What happened
On 21 August 2000, a Hughes 369E helicopter, registration G-SIVA, was conducting a private flight from a helipad in Kent. While flying at approximately 1,500 feet north of Dartford, the aircraft suffered an abrupt loss of engine power. The pilot immediately initiated an autorotative descent and issued a 'MAYDAY' call to air traffic control.
During the descent, the pilot reported that the rotor RPM remained stable at the correct speed for autorotation, though he did not recall hearing a low rotor RPM warning. The aircraft descended at a rate of roughly 2,000 feet per minute, resulting in a heavy landing at Dartford Marshes. The impact caused the landing skids to splay and the fuselage underside to crush, which subsequently ruptured the fuel tanks. While the aircraft was destroyed, it remained upright and did not catch fire. Both the pilot and the passenger sustained serious injuries.
The investigation
Following the accident, investigators examined the aircraft and specifically focused on a broken air signalling pipe. This pipe, which conveys compressor outlet pressure to the Fuel Control Unit (FCU), was found to have fractured near the FCU.
Metallurgical analysis performed at DERA Farnborough revealed that the fracture was primarily caused by fatigue. The crack originated on the outer surface of the pipe. Investigators also identified evidence of fretting and corrosion pits at the site of the failure. The presence of aluminium traces and the potential for crevice corrosion due to moisture trapped by capillary action were also noted.
Maintenance records indicated that the engine had previously experienced issues with low power and engine stagnation during starts, which had led to various investigations and the replacement of the FCU. Additionally, the aircraft had recently been noted to exhibit unusual high-frequency vibrations through the rudder pedals.