Enstrom Helicopter Corp 280C N280WR accident near Port Lavaca, Texas

Probable cause

The pilot’s improper manual engine mixture management which resulted in a loss of power. Contributing to the accident was the mechanic’s improper setting of the idle mixture adjustment and the pilot’s decision to take off with a known mechanical deficiency.

NTSB analysis

The pilot reported that he was repositioning the helicopter from his yard to a raised landing platform. When he initiated a hover to maneuver to the platform, the overboost warning light illuminated, the engine subsequently lost power, and the helicopter settled with only the right skid on the platform. The helicopter then rolled to the left, and the main rotor blades contacted the ground. The pilot reported that the overboost warning light had illuminated during a previous liftoff that day but that reducing manifold pressure had effectively extinguished the light.

According to maintenance records, the idle mixture adjustment had been adjusted during the last annual inspection about 7 flight hours before the accident. Although the Rotorcraft Flight Manual stated that the mixture control must be in the full-rich position for takeoff and landing operations (to ensure adequate engine cooling), the pilot reported that, following this maintenance, he had to manually lean the cockpit mixture control during idle and low-power operations to prevent the engine from running excessively rich. Postaccident examination of the engine found that the idle mixture adjustment was set excessively rich. No other preaccident mechanical failures or malfunctions were found with the helicopter that would have precluded normal operation.

According to Enstrom Service Information Letter (SIL) No. 0069, excessively rich idle mixture settings can result in engine roughness and engine stoppage following reductions to low-power conditions. These characteristics are consistent with the pilot’s reported need to manually lean the mixture during idle operation after the most recent maintenance.

Because the pilot reported routinely leaning the cockpit mixture control during low-power operations after the maintenance was performed, it is likely that the mixture control was not returned to the full-rich position before repositioning to the landing platform landing pad, which resulted in an overly lean condition and power loss. It is likely that the over boost light illuminated due to the lean mixture which resulted in abnormally high exhaust gas temperature and led to turbocharger overspeed.

Text from the NTSB. Open the NTSB docket for CEN24LA137 for the full record.