Bell UH 1B N486SA accident near Maxwell, California

Probable cause

A reduction in available engine power during takeoff due to a stuck hot air valve, which resulted in low rotor rpm and a forced landing. Contributing to the accident was the lack of dry film lubricant and the presence of corrosion on the hot air valve assembly, which resulted in the sticking of the valve.

NTSB analysis

The commercial pilot reported that, after loading the helicopter with chemical for the agricultural application flight, he completed a pre-takeoff checklist, noted that all instrument indications were normal, and initiated a takeoff from the loading truck platform. As the helicopter climbed over the tree canopy, the rotor and engine rpm began to rapidly decay. The pilot initiated a right turn in an attempt to gain airspeed and recover rotor rpm; however, the attempt was unsuccessful, and he initiated a forced landing within an almond orchard. The helicopter landed hard and came to rest upright.

The postaccident engine test run revealed a stuck, partially-open hot air valve used for engine inlet anti-icing. The open hot air valve allowed compressor bleed air to escape the power flow path, thereby reducing the maximum power that the engine could produce before reaching its maximum exhaust gas temperature limitations. The manufacturer of the engine reported that, with the partially-open hot air valve, the engine would only produce about 80% of its maximum power.

Further examination of the hot air valve revealed a lack of dry film lubricant on the flow surfaces in addition to corrosion buildup throughout the valve assembly. The dry film lubricant reduces friction between the sliding elements, allowing for smooth, unimpeded motion of the valve. The lack of lubricant coating in these areas would increase the sliding friction such that the closing spring would be unable to overcome the friction forces, causing the valve to remain open even when commanded closed. It is likely that the valve became stuck before the accident flight, which would have resulted in reduced power available for takeoff.

The engine overhaul manual indicated that an inspection of the hot air valve should occur with an overhaul of the engine. A review of the engine logbooks revealed that the engine was overhauled about 9 years before the accident and that the helicopter had accumulated about 598 hours since the overhaul. There were no logbook entries pertaining to the inspection of the hot air valve.

Text from the NTSB. Open the NTSB docket for WPR16LA104 for the full record.