Sikorsky S76 N761AF accident near Morrilton, Arkansas

Probable cause

The improper securing of the exhaust duct bolts, which resulted in the duct becoming unseated and substantial heat damage to the tail rotor drive shaft.

NTSB analysis

The pilot of the helicopter air ambulance flight reported that he experienced fumes in the cockpit while enroute, and shortly thereafter, received visual and aural warnings of smoke in the aft baggage compartment. The pilot initiated an emergency descent into an alternate airport, declared an emergency with air traffic control, and landed uneventfully.

Examination of the helicopter revealed that the No. 2 engine exhaust duct was disconnected and unseated, which resulted in engine exhaust entering the compartment containing the tail rotor drive shaft and substantial heat damage to the drive shaft and surrounding areas. Examination of the No. 1 engine exhaust duct revealed that its attach bolts were not tightened to the specified torque value. Additionally, examination of the operator’s other helicopter revealed that the bolts securing both of its two exhaust ducts were not tightened to the specified torque value. The maintenance history of these components was not determined, but given the available information, it is likely that they were improperly secured, which resulted in their loosening and subsequently allowed the exhaust duct to become unseated.

Text from the NTSB. Open the NTSB docket for CEN19LA146 for the full record.