Eurocopter Deutschland Gmbh MBB-BK 117 C-1 N117NC accident near Fort Washington, Pennsylvania

Probable cause

Maintenance personnel’s failure to identify and replace damaged rivet nuts at the air conditioning louver assembly, which resulted in the separation of the louver assembly from the helicopter, the louver assembly’s impact with the tail rotor, and subsequent loss of helicopter control.

NTSB analysis

The helicopter was returning to its home base following a patient transport mission when it became uncontrollable and began shuddering violently. The pilot experienced a total loss of tail rotor control with no warning. He performed an autorotation, and the helicopter came to rest on a residential lawn.

Postaccident examination of the helicopter revealed that the air conditioning louver assembly, tail rotor gearbox, and tail rotor assembly had separated in flight. A recovered louver fragment exhibited contact damage from a tail rotor blade, and both tail rotor blades were destroyed. Examination of the louver’s forward attachment points revealed stripped rivet nuts and evidence of a prior repair; bolts for fastening the louver assembly to the rivet nuts were found within the airframe. Maintenance records indicated that the louver assembly had been removed and reinstalled about 16 flight hours before the accident.

Based on the evidence, the rivet nuts for the air conditioning louver assembly were likely stripped during maintenance, resulting in the louver assembly separating from the helicopter in flight and impacting the tail rotor. The tail rotor assembly then separated from the helicopter, which led to a loss of control.

Text from the NTSB. Open the NTSB docket for ERA25LA204 for the full record.