Bell 407 Damaged During Autorotation Training (PHI, N451PH)
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.
The NTSB provide weather data but wind speed and direction are blank.
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 NTSB 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.
The reference to blowback is unusual in NTSB reports into what is a relatively common type of accident. NTSB describe main rotor blowback occurs by reference to another manufacturers’ data (MD Helicopters):
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.
However, no safety recommendations are made.
The European Safety Promotion Network Rotorcraft (ESPN-R) has a helicopter safety discussion group on LinkedIn. One ESPN-R resource is: EHEST Leaflet HE 5 Risk Management in Training. Also see: Why is autorotation management important for safety? within the EASA community site.
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