight information and communication channels in the cockpit — or to ground based personnel such as deicing or flight operations personnel involved in flight safety management/decisions.

An apparatus wherein the apparatus comprises means for converting the thickness or mass measured into a value that will be indicative of a risk of icing.

The apparatus according to the present invention is particularly suitable for use in airports, where the apparatus is preferably arranged at ground level in an air port, and whereby the apparatus comprises means for recording the measured results of the thickness or mass of the ice deposited on the surface elements, and means for visually or auditively emitting a signal regarding the measurement results to the monitoring personnel of the airport.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in further detail with reference to the drawings, wherein

Figures 1 through 4 are explanatory sketches using a schematically represented apparatus to illustrate various process steps used with the present invention;

Figure 5 is a sectional view that illustrates a vertical, sectional view through a surface element for use in the apparatus according to the present invention;

Figures 6 through 8 show a first, preferred embodiment of an apparatus for use in the invention, wherein the cover is shown in different positions;

Figure 9 shows an alternative, preferred embodiment of an apparatus for use in the invention, seen from the side;

Figure 10 is a vertical, sectional view through the apparatus shown in Figure 9 corresponding to the plane indicated by B-B;

Figure 11 illustrates a preferred embodiment of a rotor element for use in connection with the invention;

Figure 12 is a top plan view of the rotor element corresponding Figure 11 ;

Figure 13 shows a further preferred embodiment of a rotor element for use in connection with the invention; and

Figure 14 illustrates the principle and apparatus according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First with reference to Figures 1-13 a number of embodiments of submethods and components used in the present invention will be described. Later, with reference to Figure 14, the present invention will be described.

Figure 1 shows the constructive principles of an apparatus according to the invention, the apparatus including a housing or a frame 1 in which a shaft or a rotor 2 is arranged that supports two diametrically opposed surface elements 3 having predetermined surface areas 3a, the rotor and elements being rotated by a drive unit 4 in the direction of the arrow A. The rotor with one or more surface elements is also referred to as the rotor element.

Corresponding to the normal operative state of the apparatus, the shaft or the rotor is configured for being arranged in a substantially vertical position. When, in the following, the terms vertical and horizontal are used, they refer to the apparatus when arranged in such preferred position.

Additionally the apparatus comprises a weighing device 5 configured for weighing the surface elements 3, the rotor 2 and the drive unit 4, whereby a weight increase can be determined.

Besides, the apparatus comprises a movable cover, in Figures 1 through 4 shown as a dome-shaped shield consisting of two spherical quarter shells 6 that are shown in Figure 1 in a first position in which they shield the surface elements 3 and the rotor 2 and form a substantially closed space 7 around the surface elements and the rotor.

Moreover, the apparatus comprises a blower 8 configured for conveying atmospheric air towards the surface elements 3.

In the process step shown in Figure 1 , the rotor is thus rotated in the closed space 7, and as a consequence of the blower 8 generating circulation of air

in that space, the surface elements 3 are caused to assume a temperature that corresponds essentially to the ambient temperature.

Now, Figure 2 illustrates a subsequent process step in which the two spherical quarter shells 6 have been withdrawn to a second position in which the surface elements 3 on the rotor 2 rotate in the open air, and wherein — provided supercooled water is present in the air — ice formations will settle on the surface elements 3. It will appear from the figure that the cover is, in its second position, situated within the housing that is advantageously configured such that inside of the cover is protected against atmospheric precipitation settling thereon. In the embodiment shown, this could only be accomplished by the spherical shells of the cover being conveyed down into the housing through a narrow opening between the top face and lateral walls of the housing. Such configuration ensures that atmospheric precipitation cannot drip from the inside of the cover and down onto the surface elements when the cover is conveyed above them.

In accordance with the invention, the process step shown in Figure 2 may be carried out for a predefined period of time, whereby a suitable amount of ice will deposit on the surface elements, and the rotor is rotated at a velocity that, on the one hand, takes into account that the ice is not to be thrown off the rotor, but wherein other precipitation, if any, in the form of rain and snow is thrown off to a suitable extent. As shown in the figure, it is of course to be ensured that the cover can be conveyed past the various structures of the housing.

