Hughes 369 N420AT accident near Ash, North Carolina

Probable cause

A partial loss of engine power due to corrosion and erosion damage to the engine’s compressor section components.

NTSB analysis

The pilot flew the helicopter from his home base to a remote aerial application site, where he landed the helicopter on a support truck so that the ground crew could load the chemical hopper. After loading was completed, the pilot increased the throttle until the engine and rotor rpm needles were in the green range of the tachometer. Immediately after liftoff from the truck, the pilot heard the low-rotor rpm warning horn, and the helicopter began to descend. The pilot attempted to add more pitch; however, the helicopter did not respond. The helicopter subsequently descended, impacted a field, and rolled over onto its right side, resulting in substantial damage to the tailboom.

Postaccident examination of the engine revealed that it would start and run but could not reach full operating speed. Upon disassembly of the compressor section, significant erosion and corrosion damage was found on the compressor case halves and stator vanes, which also exhibited structural damage that ranged from being fully rolled over to being severely distorted or twisted. Other stator vanes were missing. The polymer coating on the inside of the compressor case was eroded in several places. Many of the rotating blades of the compressor were mechanically damaged.

Since 1986, the engine manufacturer has published guidance instructing operators to perform a daily water rinse of engine components if the engine was operating in a corrosive environment, to reduce the possibility of component damage from corrosion and erosion. The aerial application of agricultural chemicals creates such a corrosive environment. The damage to the blades and polymer coating were consistent with a buildup of chemicals that the daily wash would likely have mitigated. The significant erosion and corrosion damage on the blades and the polymer coating likely began at some point before the accident and resulted in a partial loss of engine power during the accident flight.

Text from the NTSB. Open the NTSB docket for ERA21LA368 for the full record.