Accident Overview
On May 9, 2021, at about 1440 Alaska daylight time, an Enstrom F-28F (registration N8627J) experienced a belt clutch disengagement while on a sightseeing tour flight near Palmer, Alaska. The helicopter was operated by Talahiem Air Service as part of the Great Alaska Aviation Gathering at the fairgrounds. The pilot had made ten previous flights that day, each lasting about seven minutes, before the accident flight.
Flight and Mechanical Events
During the return to the fairgrounds, the clutch disengage light illuminated dimly, flickered, then fully illuminated. The pilot reported no vibration or other indication of a problem but prepared for a sudden clutch release. While descending for approach at about 200 feet above ground level and 80 mph, the belt clutch disengaged, separating engine rpm from transmission rpm and causing loss of engine power to the rotor system.
Forced Landing and Damage
The pilot reduced throttle, lowered collective, and entered an autorotation. The approach and flare were normal, with the helicopter touching down in an open field with a tailwind of about 13 mph and a ground speed of about 15 mph on plowed dry dirt. After touchdown, the helicopter nosed down; a main rotor blade struck the ground in front, and another blade impacted the tail section just forward of the stabilizers. The tail boom partially separated from the fuselage, and the fuselage sustained substantial damage. The pilot and two passengers exited without further incident.
Clutch Mechanism and Postaccident Examination
The clutch lever in the cabin is used to engage the belt. When fully engaged, the belt tensioning mechanism locks over-center, and the lever is stowed on the floor. Postaccident examination revealed that with the handle in the stowed position, it was not firmly on the floor and could be moved enough to unlock the belt engagement mechanism.
Further examination by Enstrom Helicopter Corporation and FAA inspectors found no evidence of any situation causing clutch disengagement unless the cable moved the bellcrank out of the over-center locked position. Wear marks on the clutch handle, pilot statements, and follow-on investigation indicated that the handle could contact a roll pin in the bellcrank when disengaged from the bellcrank, allowing handle movement to move the cable. The manufacturer issued a service bulletin in December 2021 clarifying rigging requirements for the clutch handle.
Injuries and Damage
The pilot and two passengers were not injured. The helicopter sustained substantial damage to the fuselage and tail boom.