Nephelometer: a Flight Test Probe for Cloud Characterisation

Sandrine Roques, Airbus France, Toulouse, France

Abstract

Until nowadays, during icing certification flight tests, Airbus used meteorological probes, FSSP and King probe, to determine cloud granulometry. The measurements help to find the different conditions asked by Appendix C. Airbus France has developed a new probe, called Nephelometer, measuring particle sizes between 10 and 500 microns, adapted to operational requirements, to be used on FSSP and King probe's behalf. The Nephelometer is based on an optical principle called "coherent shadowgraphy": droplets and crystals going through the measurement area of the probe are lightened by a low coherency punctual source. The number and the size of the droplets are measured from their shadows on a CCD. A pulsed laser coupled to a fast camera freezes the movement. Image processing is done in real time and results are provided to the flight test engineer. All data and images are recorded in order to allow on-ground complementary analysis if necessary. The probe is designed to be easily installed through a dummy window. It is retractable which allows the aircraft to fly at VMO (Maximum Operating Limit Speed). Nephelometer now exists in triplicate. During the fine tuning phase, experiments in icing tunnel and on plane have been held, involving FSSP, King Probe and Nephelometer. The results of the different probes are herein compared.