An Experimental Study of the Production of Ice Crystals by a Twin-Turboprop Aircraft

Robert D. Kelly Department of Atmospheric Science, University of Wyoming, Laramie, Wyoming

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Gabor Vali Department of Atmospheric Science, University of Wyoming, Laramie, Wyoming

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Abstract

The University of Wyoming King Air (KA) research aircraft was used in controlled, in situ experiments to determine whether or not, and under what cloud and aircraft operating conditions, a twin-turboprop aircraft would itself produce ice crystals during passage through clouds containing supercooled liquid water. Such crystals are termed “Aircraft Produced Ice Particles” (APIPs). Computer-aided, air-relative navigation was used to pilot the KA back through the diffusion volumes of earlier flight segments. To protect against false-positive and false-negative conclusions, large concentrations of artificially nucleated ice crystals were used as tracers at one or two points along each flight segment. These tracers should have the same diffusion and sedimentation characteristics as the APIPs, and their detection should indicate that any APIPs, if present, would also be detected.

The results of 15 experiments in which the cloud volume affected by the aircraft was subsequently sampled suggest that the KA produces APIPs only in a limited range of cloud and operating conditions. The range of experimental conditions included temperatures from −3° to −25°C, liquid-water contents up to 0.5 g m−3 mean drop diameters 7–20 μm, maximum drop diameters up to 30 μm, true airspeeds 80–110 m s−1, engine speeds 1700–1900 rpm, and low to heavy airframe icing. For the single case in which APIPs were detected, the conditions were − 12°C temperature, 0.5 mg cm−3 liquid-water content, 20 μm mean diameter, 27-μm maximum diameter and heavy airframe icing. An experiment with very similar conditions, but with little or no airframe ice, generated no APIPs. The two most plausible APIP generation mechanisms consistent with these results are splinter production during airframe icing and/or enhanced ice nucleation rates due to adiabatic cooling in propeller tip or wing tip vortices.

Abstract

The University of Wyoming King Air (KA) research aircraft was used in controlled, in situ experiments to determine whether or not, and under what cloud and aircraft operating conditions, a twin-turboprop aircraft would itself produce ice crystals during passage through clouds containing supercooled liquid water. Such crystals are termed “Aircraft Produced Ice Particles” (APIPs). Computer-aided, air-relative navigation was used to pilot the KA back through the diffusion volumes of earlier flight segments. To protect against false-positive and false-negative conclusions, large concentrations of artificially nucleated ice crystals were used as tracers at one or two points along each flight segment. These tracers should have the same diffusion and sedimentation characteristics as the APIPs, and their detection should indicate that any APIPs, if present, would also be detected.

The results of 15 experiments in which the cloud volume affected by the aircraft was subsequently sampled suggest that the KA produces APIPs only in a limited range of cloud and operating conditions. The range of experimental conditions included temperatures from −3° to −25°C, liquid-water contents up to 0.5 g m−3 mean drop diameters 7–20 μm, maximum drop diameters up to 30 μm, true airspeeds 80–110 m s−1, engine speeds 1700–1900 rpm, and low to heavy airframe icing. For the single case in which APIPs were detected, the conditions were − 12°C temperature, 0.5 mg cm−3 liquid-water content, 20 μm mean diameter, 27-μm maximum diameter and heavy airframe icing. An experiment with very similar conditions, but with little or no airframe ice, generated no APIPs. The two most plausible APIP generation mechanisms consistent with these results are splinter production during airframe icing and/or enhanced ice nucleation rates due to adiabatic cooling in propeller tip or wing tip vortices.

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