Icing Characteristics of Low Altitude, Supercooled Layer Clouds

A limited amount of new data has been obtained on the icing environment during initial airborne measurements aimed at developing environmental icing criteria for use in certifying helicopters for flight into icing conditions. Super-cooled cloud characteristics are reported for 12 icing events encountered at temperatures from -10 C to 0 C at altitudes from 3500 to 6500 ft above the surface of Lake Erie and Lake Michigan. Recorded droplet size spectra from a Particle Measuring Systems' Axially Scattering Probe (ASSP) were used to compute droplet mass (volume) median diameter (MMD) and, in addition to a Johnson-Williams LWC Indicator, the liquid water content (LWC). A review of available historical data from 1944-1950, upon which the atmospheric icing standards of Appendix C, FAR 25 were based, reveals that data obtained from measurements of ice accretion on multidiameter cylinders are subject to a number of significant errors of both signs. These probable errors, which will continue to be evaluated, may be responsible for the conclusions that (1) the historical LWC values are generally larger than those observed in the flights described in this report, (2) the historical MMDs appear to be generally too small for all values of LWC and (3) the historical droplet size distributions are unreliable, as is acknowledged in the later historical literature.