On April 20, 1991, a Bell 47-G3B1 (registration N1166W) operated by Firefly Leasing, Inc. was involved in an aviation accident near Harwich, MA. Investigators recorded the probable cause as: Maintenance personnel's failure to follow overhaul manual procedures when reassembling the clutch, resulting in insufficient shoe-to-drum contact. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A helicopter suffered a loss of lift from the main rotor system, resulting in a hard landing. Investigation revealed the clutch had been reassembled with insufficient shoe-to-drum contact due to maintenance procedural failures.
What happened
The helicopter experienced a sudden loss of lift through its main rotor system while in flight. The pilot was unable to maintain controlled flight, and the aircraft subsequently made a hard landing.
The investigation
A post-crash examination of the aircraft revealed significant maintenance irregularities. The clutch had recently been removed and disassembled for inspection purposes. However, when the component was returned to service, it exhibited only 25 percent shoe-to-drum contact. This level of engagement was insufficient for proper operation.
Findings
The investigation determined that maintenance personnel failed to follow the procedures contained in the overhaul manual during the reassembly process. The improper reinstallation of the clutch directly contributed to the loss of lift and the subsequent hard landing.
Probable cause
Maintenance personnel's failure to follow overhaul manual procedures when reassembling the clutch, resulting in insufficient shoe-to-drum contact.