Hereby a stable determination of weight for the drive unit, the rotor, the surface elements and the ice frozen thereon is obtained.

Following the process step shown in Figure 2, a subsequent step is shown in Figure 3 wherein the cover has been returned to its first position. In order to

ensure that the surface elements are substantially free of atmospheric precipitation other than ice before the amount thereof is determined, they can advantageously be rotated for a predetermined period of time after the cover has reverted to its first position.

Now the surface elements are brought to a halt, and the weight increase of the surface elements resulting from the ice deposited on its surfaces is recorded by means of the weighing device 5, and on the basis of the value measured, a signal can be generated for showing a risk of engine icing or run-up procedure recommended; however, the drawing does not feature equipment for this use since it will be obvious to the person skilled in the art to configure such equipment on the basis of the present description.

Now a subsequent process step is shown in Figure 4 wherein the two spherical quarter shells 6 have been moved across the surface elements 3 and shield them so as to form yet again the closed space 7. In this process step the rotor is caused to rotate, and a quick heating of the closed space 7 is carried out by means of the blower 8 and a heater element 9 whereby the ice deposited on the surface elements is melted and thrown off by the rotation of the rotor, whereby the combined weight of the rotor 2 and the surface elements 3 is caused to revert to the initial weight.

Now the process step shown in Figure 1 can proceed, and the apparatus according to the invention is thus very suitable for carrying out repeated measurements with a very high degree of accuracy in standardised conditions.

In order to obtain the highest possible accuracy of measurement, the surface elements 3 may be configured with the largest possible surface for adhesion of ice. Figure 5 illustrates an embodiment wherein a surface element is constructed as a grid, as seen in a vertically sectional view through this, and

from which it will appear that the surface element that is moved in the direction of the arrow B has a front 13 and a back 14; and wherein walls 11 ,12 combine to form passageways 10 that extend from the front 13 to the back 14.

Hereby the surface element forms a relatively large surface compared to the indigenous weight of the surface element which means that a relatively quick formation of a suitable and measurable amount of ice is accomplished on the surfaces of the surface element 3, and such that the surface element can quickly be caused to assume the desired temperatures by the heating as shown in Figure 4 and the cooling as shown in Figure 1.

Both walls 11 ,12 having a downwardly tapering course means that, on the one hand, atmospheric precipitation other than supercooled water or mist is very like to slide or flow off the surface element 3, but also that supercooled water droplets or mist are very like to settle on the surfaces of the surface element without having to pass all the way through the surface element.

The surface elements shown in Figures 1 through 4 are, for the sake of clarity, shown to be very small, but according to a preferred embodiment they are dimensioned to essentially fill the space 7, which means that the surface elements as shown in Figures 1 through 4 join to constitute an approximate semicircle. Hereby it is ensured that the apparatus can be configured with the smallest possible outer dimensions.

As described above, it should be ensured that the inside of the cover is protected against atmospheric precipitation in all process steps; and this is to ensure accurate determination of the amount of adhered there on.

Figures 6 through 8 show an embodiment in which an apparatus comprises a housing 1 , a cover in the form of two spherical quarter shells 6, two storage

units 41 for the two spherical quarter shells and a platform 40 on which the storage units have been arranged. Furthermore the apparatus comprises a rotor element, a drive unit and a (not shown) weighing device as described above.

In a first position the cover 6A covers the rotor element and forms a substantially closed space; in Figure 6 this is outlined with dotted lines. When it is desired to perform a measurement, the elements of the cover are, as shown in Figures 6, 7 and 8, moved to their second position where they are stored for protection in the storage units 41. Since it is in particular the inside of the cover that is to be protected against atmospheric precipitation, it can be chosen to allow the storage units to be upwardly open, which would simplify the construction. Once the first part of the measurement is accomplished, the cover reverts to its first position.

Above it has be outlined schematically how protection of the cover inside against the weather can be accomplished, but of course it is possible to select the configuration, shape and the mutual relations of the cover and the storage units on the basis of many considerations.

