Photogrammetric Frost Roughness Measurements in Cold-Soaked Conditions 2019-01-1970

Cold-soaked fuel frost (CSFF) is a form of aircraft wing contamination that occurs when a vehicle caries sufficient fuel for multiple trips or take-offs and landings. Following the first trip, which may reach altitudes above 10,000 m (33,000 ft), the fuel for the subsequent trips is carried in the wing tanks and may reach temperatures below -25 °C. In certain times of the year at some airports, temperatures and humidity levels will form CSFF on the aircraft wing surfaces over the fuel tanks. Unless an exemption is granted for the specific aircraft model, aircraft are not allowed to takeoff if the wing surfaces are contaminated by frost. Because aircraft operators desire to minimize vehicle time spent at airports, aircraft manufacturers are expected to pursue designs that safely operate with CSFF at takeoff and to pursue certification exemptions for aircraft models enabling CSFF takeoffs. To assist manufacturers in the design of future aircraft and to assist regulators in evaluating certification exemption requests, more information about frost roughness characteristics and evolution in CSFF conditions is required. However, because of the material and optical properties of frost, measuring CSFF properties using traditional roughness measurement presents challenges. For this investigation, a photogrammetric approach based on “structure-from-motion” algorithms traditionally employed in aerial surveying was developed to characterize the evolution of CSFF roughness. Using the approach, measurements of frost roughness evolution were performed in the Baylor Frost Tunnel (BFT) for two basic air temperature, humidity, surface temperature, and velocity conditions with frost time up to two hours. An analysis of variance approach was used to determine the sensitivity frost growth rates to the environmental conditions. The resulting measurements demonstrate different evolution histories with air velocity and air temperature being the most important factors governing roughness growth.