Combination Cloud Probe (CCP) History
Aircraft in-flight icing has been an aviation hazard ever since aircraft started to fly through supercooled liquid water clouds. During the 1940s and 50s, a great deal of research was performed documenting the characteristics of clouds where icing occurred. This led to aircraft icing certification criteria such as the FAA’s FAR 25 Appendix C, which describes the cloud characteristics that aircraft must be capable of withstanding if they are to fly in icing conditions. The criteria are defined in terms of cloud droplet mean effective diameter, cloud liquid water content, and temperature over a specific distance. Most commercial aircraft are now designed with de-icing or anti-icing systems, which allow them to fly into such clouds.
In October of 1994, an ATR-72 commuter aircraft crashed near Roselawn Indiana, killing all 68 persons on board. The cause of the accident has been attributed to icing associated with supercooled large (diameter>50 microns) droplets (SLD). A ridge of ice built up behind the de-icing boots, which led to roll instability in the aircraft, and ultimately to the aircraft crash. The accident focused attention on the fact that the test environments used by regulatory agencies such as FAA and JAA for certifying aircraft for flight into icing conditions do not include freezing drizzle or rain. The test environments only extend to cloud droplet mean effective diameters of 40 microns, while mean effective volume diameters as large as 1000 microns have been measured. This implies that aircraft have been certified for flight into icing conditions without actually having been proven to operate safely under the severe icing conditions that can be associated with freezing precipitation.
Certification of aircraft for performance under icing conditions have always incorporated a basic set of three sensors to establish that the certification flights occur within the FAR 25 prescribed range of temperatures, liquid water content and mean effective droplet diameter. In recent years this has usually meant a Rosemount temperature sensor, a hotwire liquid water probe and an forward-scattering spectrometer probe (FSSP). This set of measurements, however, is greatly limited for accurately portraying the actual environmental conditions for the following reasons:
1) By definition, the effective droplet diameter is the diameter where 50% of the mass is found at larger diameters. The maximum size measured by the FSSP is 47 μm, hence, for the FAR criterion of 40 μm effective diameter, it will be virtually impossible to meet with this instrument.
2) Hot wire probes are able to measure water content accurately only when the effective diameter is 20 μm or less. The reason is that when the water is found at larger diameters, the evaporation occurs inefficiently and water content is underestimated. Hence, much higher environmental water contents are needed in order for the hot-wires to measure an amount that meets the FAR criterion.
3) The icing research community is unanimous in its conclusion that super-cooled drizzle drops pose a large hazard to aircraft. The FAA acknowledges this danger and activity is ongoing at this agency to revise the FAR 25, Part C to include a certification criterion for measuring drops larger than 50 μm.
The DMT Cloud Combination Probe (CCP) incorporates the Cloud Droplet Probe (CDP 2-50μm), the Cloud Imaging Probe (CIP 25-1550 μm), and an LWC hotwire sensor, as well as Pitot tube and temperature sensor for TAS determination. The CCP is DMT’s solution, in a single package, to fully cover the particle range that we expect FAA will be requiring for aircraft icing certifications in the near future.
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