Robinson R22 N259TW accident near Goshen, New York

Probable cause

The pilot’s failure to adhere to published preflight checklist limitations and procedures, resulting in excessive slack and subsequent failure of the power transmission belts and a total loss of rotor system drive.

NTSB analysis

According to the pilot, following clutch engagement after engine start and warm up, it took 20 to 25 seconds for the main rotor to start turning. About 15 minutes into the flight and while cruising at 700 feet above ground level, the pilot heard a “flapping” noise and smelled burning rubber. He then “sensed” that the engine was losing power, observed numerous caution lights illuminate, and entered an autorotation. The helicopter subsequently impacted uneven terrain, which resulted in damage to the fuselage, as well as the main and tail rotor blades. Postaccident examination found that both power transmission belts had been ripped, mostly through the center of the backing, separating the two sides of the belt. A review of maintenance records showed that an airframe overhaul was completed about 4 months before the accident, which included replacement of both v-belts.According to the manufacturer’s maintenance manual, a delay of more than 5 seconds between clutch switch engagement and rotor turning indicates excessive belt slack. Power transmission belts that are too loose can jump out of their grooves during start up and jump forward into the next groove or off the forward edge of the pulley. The pre-engine start checklist instructs pilots to adjust the slack of the power transmission belt system so that the rotor begins turning within 5 seconds of clutch engagement. Evidence suggests that the pilot did not perform the recommended check.

Text from the NTSB. Open the NTSB docket for ERA11LA204 for the full record.