Comparability of Hot-Wire Estimates of Liquid Water Content in SLD Conditions

Future compliance to FAA 14 CFR Part 25 and EASA CS-25 Appendix O conditions has required icing wind tunnels to expand their cloud simulation envelope, and demonstrate accurate calibration of liquid water content and droplet particle size distributions under these conditions. This has led to a renewed community interest in the accuracy of these calibrations, and the potential inter-facility bias due to the choice of instrumentation and processing methods. This article provides a comparison of the response of various hot-wire liquid water content instruments under Appendix C and supercooled large droplet conditions, after an independent similar analysis at other wind tunnel facilities. The instruments are being used, or are under consideration for use, by facilities collaborating in the ICE GENESIS program. For droplet median volume diameters (MVDs) between about 15 and 250 μm, cylindrical hot wire LWC sensors were found to consistently and increasingly under-read measurements from conical and trough TWC sensors as MVD increased, and were not considered further. Of the remaining TWC sensors, the specific instruments investigated were found to agree within about ± 20% of their average test point response for the range of conditions tested, but systematic scale differences between instruments were found to reach about a factor of 1.4. Sensitivity to increasing droplet MVD was concluded to be similar amongst different instruments given the uncertainties, except for two that exhibited notable roll-off with MVD relative to the others.