Certification of the Sikorsky S-92A® Helicopter Ice Protection System: Meteorological Aspects of Tanker Tests and Natural Icing Flights 2007-01-3329

Sikorsky Aircraft Corporation flew the S-92A® helicopter into natural and tanker artificial icing conditions as part of the certification program for the rotor ice protection system (RIPS). Icing tanker tests were conducted during the late winter of 2004, and natural icing flights were made during the spring of 2004, winter of 2004-05, and early fall of 2005. One goal of the natural icing flight program was to obtain water-dominated icing encounters at temperatures between -15 and -23.33°C. Past studies have shown that mixed-phase icing is often found in this temperature range, and that most icing occurs at temperatures warmer than -15°C. Tanker tests were conducted at specific temperatures at altitudes below about 3 km (10,000 ft), and under prolonged weather conditions meeting visual flight rules (VFR). This represents a challenging combination of conditions to find within a small radius of operation from a given airport.

Weather climatology and forecasting support was provided in an effort to maximize the efficiency of the flight program. During tanker tests, short-term and long-term daily forecasts of temperatures aloft, ceiling, and visibility were needed. They were used to determine if VFR conditions were expected and to optimize the choice of test plan conditions to be attempted. In advance of the natural icing flight campaigns, climatological information was used to aid in the selection of base locations.

Once the aircraft was deployed for flight in natural icing conditions, daily briefings on the expected location, altitude, timing, probability, consistency, temperature, liquid water content, and drop size of the icing conditions were provided. After takeoff, the weather conditions were monitored and provided to the crew in an effort to make the most of the conditions that were available for a given flight. A wide variety of natural icing conditions were sampled in stratiform and stratocumulus clouds over the twenty-month period of the flight campaign, covering substantial portions of the certification envelope. Several rather difficult points were acquired, including those with liquid water contents as high as 1.0 gm^-3 and temperatures as cold as -23°C.