SLD and Ice Crystal Discrimination with the Optical Ice Detector 2019-01-1934

In response to new safety regulations regarding aircraft icing, Collins Aerospace has developed and tested an Optical Ice Detector (OID) capable of discriminating among icing conditions appropriate to Appendix C and Appendix O of 14 CFR Part 25 and Appendix D of Part 33. The OID is a short-range, polarimetric lidar that samples the airstream up to ten meters beyond the skin of the aircraft. The intensity and extinction of the backscatter light correlate with bulk properties of the cloud, such as water content and phase. Backscatter scintillation (combined with the outside air temperature from another probe) signals the presence of supercooled large droplets (SLD) within the cloud-a capability incorporated into the OID to meet the requirements of Appendix O.

Recent laboratory and flight tests of the Optical Ice Detector have confirmed the efficacy of the OID to discriminate among the various icing conditions. Drizzle-sized droplets, mixed with a small droplet cloud in the Collins Cloud Chamber, appear as scintillations in the lidar signal when it is processed pulse-by-pulse. Averaging the signal over multiple pulses, causes large droplets to become obscured by the small droplet background. In addition, the OID has discriminated and quantified mixed phase in a flight test aboard the NASA DC-8 Airborne Science Laboratory. The threshold for ice water quantification is less than 0.5 g/m³ IWC, while that for liquid water cloud detection is less than 0.05 g/m³ LWC.