Optical fibre ice sensor poised to improve aircraft safety

Citation

(2004), "Optical fibre ice sensor poised to improve aircraft safety", Sensor Review, Vol. 24 No. 2. https://doi.org/10.1108/sr.2004.08724baf.001

Optical fibre ice sensor poised to improve aircraft safety

Optical fibre ice sensor poised to improve aircraft safety

Keywords: Infrared radiation, Sensors, Aircraft, Fibre optics

This paper describes an infrared, fibre optic sensor that can detect the formation of thin layers of ice on aircraft surfaces such as tailplanes and rotor blades. It is being developed by Cambridge Optical Sciences (COS) under a collaborative European Framework Five project (Plate 1).

The potentially disastrous consequences of ice formation on an aircraft's flight surfaces are well known: the increase in drag and decrease in lift can be catastrophic. It is estimated that around 35 serious air accidents have been due to this, including a case at Canada's Gander Airport in 1985, where a McDonnell Douglas DC-8 passenger jet carrying 248 passengers crashed just after take-off. Consequently, commercial jets are usually de-iced prior to take-off by spraying them with hundreds of litres of glycol, which involves large teams of ground staff who inspect the aircraft and take weather conditions into account. As this is clearly not possible once the plane is airborne, many aircraft, are fitted with ice warning sensors. These are electro-mechanical devices whereby ice formation influences the dynamics of moving structures such as vibrating wires or rotating spindles. However, these tend to be large, which can cause severe disturbances to the airflow on leading surfaces, and can weigh around 1 kg or more, which limits their use on components such as light aircraft tailplanes and helicopter rotor blades.

Plate 1 Prototype ice sensor undergoing tests

With these considerations in mind, a 3-year European Framework Five project dubbed Air Conformal Ice Detection System (ACIDS), part of the “Competitive and Sustainable Growth” programme, was initiated in 2002, with the objective of developing an improved ice warning sensor system. The development programme focuses on three main areas: the determination of appropriate optical sensor architectures in fibres; their integration into the existing rotor blade and wing structures; and the miniaturisation and integration of the optical modules with the data acquisition hardware and algorithms interfaced with localised de-icing procedures. Further, rather than just responding to the presence of ice, the aim was also to determine the type of ice and its thickness.

The sensor is under development by UK optics specialist COS and the other project partners are Aerospace Composite Technologies, Hellenic Aerospace Industry, the University of Limerick, AOS Technology, AxonTec and Eurocopter Deutschland. The original sensing concept arose from the earlier work by COS on non-contacting surface analysis for the UK's National Physical Laboratory and relies on the analysis of an IR signal, reflected from an abrasion resistant window onto which the ice forms. The sensor comprises of an IR semiconductor laser (the source), a parallel array of seven optical fibres, the detection and analysis electronics and an abrasion-resistant sapphire window. Light from the laser is coupled into the central fibre and directed to the window where it interacts with any ice present on the surface. This alters the characteristics of the reflected light which is collected by the neighbouring fibres and analysed by the electronics.

In addition to being more rugged and sensitive than existing devices, detecting ice layers as thin as 100 mm, the sensor is lighter too, weighing less than 50 g. Further, it can distinguish between water and different types of ice; glazed ice with a smooth surface reflects the light very strongly, whilst rime ice, which is composed of spiky crystals, gives far more scatter and a weaker reflection. Reliability is further enhanced by inbuilt, self-test and self-calibration facilities. The system will be capable of directly measuring ice formation on the leading surfaces, raise an alarm and automatically activate the aircraft's localised de-icing elements. This has the potential to reduce engine power consumption diverted for de-icing in difficult flying conditions and should, therefore, contribute to the safety of the aircraft.

The sensor is presently at an advanced stage of development and was shown at the 2003 Paris Air Show. It is presently undergoing extensive evaluation which will be followed by flight approval testing. COS is currently working on miniaturising the sensor further with Luton-based ACT, a division of AgustaWestland, where wing sections can be tested in a wind tunnel equipped with an icing facility. Martin Lawrence, Managing Director of COS, said that the sensor will be making its first in-flight appearance on the supersonic rotor blades of AgustaWestland's EH1O1 Merlin helicopter next year.

Although ice detection is not yet mandatory in general aviation, the US Federal Aviation Authority (FAA) is pushing for all aircraft to be equipped with icing alert system; a ruling which is also likely to be adopted by the European Joint Airworthiness Authorities. Further, the FAA is expected to require that, from 2005, all new light aircraft are fitted with tailplane ice sensors. This could, therefore, turn out to be a lucrative development for COS, as an aircraft may employ ten or more ice sensors.

For further information, please contact: Martin Lawrence, Managing Director, Cambridge Optical Sciences Ltd, Ipswich Road, Needham Market, Suffolk IP6 8EK, UK. Tel: +44 (0)1449 721002; E-mail: sales@cos.co.uk