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For nearly a century, ice build-up on aircraft surfaces has presented a safety concern for the aviation industry. Pilot observations of visible moisture and temperature has been used a primary means to detect conditions conducive to ice accretion on aircraft critical surfaces. To help relieve flight crew workload and improve aircraft safety, various ice detection systems have been developed. Some ice detection systems have been successfully certified as the primary means of detecting ice, negating the need for the flight crew to actively monitor for icing conditions. To achieve certification as a Primary ice detection system requires detailed substantiation of ice detector performance over the full range of icing conditions and aircraft flight conditions. Some notable events in the aviation industry have highlighted certain areas of the icing envelope that require special attention.

The Collins Aerospace Optical Ice Detector is a short-range polarimetric cloud lidar designed to detect and discriminate among all types of icing conditions with the use of a single sensor. Recent flight tests of the Optical Ice Detector (OID) aboard a fully instrumented atmospheric research aircraft have allowed comparisons of measurements made by the OID with those of standard cloud research probes. The tests included some icing conditions appropriate to the most recent updates to the icing regulations. Cloud detection, discrimination of mixed phase, and quantification of cloud liquid water content for a cloud within the realm of Appendix C were all demonstrated. The duration of the tests (eight hours total) has allowed the compilation of data from the OID and cloud probes for a more comprehensive comparison. The OID measurements and those of the research probes agree favorably given the uncertainties inherent in these instruments.

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.

Aircraft icing has been a focus of the aviation industry for many years. While regulations existed for the certification of aircraft and engine ice protection systems (IPS), no FAA or EASA regulations pertaining to certification of ice detection systems existed for much of this time. Interim policy on ice detection systems has been issued through the form of AC 20-73A as well as FAA Issue Papers and EASA Certification Review Items to deal mainly with Primary Ice Detection Systems. A few years ago, the FAA released an update to 14 CFR 25.1419 through Amendment 25-129 which provided the framework for the usage of ice detection systems on aircraft. As a result of the ATR-72 crash in Roselawn, Indiana due to Supercooled Large Droplets (SLD) along with the Air France Flight 447 accident and numerous engine flame-outs due to ice crystals, both the FAA and EASA have developed new regulations to address these concerns.

Ice protection systems were installed on DC-3s, B-29s and other aircraft during World War II. Initially ice detection was not considered a requirement for aircraft safety. One of the earliest ice detectors installed on a production aircraft (C-130) was pneumatic. Subsequently many other technologies have been applied to detect the presence of meteorological icing. Ice detectors employing visible light, infrared light, alpha radiation, natural resonance frequency, capacitance, speed of sound, heat of fusion, and microwaves have all been developed over the years. This paper will review the different ice detection technologies and explore some of the similarities, differences, benefits and drawbacks for the purposes of performing their intended function. One of the drivers for developing new ice detection technology has been aircraft icing accidents and incidents.

Understanding the performance of ice detectors in various icing conditions is an important part of helping protect an aircraft against icing. Ice detector annunciation helps make pilots aware of ice accretion, so that ice protection systems may be activated to prevent hazardous ice formation on aircraft critical surfaces. Icing conditions can vary significantly from conditions with low liquid water content to supercooled large droplet (SLD) conditions. It is important to know if low liquid water contents can be detected by ice detectors, or if dynamic heating and other phenomena prohibit such detection. The possibility that droplet splashing in SLD conditions could result in a reduction of ice accretion on the ice detector also needs to be addressed. Sensor Systems, Goodrich Corporation (Rosemount Aerospace Inc.) has undertaken an investigation into the performance of selective ice detector models in the icing conditions described above.