Incident Overview
A helicopter landed at a remote site due to poor weather conditions. During the ground stop, snow fell and accumulated on the airframe. The helicopter was not equipped with external engine snow covers. The air induction system included snow baffles and particle separators, but the engine lacked an optional auto re-light ignition system. The plenum at the engine intake was inaccessible to the pilot for removal of any snow that may have accumulated.
Ground Operations
Approximately one hour after landing, the pilot restarted the engine and ran it for about 15 to 20 minutes. About another hour later, the engine was restarted again and run for approximately 5 minutes. The pilot then discussed the situation with the director of operations by radio.
Takeoff and Power Loss
The pilot subsequently took off with a gross weight of about 2940 lbs and at a density altitude of about 7000 feet. Shortly after lift-off, the engine lost power. The pilot performed an autorotation; during this maneuver, the helicopter impacted on a 30-degree slope and rolled over.
Flight Manual Limitations
The flight manual indicated that at a gross weight of 2940 lbs, the maximum density altitude for a safe autorotative landing was 2500 feet. Additionally, the flight manual limited hover flight in falling and/or blowing snow to a duration of 20 minutes, after which the helicopter must be landed and checked for snow and/or ice accumulation. However, ground operation in these conditions was not addressed.
Engine History
Tests of snow and ice ingestion conducted on an earlier model of the same engine (Allison 250-C18) resulted in flameouts with snow slugs as small as 30 grams.