Figure 9 shows a further embodiment wherein the apparatus comprises a housing 101 with a drive unit, a frame 110, a control unit 115, a cover 106, and a rotor element 103. The housing 101 is configured as a closed and approximately semicylindrical object cut-off in correspondence with the cylinder axis; the housing is mounted in a frame 110 such that the cylinder axis is substantially embedded horizontally. The cover 106 is also configured as approximately as a semicylinder cut-off in correspondence with its cylinder axis and open in correspondence with the cut edge. The cover is mounted on the frame 110 in such a manner that the cylinder axis of the cover essentially coincides with the cylinder axis of the housing. The cover is configured with a width that is wider (corresponding to the length of the cylinder axis) and a

cylinder radius that is larger than the housing, and pivotally mounted on the frame such that the cylinder axis of the cover also constitutes its axis of rotation. This configuration of the cover and housing makes it possible for the cover, upon a 180 degree rotation about its axis of rotation, to be conveyed from its first position as shown in Figure 9 to a second position underneath the housing, and such that the housing is essentially enclosed in the cover. In Figure 9, the end face of the cover towards the viewer has been removed such that the rotor element 103 arranged on top of the housing is visible. When the cover is turned away as described above, the rotor element will be uncovered and a measurement can be initiated. As also described above, this device will ensure that the inside of the cover is protected against atmospheric precipitation while in its second position.

Study of Figure 10 will now reveal a section through the apparatus shown in Figure 9 corresponding to the plane indicated by B-B. As will appear, the housing is — by means of two fittings 112,113 — mounted in a frame consisting of two posts 110,111. As described above, the housing as well as the cover are mounted with their respective cylinder axes about a common axis 102. The rotor element 103 comprises a rotor shaft 120 and a system of surface elements of which only the top 121 and bottom ones are shown. The overall cylindrical shape of the rotor element is outlined with dotted lines. In the space 107 between the cover and the housing, a sealing is advantageously arranged whereby it is prevented both that atmospheric precipitation penetrates into the space between the cover and the housing and that the atmospheric precipitation that has found its way to the space, if any, will be removed from the inside of the cover when it is conveyed from its second to its first position. As shown in Figures 9 and 10, the top face of the housing is configured as an upwardly conical face, whereby it is ensured that eg melt water is, upon heating of the surface elements, conveyed away from the housing and in particular away from the rotor shaft passage.

Between the posts a control unit 115 is arranged for controlling the drive unit 104, the weighing unit 105, the blower 108, and the heater element 109, and for collection, storage and optionally transmission of measurement data. Such units are configured for functioning in a manner similar to the one described above with reference to Figures 1 through 4. In Figure 10 it is indicated that air is, by means of the blower 8, blown into the rotor shaft and from there conveyed out through the surface elements as also described above. Between the top part of the housing and the rotor shaft, a (not shown) bearing device can advantageously be arranged for supporting and guiding the shaft. According to a preferred embodiment, the apparatus further comprises (not shown) means for automatically shifting the cover between its first and second positions. Obviously, the apparatus comprises a device for securing the cover in its first position; preferably also in its second position.

A preferred embodiment of a rotor element for use in an apparatus according to the invention will now be described with reference to Figures 11 , 12 and 13.

In Figure 11 , the rotor element comprises a shaft 20 on which four stacks are arranged that each consists of a number of surface elements arranged at a mutual distance opposite each other with the respective top surface elements 21 ,22,23,24 arranged in an uppermost plane. Each surface element has a free outer end and an inner end secured to the shaft. The uppermost face elements are arranged with a constant mutual angular distance that corresponds to an angle of 90 degrees as far as four top surface elements are concerned.

As will appear from the figure, the surface elements in the individual stack are displaced relative to each other with the outer free ends arranged so as to generally form a helix. For each stack this helix extends at least over an angle corresponding to the angle between two successively arranged stacks.

For an embodiment with four stacks a helix thus extends over at least 90 degrees. In a particular case where only one stack is used, such stack will be able to extend a full rotation about the shaft, ie 360 degrees.

