The Cloud Detectability Conundrum 2019-01-1932

Since the beginning of aviation, aircraft designers, researchers, and pilots have monitored the skies looking for clouds to determine when and where to fly as well as when to deice aircraft surfaces. Seeing a cloud has generally consisted of looking for a white / grey puffy orb floating in the sky, indicating the presence of moisture. A simple monitoring of a temperature gauge or dew point sensor was used to help determine if precipitation was likely or accumulation of ice / snow on the airframe could occur.

Various instruments have been introduced over the years to identify the presence of clouds and characterize them for the purposes of air traffic control weather awareness, icing flight test measurements, and production aircraft ice detection. These instruments have included oil slides, illuminated rods, vibrating probes, hot wires, LIDAR, RADAR, and several other measurement techniques. Each technology has its own strength and weakness including the particle size range and water content that can be measured and its ability (or lack thereof) to discriminate different types of icing conditions.

The FAA release of 14 CFR Part 25 Appendix O and 14 CFR Part 33 Appendix D regulations for SLD and ice crystals has spawned an increased need for detecting and differentiating these icing conditions from the traditional Appendix C clouds. In order to perform these functions, changes to the measurement technologies and flight crew identification methods are needed. To assist ice detector and aircraft manufacturers in the design and certification of systems with these expanded functionalities, an update to ED-103 (AS5498) was recently released. Revision A of this document now provides requirements for the detection and differentiation of Appendix C, O, and D clouds.

In 2009, the FAA released Amendment 25-129 which added paragraphs (e) through (h) to § 25.1419 adding focus to the operation of ice protection systems. Amendment 25-140 was released a few years ago adding the Appendix D and O icing environments. While the use of primary and advisory ice detection systems to meet the requirements of § 25.1419 (e) and (g) have steadily increased, the addition of the new icing envelopes has substantially increased the performance demonstrations required. Performance verification is typically performed through icing wind tunnel tests, icing flight tests and comparison to reference instrumentation to show compliance with FAA requirements via the methods described in AS5498A.

Demonstrating compliance to these new detection and differentiation requirements over the wide variety of icing conditions presents a significant challenge - particularly as particle sizes increase and water content decreases. The capabilities of facilities and instrumentation used in demonstrating performance have limitations that complicate the evaluation. Some have assumed that an ice detection system failing to meet all expectations would be certified as advisory, giving the flight crew the primary responsibility for detecting icing conditions. This strategy, however, is not clear cut and has its own issues. The discussion herein is intended to shed some light on the certification challenges that exist for verifying the means to detect / differentiate all types of clouds and offers some suggestions on how to resolve this conundrum.