What happened
On 24 June 2020, a Bell 407 helicopter, registration N120HH, was performing a private sightseeing flight near Long Marston, Warwickshire. The flight, which included a pilot and one passenger, was returning to Thrupton Aerodrome following a trip over the Malvern Hills when the engine began to malfunction.
Witnesses observed smoke and a trail trailing from the aircraft before a sudden mechanical noise and a violent yaw occurred. The pilot experienced a sudden drop in torque and an engine-out warning, prompting an immediate transition into autorotation. The pilot successfully navigated the aircraft to a field near Long Marston, performing a controlled landing. However, upon touchdown, the pilot rapidly lowered the collective, which caused a main rotor blade to strike the tail boom, severing it from the fuselage.
Following the landing, a fire broke out in the engine compartment. The occupants were able to exit the aircraft without injury, but the helicopter was subsequently destroyed by the flames.
The investigation
The AAIB investigation focused on the engine's failure and the subsequent fire. Investigators examined the engine components and the cockpit instrumentation, noting that while the pilot managed a safe landing, the aircraft's fuel valve remained in the open position. This allowed the booster pumps to continue delivering fuel to the engine compartment, feeding the fire.
Technical analysis of the engine revealed that the gas-producing turbine disc had suffered an uncontained failure. The investigation also looked into the rotor dynamics during the autorotation, noting that the rotor RPM (NR) had dropped below the manufacturer's specified range, though the pilot was still able to complete the landing safely.
Findings
- The primary cause of the engine failure was an uncontained failure of the gas-producing turbine disc.
- This catastrophic failure was triggered by insufficient oil reaching the engine bearings due to an unidentified oil leak.
- The failure of the turbine disc damaged fuel and oil lines, allowing leaking fluids to ignite in the hot engine compartment.
- The tail boom was severed during the landing because the pilot rapidly lowered the collective immediately after touchdown.
- The fire was intensified because the fuel shutoff valve had not been closed, allowing fuel to continue flowing from the tanks to the engine area.