This invention relates to ice detectors for detecting ice on structures, especially air vehicle structures such as aircraft and helicopters.

Ice build-up on air vehicle structures is caused by air vehicles flying through saturated air at low temperatures, and this has been responsible for a number of accidents. To alleviate the problem, de-icing fluids are sprayed over the air vehicle at risk before take-off, and heaters are built into the skin of the air vehicle at key points of vulnerability (leading edges and engine air intakes) . However, ice is not always detected to allow the timely application of de-icing fluid and the operation of heaters. Heaters are not run continuously as the considerable power drain is wasteful of fuel and reduces range/load capacity.

A typical method of detecting ice is for a crew member to observe the bolts holding in a flight deck window with a torch. If the bolt heads are obscured, then ice is thought to be present. However, with such a method, the inspection is restricted and the window is not in any case a key area.

SUBSTITUTESHEET

The invention provides an ice detector for detecting ice on structures, comprising a camera for imaging a pattern on the structure, and means for detecting variation in the resolution of the image.

With such an arrangement ice build-up can be detected simply from the impairment of the resolution of the image.

Impairment of the resolution of the image due to ice build-up may be detected by employing a video camera and examining the high frequency content of the video signal, or it may be detected by comparing the instantaneous image of the pattern with a stored representation of the pattern.

An ice detector for detecting ice on air vehicle structures constructed in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure l is a schematic representation of a detector imaging a target on the air vehicle structure; and

Figure 2 is a view in the direction of Arrow A in Figure 1, showing a typical target.

UBSTITUTE SHEET

A video camera 1 (not to scale) on the body of the air vehicle structure (not shown) images a target 2 on the leading edge of a wing 3 of the air vehicle.

The effect of ice build-up is to make the image of the target 2 in the camera "fuzzy" i.e. there is no longer a sharp transition between the black lines on the surrounding white surface, but the transition is more gradual, and the image of the lines is spread out and gradually rises in intensity over the wider region of the line. The image has the appearance of a blurred image. This effect is used to determine when ice has been deposited on the wing surface.

A processor 4 analyses the frequency content of the image from the camera and sets off an alarm 5 when the high frequency content of the video signal is reduced below a predetermined level. It will be appreciated that high frequencies are only present if the target pattern is in sharp focus, and the build-up of ice will result in only lower frequencies being present.

A heater 6 is located underneath a part of the pattern, and the image of that part of the pattern is separately examined. This could inhibit the alarm if the

SUBSTITUTE SHEET

frequency content of the heated region is the same as that of the remainder of the target, because this would simply indicate that the loss of detail was caused by dirt or rain or cloud.

As an alternative, the heated area can be used to set a threshold against which the resolution of the remainder of the target is compared, so that the alarm is triggered when the high frequency content of the image of the remainder of the target falls a predetermined amount below the high frequency content of the image of the heated part of the target.

A cooling means 7, for example a thermoelectric device such as a peltier cell arrangement, may be located underneath the pattern and arranged to cool the pattern down to a temperature just above that at which icing will occur. Any icing conditions will therefore quickly cause obscuration of the pattern in advance of ice being deposited upon other parts of the vehicle structure, so that heaters can be applied in timely fashion. Peltier cells can be operated in one sense to cause a cooling effect, and in another sense to cause a heating effect which can be used to melt ice which has obscured the pattern.

As an alternative to examining the high frequency content of the video image, the high frequency content of the image could be compared with a stored representation of the pattern, and the alarm could depend on a predetermined difference from the stored pattern. Indeed, other types of sensors then a video camera could be used in this case, and the build-up of ice could be detected by examining unit areas in the vicinity of the lines in order to ascertain how wide the image is and how great are the differences between the intensity of the unit areas from one side to the other of the image of the line.

As an alternative, or in addition, to the alarm, the processor may be operative to automatically switch on heaters at vulnerable parts of the aircraft.

With the invention, it will be possible to image various areas of the aircraft at risk of icing.

The ice detector need not be restricted to use on air vehicles, and it could also be used on the masts of ships, transmitter masts, power pylons, bridges or any other structure that could be hazarded by the weight of ice built up on it.

SUBSTITUTE SHEET