The individual surface elements are configured and arranged such that, in correspondence with their projection on a horizontal face, they overlap the surface element(s) that are adjacent to or arranged between, respectively, the individual surface elements, so as to eliminate spaces between the individual surface elements when the apparatus is viewed from above. This is shown in Figure 12 that illustrates the apparatus according to Figure 11 , seen from above. Hereby it is obtained that atmospheric precipitation falling within the expanse of the apparatus hits the surface elements and is thus able to settle in the form of ice. The larger the overlap between the individual surface elements, the larger a deviation from vertically falling atmospheric precipitation can be tolerated, while ensuring this.

In correspondence with the above teachings, the surface elements in the embodiment shown can advantageously be configured and arranged such that the individual surface elements corresponding to their projection on a vertical face abut on or overlap the surface elements (5) that adjoin or surround, respectively, the individual surface element so as to eliminate a space, if any, between the individual surface elements when the apparatus is seen from the side. Hereby it is obtained that the atmospheric air conveyed across the surface elements by the relative movement between the atmospheric air and the surface elements hits a surface element and is thereby able to deposit the water contained therein as ice. This is in accordance with the disclosures above with reference to Figure 5.

In an embodiment as shown in Figures 11 and 12 it is further ensured that the apparatus can be configured with the smallest possible outer dimensions.

For instance, an embodiment with four stacks of each eleven surface elements and a diameter of 70 cm has proven to be convenient.

As will also appear from Figure 11 , the individual surface elements are provided with a number of passageways 25 through which air can be blown, heated air to deice the surface elements and air with ambient temperature for tempering the surface elements prior to the next measurement, respectively. According to a convenient embodiment the air is conveyed from the basis of the apparatus to the surface elements through the shaft 20.

In accordance with the embodiment shown in Figure 11 , the surface elements are attached only to the shaft 20 in correspondence with their one end, and this is why the individual surface element is dimensioned to support itself and resist the forces that will occur during the intended use of the apparatus. In an alternative embodiment shown in Figure 13, a large number of rather thin surface elements are used that will be secured partly in correspondence with their inner end to the shaft, partly secured with their outer end to a support structure. This support structure comprises an upper support element 30 secured to the shaft by means of upper connecting means 31 and a lower support member 32 secured to the shaft by means of lower connecting elements 33. Between the upper and the lower support elements a number of outer support elements 34 are arranged, to which the outer end of the surface elements are secured. Corresponding to the above- described embodiment, four stacks of surface elements are also used herein that are in a corresponding manner supported by four outer support elements 34.

Corresponding to the embodiment shown in Figure 11 , the surface elements have equal lengths, which means that each helix extends in a manner that corresponds to the surface of a cylinder and, likewise, the overall shape of the rotor element will be cylindrical. In the alternative embodiment shown in

Figure 13, for each stack surface elements are used that have decreasing length towards the top of the apparatus; if the length decreases linearly the individual outer support element 34 will have a course corresponding to a helix on the surface of a cone, and - likewise - the overall shape of the rotor element will in that case be a cone. As stated above, the overall shape and dimensions of the rotor element and the cover will conveniently correspond to each other. For an embodiment of the apparatus like the one shown in Figures 6 through 8, the rotor element should thus be semispherical.

Of course, it is possible in case of both the described embodiments to arrange surface elements such that the helix-shape described is not a 'perfect' geometrical helix but merely what the person skilled in the art describes as "spiral-shaped".

Obviously, on the basis of the above explanation of the principle underlying the invention, the person skilled in the art will readily be able to point to various embodiments of the construction of an apparatus that is able to perform said process steps as well as to identify process parameters that are suitable therefor with due regard to the configuration of the apparatus, such as the period of time during which the surface elements are to be moved through the open air as shown in Figure 2, and the rate at which the surface elements 3 are to be moved through the air in order to obtain suitable measurement results that are not considerably influenced in case atmospheric precipitation other than supercooled water and mist is deposited on the surface elements 3. Obviously, there is a correlation between the configuration of the apparatus itself and the operation parameters that ensure that the desired measurement results are obtained.

