The accident happened although most aircraft flying at 740 km/h (400 knots) or greater, which includes most jets, generally do not have icing problems because they typically have heated leading edges and fly above most icing. However, jets on approach and departure, turboprops, piston aircraft, and helicopters are all susceptible. Turboprops fly exclusively at lower altitudes and are thus exposed to ice for extended periods. A few light piston-engine aircraft have deicing capability. Helicopters are probably the most threatened of all aircraft because of their unique aerodynamics, their mission requirements, and because they typically lack deicing capability (Ryerson C. C., 2000).
We mention also that at least certain types of UAVs are susceptible to icing too. Usually the UAVs have high efficiency airfoils, which makes them more prone to icing accidents. The report [Safety Report 2015] indicates that during the last five years there have been a total of 31 loss-of-control inflight accidents (30 involved fatalities), with an average of approximately six such accidents per year. Turboprop aircraft contributed to 68% of the accidents. The accident rate for the 5-year period was 0.18 accidents per million sectors (Sector: the operation of an aircraft between takeoff at one location and landing at another). The breakdown is 0.07 for jets and 0.69 for turboprops. Weather is a key contributing factor to these accidents, with 32% of loss-of-control accidents having occurred in degraded meteorological conditions, usually involving thunderstorms and icing.
Helicopters are even more susceptible to inflight icing danger. Two problems that are specific to helicopters are the change of the shape of the airfoil and loss of the efficiency of the rotor system. Helicopters are lightweight machines and ice adds significant weight, with a much higher ice- weight-to-aircraft-weight ratio than in a typical fixed-wing aircraft (Huber, 2013). Helicopters often fly critical missions such as search and rescue or medical flights, and ice can accumulate on the aircraft in a matter of seconds.
We will present key analyses and basic design studies of a practical airborne passive microwave sensor for detection and avoidance of hazardous aircraft icing conditions. The analyses consisted in part of optimal channel selection using a scattering-based radiative transfer model and statistically realistic cloud profiles containing either cloud ice (which is benign for flight) or SCL. The model results along with size, weight, power and cost of hardware limitations yielded a set of 16 channels suitable for a sensor operating within three radiometric bands (~70, ~150, and ~230 GHz). Using a few subsets of these channels an optimal binary inverse covariance detection algorithm was developed based on a likelihood ratio test to determine the probabilities of detection and false alarm, along with the receiver operating characteristics for icing detection. The detection algorithm could be used for an inflight warning system. This analysis further narrowed the range of useful channel sets through quantitative ranking of their detection performances. The results confirm the ability of the dual-polarized 16-channel set and several subsets to perform adequately for inflight icing detection under a typical flight scenario. A relatively simple instrument, called the Microwave Radiometer for Aviation Safety (MRAS), with three dual-polarized radiometers can achieve a probability of detection better than 95% and a false alarm rate less than 15% for all the simulated atmospheric conditions.
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