Bell Helicopter Textron Canada 407 N451PH accident near Abbeville, Louisiana

Probable cause

The pilot’s failure to maintain proper helicopter control during autorotation that resulted in an abnormal ground contact which caused the subsequent main rotor strike on the tail boom that severed the tail rotor driveshaft. Contributing to the accident was the main rotor blowback condition, due to the aft tilting of the main rotor disk.

NTSB analysis

The check pilot and the pilot receiving instruction were performing initial new hire training for the commercial operator. The pilot previously performed three practice 180° autorotations, terminating with a power recovery. The pilot then performed a practice, straight-in, full down autorotation to touchdown on the sod area parallel to the runway. During the touchdown, the two pilots heard a “loud bang.” The helicopter came to rest upright on the sod area and both pilots were able to egress from the helicopter without further incident. A postflight inspection revealed that the main rotor blades struck the tail boom, severing the tail rotor driveshaft. The main rotor blades, the tail boom, and the tail rotor system sustained substantial damage.

The operator reported there were no preimpact mechanical malfunctions or failures with the airframe or the engine that would have precluded normal operation. According to another helicopter manufacturer, main rotor blowback occurs when the forward portion of the helicopter’s main rotor disk is displaced upward, while the rear portion of the main rotor disk is displaced downward. If the resulting blowback is excessive, the main rotor blades may impact the tail boom. A review of the accident helicopter rotorcraft flight manual (RFM) found no information listed to provide awareness to pilots about the main rotor blowback condition.

Text from the NTSB. Open the NTSB docket for CEN23LA199 for the full record.