Reference is now being made to Figure 14 for an explanation of the 30 measurement principle according to the invention.

By reference numeral 40 an apparatus is shown corresponding to what was explained above in the context of figures 1-13. A substantial part of the invention consists precisely in selecting such type of apparatus for the measurement of the conditions prevailing in jet engines, albeit the apparatus was originally constructed for measuring precipitation for other purposes. It is another important aspect of the invention that it features a database 41 containing information from the various engine manufacturers on the point in time when it is recommended to perform run-up, inspection, or when danger is imminent.

By means of 'engine choice' 42 the pilot may enter the type of engine of the airplane, following which the computer 43 is configured for calculating the respective run-up, inspection, and danger time periods, see the information shown by 44, 45 and 46, respectively.

The computer 43 may also contain other information, such as empiric correction factors. Albeit it is entirely crucial to the invention that the best possible result is obtained by use of a detector that moves through the air in the same way as the rotor blades of the jet engine, a very specific construction of the rotor, eg like the one shown in Figure 11 or Figure 13, will provide an ideal measure for one engine construction of rotor blades, while there may be a small difference compared to another brand of jet engine. Such small difference may be taken into account by incorporation of correction factors in the computer 43.

In brief, the invention thus provides a so far unprecedented degree of measurement accuracy.

This can be exemplified by means of the below tables that show minute intervals for the run-up procedure. A so-called Ice Factor is used which is determined by means of an apparatus like the one described above in

connection with figure 1-13. The Ice Factor is found on the basis of a measurement of the ice which was formed on the rotating surface element, see the above description. The Ice Factor is directly proportionate to the collected amount of ice/ice thickness; this means that, at Ice Factor 10, twice as much ice is collected as is the case of Ice Factor 5.

It will appear that the run-up intervals depend highly on the amount collected. For instance, 45 minutes at Ice Factor 10 corresponds to 90 minutes at Ice factor 5; and that, here, it is very important to know exactly how much ice is deposited. This is read by means of the invention.

The run-up speed depends on the force it takes to shed the deposited ice. If the run-up intervals are utilized maximally it means that, at all times, it is the same amount of ice that is to be shed and hence the run-up rate depends exclusively on the temperature.

The attachment of the ice increases with decreasing temperature and therefore it takes a higher number of revolutions to shed it off. This will appear from the below table, wherein N1 = the full number of revolutions of the engine.

The latter circumstance shows, in another manner, that it is important to know exactly how much ice is deposited. As mentioned, this is read by means of the invention.

For comparison with the above example of the invention, it may be liked to the following example of the prior art. It comes from Boeing Model 777-200 and -300 series of airplanes equipped with Rolls-Royce RB211 TRENT 800 Engines:

Airplane Flight Manual (AFM) Revision:

"GROUND OPERATIONS IN FREEZING FOG WITH VISIBILITY OF 300 METERS OR LESS

When freezing fog with visibility of 300 meters or less is reported and (a) The OAT is 0 degrees C to -6 degrees C then run up the engines to 50 % N1 for 1 minute every 45 minutes taxi time, or

(b) The OAT is -7 degrees C to 13 degrees C then run up the engines to 59% N1 for 1 minute for every 45 minutes taxi time, or

(c) The OAT is colder than -13 degrees C and taxi time exceeds 45 minutes, there is no run-up procedure; the engines must be manually de-iced.

It will appear that very wide intervals are concerned and that the intervals can be made both more reliable and more specific by the invention; see the above table.

One option of exercising the invention may be that the Ice Factor is transmitted from the measurement apparatus according to the invention directly to a given engine manufacturer to the effect that he may transmit the most recent, available knowledge on the run-up needs of current motor type back to the airplane.

A further aspect of the invention could be that the engine manufacturers employ an apparatus of the kind described above for defining the threshold values for run-up, inspection and danger. Use of the same apparatus for defining the threshold values as the apparatus used for measuring the

concentration of freezing precipitation at the airport would enable complete safeguarding against the risk of ice formation in the engine.