Schweizer 269C N656RB accident near Montrose, Colorado

Probable cause

A loss of engine power during a demonstrated autorotation due to an improperly adjusted air-fuel mixture or idle setting. Contributing to the accident was the pilot’s failure to perform the required idle check during previous flights in higher elevation areas.

NTSB analysis

The pilot stated that during an autorotation demonstration, the helicopter exhibited a complete loss of engine power. The pilot had set up the demonstration about 1,500 ft above ground level and about 60 knots indicated airspeed with hard packed gravel chosen for the touchdown point. The pilot stated that the engine rpm went to zero as soon as he rolled off the throttle to start the autorotation. He attempted to restart the engine with no success and conducted an autorotation to the predetermined touchdown area. During the ground run, the left skid collapsed and the helicopter rolled onto its left side, which resulted in substantial damage to the main rotor system, tail boom, and tail rotor system.

An examination of the engine revealed no evidence of mechanical malfunctions or failures that would have precluded normal operation. The engine was started multiple times and ran through various rpm settings. Both fuselage fuel tanks were intact and contained fuel consistent with 100LL Avgas.

Discussions with the pilot revealed that he had recently purchased the helicopter and was now operating it in high elevation areas. Review of the rotorcraft flight manual revealed a required check of the idle mixture, idle speed, and fuel boost pump at the end of the last flight of each day. When interviewed, the pilot stated that he had never performed this check, did not witness the owner perform the check, and was not taught to perform the check during his initial flight training in the make and model helicopter 12 years earlier.

According to the airframe manufacturer, failure to perform the idle check can allow an improperly adjusted idle speed, throttle linkage, or air-fuel mixture setting to go undetected. Under those conditions, rolling the throttle to idle or a rapid power reduction—such as during a simulated autorotation—can result in the engine rpm decaying below sustainable idle limits, leading to a total loss of engine power.

Based on available evidence, it is likely that the air-fuel mixture or idle setting had not been adjusted for flight operations in the high elevation area and went undetected due to the pilot’s failure to perform the required post flight check.

Text from the NTSB. Open the NTSB docket for CEN25LA003 for the full record.