What happened
On 2 April 2021, a Rotorway 162F helicopter, registration G-JDHN, was conducting a private flight near Ledbury, Herefordshire. While cruising at 1,500 ft, the pilot heard a massive bang, described by ground witnesses as an explosion, which appeared to originate from the rear of the aircraft. This event caused the helicopter to twitch in a rightward yaw and pitch the nose upward rapidly.
Believing he had lost tail rotor drive, the pilot initiated an autorotation. During the descent, the pilot managed to avoid polytunnels but noted a lack of response to left pedal inputs. The pilot executed a landing in a fallow field, touching down at a walking pace. However, during the ground run, the left skid caught in a rut in the uneven terrain, causing the aircraft to pitch forward and roll onto its left side. Both the pilot and the passenger sustained minor injuries.
The investigation
The AAIB examined the wreckage and the engine components. Investigators found that the tail boom structure had been severely damaged, with the tail rotor assembly and vertical fin breaking off upon impact. The investigation also focused on the engine, where oil was found in the inlet manifold.
Detailed microscopic examination of the engine revealed surface defects on the inlet valve of cylinder 2 and the exhaust valve of cylinder 3. While a recent annual inspection had shown passing compression results, the post-accident testing revealed significant air leaks. The investigation also reviewed the flight data from the Dual Engine Control Units (DECUs), which confirmed a sudden throttle closure and subsequent engine limiter activation following the initial noise.
Findings
- The primary cause of the initial disturbance was likely an after-fire in the exhaust system, caused by unburnt fuel accumulating and igniting due to defects in the cylinder 3 exhaust valve sealing.
- The sudden noise and subsequent pitch-up may have caused a startle effect, leading the pilot to react to unexpected aircraft behavior.
- The pilot's decision to enter autorotation was influenced by the uncommanded yaw, though the aircraft's tail rotor remained operational during the descent.
- The rollover during the landing phase was caused by the left skid digging into a rut in the uneven ground.