Aircraft and Flight Details
The accident involved a McDonnell Douglas Helicopters Hughes 369E helicopter, registration G-KSWI, manufactured in 1986. It was powered by a single Rolls Royce Allison C250-C20B turboshaft engine. The private flight departed from Draycot Airfield near Swindon at approximately 1250 hrs on 19 June 2011, with the pilot as the sole occupant. The pilot held a Private Pilot's Licence and had 730 total flying hours, of which 720 were on type.
History of the Flight
The pilot flew from Cornwall to Draycot Airfield earlier that morning. After a two-hour stop, he departed westward for the return flight to Cornwall. At 1255 hrs, while cruising at 2,200 ft, the pilot contacted Bristol ATC for a Basic Service. At 1317 hrs, Bristol ATC called G-KSWI without response, and radar showed the helicopter descending. The radar return disappeared near Glastonbury Tor.
Witness Observations
Witnesses near Glastonbury saw the helicopter descending in a westerly direction. They reported hearing an abnormal loud clunking sound. The helicopter descended to low level, flew one right orbit, and at about 50 ft altitude began spinning clockwise. Witnesses reported two to six rotations before the noise reduced and the helicopter dropped to the ground. A teacher nearby observed the helicopter fly low over her house, orbit a field containing cows, then spin clockwise at least twice before descending rapidly. Her husband heard a loud mechanical noise and found the pilot unconscious, hanging from his seatbelt, with a strong fuel smell.
Accident Site and Damage
The helicopter came to rest on a heading of 060° in a grass field. The tail section had separated from the fuselage, with the blue tail rotor blade resting on the ground. The tail cone attached to the fuselage was bent approximately 90° anti-clockwise. Main rotor blade strike marks were present on the ground. The outer section of the green tail rotor blade was found 90 m away. The helicopter was destroyed, but there was no fire.
Recorded Data
The helicopter was equipped with a FlymapL GPS that recorded flight data. The last two minutes showed a descent from 2,200 ft with a rate of descent averaging 1,800 ft/min initially, then 650 ft/min. Groundspeed reduced to below 30 kt. The final recorded groundspeed was 10 kt. Radar data from Clee Hill was lost below 1,500 ft.
Investigation Findings
The investigation established that fatigue cracks emanating from corrosion pits were present on the tail rotor blade pitch horn of one blade, which led to its failure. The associated tail rotor pitch link had also failed. The sequence of the two failures could not be determined, but either could explain the helicopter's behaviour. Neither the failed pitch horn section nor the pitch link were recovered. The loss of pitch control to one tail rotor blade likely caused the loss of directional control, leading to the crash.
Probable Cause
The probable cause was fatigue cracks emanating from corrosion pits on one tail rotor blade pitch horn, leading to its structural failure and subsequent loss of tail rotor pitch control.