ENSTROM F-28F near Palmer, AK — 2021-05-09

Final reportANC21LA038

Date: 2021-05-09
Location: Palmer, AK, USA
Aircraft: ENSTROM F-28F
Registration: N8627J
Category: Helicopter
Highest injury: None
Fatalities: 0
Phase of flight: Descent

Probable cause

A loss of drive to the rotor system due to the disengagement of the main belt drive tensioning system, which resulted in a forced landing. The disengagement was due to inadequate clearance between the clutch handle and the roll pin in the handle bellcrank when the handle was in the stowed position.

NTSB narrative

During a helicopter tour flight, the clutch disengage light dimly illuminated, flickered, then fully illuminated. As the pilot began to descend for landing, he felt the belt clutch disengage, and the engine rpm separated from the transmission rpm indicating the helicopter had lost engine power to the rotor system. The pilot entered an autorotation and made a forced landing in a nearby open field. The helicopter touched down with a tailwind of about 13 mph, slid forward and nosed down. A main rotor blade impacted the ground in front of the helicopter followed by a main rotor blade impacting the tail section just forward of the stabilizers, which resulted in substantial damage to the fuselage. A postaccident examination of the helicopter revealed that the main belt drive tensioning system (belt clutch) inadvertently disengaged in flight. Testing found that the disengagement occurred due to inadequate clearance between the clutch handle and the roll pin in the handle bellcrank when the handle was in the stowed position. As a result of this accident, the helicopter manufacturer issued a service bulletin that clarified the rigging requirements for the clutch handle and allowed for a minor modification to the handle to increase clearance between the handle and the roll pin in the bellcrank.