Detection and differentiation of supercooled large droplet icing conditions

Kurzfassung

Icing conditions that contain supercooled large droplets (SLDs) represent a hazard for aviation. Due to their large inertia, SLDs impact behind the extent of aircraft ice protection systems where the developing ice accretion cannot be removed. At present, aircraft that are vulnerable to SLD icing need to avoid severe icing conditions in general, leading to increased operating costs. New American and European aviation regulations allow the passage through such conditions if the aircraft carries instruments that can detect the presence of SLDs. Upon detection, the pilots can exit the icing cloud before the situation becomes hazardous. The mass of liquid water contained in an SLD is several orders of magnitude higher than the mass contained in a typical small cloud droplet. However, the number concentration of SLDs is much lower than that of small cloud droplets, consequently, it is challenging to detect SLDs with instruments. As of now, no instruments for the detection of SLD icing conditions are in use on commercial aircraft. This thesis investigates a combination of two instruments for the detection of SLD icing conditions. The first instrument is the Nevzorov probe for the measurement of liquid and total water content and the second instrument is the Backscatter Cloud Probe with Polarization Detection (BCPD), a non-invasive laser backscatter probe that measures the size and shape of cloud particles between 2 and 42 µm. The Nevzorov probe used in this work carried a new, 12 mm diameter total water content sensor that was added to the instrument specifically for the measurement of SLD icing conditions. Both instruments, the Nevzorov probe and the BCPD, are first analyzed individually in icing wind tunnel tests. The findings from the tests show that the new 12 mm sensor of the Nevzorov probe captures SLDs effectively. No indication was found of SLD being incompletely evaporated due to splashing or of water being swept out of the cone. The collision efficiency of small droplets with the sensor is low and can be compensated with a correction derived in this work. Intensive atmospheric testing ensued the icing wind tunnel measurements. During measurements in Arctic clouds, it could be shown that the fraction of liquid and glaciated particles can be estimated from the BCPD. Measurements in the South of France were able to demonstrate that the detection and discrimination of SLD icing conditions is possible with the Nevzorov probe and the BCPD for SLD icing encounters that are sufficiently long and contain a high number of SLDs. The results of this work allow future flight campaigns to use the 12 mm sensor of the Nevzorov probe and benefit from its capture efficiency and better sampling statistics. The comparisons of the Nevzorov probe to other instruments can help scientists choose suitable instrumentation for future icing wind tunnel and flight campaigns. Concerning the BCPD, a new method developed in this thesis to estimate the number of ice and water particles could, with small modifications, also be employed for other instruments that use polarization filters. For the detection and discrimination of SLD conditions, future work should focus on extending the sample area of the BCPD further outward from the aircraft skin to measure particle size distributions that are unaffected by the aircraft boundary layer. Furthermore, the false alarm rate of the system could be reduced by incorporating an instrument similar to the BCPD but with a larger size range and larger sample area for the direct detection of SLDs.