Schweizer 269C N3625Z accident near Kindred, North Dakota

Probable cause

The pilot’s failure to adequately inspect the lower coupling driveshaft during his preflight inspection and his non-compliance with an applicable airworthiness directive, which resulted in the failure of the forward drive splines of the lower coupling driveshaft during cruise flight. Co-causal to the accident was the pilot’s failure to maintain control of the helicopter during the autorotation landing.

NTSB analysis

The pilot was conducting a personal flight when he heard a loud "snap" that was followed by a significant yaw of the helicopter during cruise flight. He stated that the throttle still controlled engine speed, but the main rotor speed did not respond to throttle changes. He immediately entered an autorotation and landed the helicopter level on its skids in a nearby wheat field. The helicopter still had forward ground speed at touchdown, it pitched up briefly, and then slowly rolled onto its right side. The pilot moved the mixture control into the fuel cutoff position to shut down the engine after the accident. The helicopter caught fire a few minutes after the accident and was subsequently destroyed.

Postaccident examination revealed that the forward drive splines of the lower coupling drive shaft were missing and exhibited evidence of significant rotational damage. The pilot was unaware of the flight manual’s preflight checklist requirement to check the free-play and to verify the proper lubrication of the lower coupling drive shaft, nor had he ever completed those preflight inspection tasks during the 18 years that he had owned the helicopter. Had he completed an adequate preflight inspection of the helicopter, the pilot would have been aware of the lack of lubrication and/or excessive free-play of the lower coupling driveshaft and that maintenance was required before the helicopter could be flown safely.

A review of available maintenance documentation revealed that the helicopter was not in compliance with an airworthiness directive that required the disassembly of the lower coupling drive shaft components for a recurring inspection. The helicopter had exceeded the initial 25-hour time-in-service inspection requirement by about 44 hours when the accident occurred. The pilot, who was the sole owner and operator of the helicopter, was responsible for ensuring compliance with the airworthiness directive. The pilot’s non-compliance with the required inspection contributed to the failure of the lower coupling drive shaft.

Text from the NTSB. Open the NTSB docket for CEN18LA334 for the full record.