Ice Crystal Erosion in Aircraft Icing and Airborne Cloud Measurements

Very little is known about the effect of erosion of ice accreted on aircraft surfaces due to impact with ice crystals. This issue's primary importance in cloud physics is probably in the use of the Rosemount Ice Detector to indicate the presence of supercooled liquid water content in airborne cloud measurements. In mixed phase situations dominated by ice crystals, the interpretation of conventional cloud particle spectrometers and hot wires becomes ambiguous due to the response of these probes to both water and ice, and in many cases researchers have relied on the Rosemount Ice Detector as the primary indication of the presence of supercooled liquid water, particularly when only trace amounts are present. The identification of even trace amount of LWC is important in understanding the mixed phase process and the evolution of condensed water in clouds. The potential erosion of ice accreted on the Rosemount Ice Detector by ice crystal impacts at aircraft speeds thus becomes a potential complication in proper understanding of our cloud mixed-phase measurements.

Environment Canada and the National Research Council of Canada have recently collaborated in high speed tunnel testing of the performance of airborne cloud probes at the NRC engine test cell. This work supports ongoing research into the effect of ice particle ingestion into jet engines. The engine test cell creates simulated ice particle clouds with calibrated IWC values up to about 9 gm-3 at 150 ms-1. During the winter of 2008/2009 a series of mixed-phase tests were performed to characterize the effect of ice crystal erosion on the Rosemount Ice Detector. Results show that the effect of ice crystal erosion can completely negate the accretion of ice under certain conditions. The paper will summarize these results and report on other unusual observations of the ice accretion process in mixed-phase conditions.

The application of this work is also important to conventional aircraft icing and jet-engine icing. Researchers are investigating how icing can occur in an initially warm jet engine, and it is important to know whether supercooled LWC is present in the atmosphere under these mixed phase conditions highly dominated by ice crystals. In addition, ice crystal erosion may also some day be a consideration in establishing the requirements for Extended Twin Engine Operations (ETOPS) over the oceans, where manufacturers must demonstrate the capability of their aircraft to operate safely in icing conditions in the event of an engine failure and a descent to low altitude.