Robinson R22 N566BC accident near Winder, Georgia

Probable cause

Maintenance personnel’s failure to properly secure the engine cooling fan attachment bolts, leading to vibration of the cooling fan, the overextension of the tension belt actuator, the failure of the drive system vee-belts, and the loss of power to the rotor system. Contributing to the accident was the failure of the operator to incorporate the manufacturer’s service bulletin addressing the prevention of belt tension actuator overextension, and the failure of maintenance personnel and the pilot to observe the loose fanwheel bolts during the most recent 100-hour inspection and during the preflight inspection of the helicopter (respectively).

NTSB analysis

The pilot was practicing touch-and-go landings during a personal flight in a helicopter. As the helicopter was turning onto a left crosswind during the fifth circuit around the pattern, the pilot heard a “metal against metal” noise, and the helicopter “jerked.” As he began to look for a clearing to land the helicopter, it jerked again, and the pilot thought that the helicopter was losing power. The pilot was unable to reach a clear area, and the helicopter descended “straight down” through trees and collided with the ground, resulting in substantial damage to the helicopter’s fuselage, tailboom, main rotor, and tail rotor and serious injuries to the pilot.

Postaccident examination of the wreckage found that most of the drive system vee-belts were missing; only a small 2-ft portion of one vee-belt was found. Further examination of the wreckage revealed that the engine cooling fan attachment bolts were loose and could be easily turned with a wrench. The paint around the fanwheel-to-hub hardware was displaced, and surface corrosion and fretting residue were found on the exposed metal around the hardware. This evidence was indictive of the bolts having been loose for a period of time, and it is likely that the condition of the bolts would have resulted in the cooling fan imparting vibratory loads to the helicopter. The cooling fan vibration likely allowed the belt tension actuator to incrementally overextend, resulting in the failure of the vee-belts and the loss of power to the rotor system.

The helicopter operator had not complied with a service bulletin that addressed an issue with the belt tension actuator. According to the airframe manufacturer, taking the action described in the service bulletin would have prevented the actuator from overextending by incorporating an electronic time delay. Also, there was no record of fanwheel replacement at the most recent overhaul, which was required by the manufacturer. The fanwheel hardware should also have been inspected by maintenance personnel at the last 100-hour inspection and by the pilot during the preflight inspection.

Text from the NTSB. Open the NTSB docket for ERA21LA024 for the full record.