Synopsis
The aircraft, a Schweizer 269C registered G-TASS, was engaged in a mountain flying instructional flight on 10 May 2004 when it suffered an engine failure. The helicopter was at a point where insufficient height remained to reach the valley floor for a safe landing. During the attempted landing, the tail rotor struck the ground, followed by a heavy impact with the hillside, and the aircraft rolled onto its right side. The two crew members sustained serious injuries.
History of Flight
The helicopter owner, who held a Private Pilot's Licence (Helicopter), was undertaking mountain flying training with an experienced instructor over a planned two-day period. Their first flight from Leeds to Carlisle Airport was uneventful, with several practice forced landings initiated by the instructor. After refueling at Carlisle and a lunch break, they flew back toward the Lake District intending to land at Newcastle Airport. Approximately 20 minutes after departure, while flying along a shoulder of high ground, the owner asked the instructor about actions in the event of engine failure. The instructor replied they would not make the valley floor. The owner, who was flying, initiated a gentle right turn toward the valley floor and began a descent by lowering the collective lever. Shortly after, engine RPM fell rapidly to zero and engine noise ceased. The instructor took control and entered autorotation, continuing the turn toward the valley floor. The owner attempted an engine restart unsuccessfully. After less than 10 seconds in autorotation, the instructor pulled full collective to reduce descent rate at impact. The aircraft came to rest on its right side; the owner exited unaided, while the instructor was extricated with help from hill-walkers.
Witnesses
An eyewitness on the opposite side of the valley observed a level portion of flight and then lost sight of the helicopter behind trees before hearing the impact. Using visual features, the witness estimated the helicopter's height at 100 metres above the valley floor, but lateral distance could not be determined.
Autorotation
The exact position and height at engine failure are unknown. Estimated all-up weight was approximately 1,700 kg. In calm winds, engine failure autorotation rate of descent would be between 1,500 and 2,000 feet per minute; at 60 kt, the descent angle would be 14° to 18°. No suitable landing sites existed apart from the valley floor.
Mountain Flying Instruction
The helicopter owner was the commander on the flight. Instruction was provided by a 57-year-old Qualified Helicopter Instructor with over 16,000 hours total helicopter experience, including approximately 150 hours on type.
Accident Site Details
The aircraft was flying westerly along the valley close to the north-facing slope when the engine failed. Ground marks indicated a turn onto a track of about 040°. Initial contact was by the tail rotor; paint flakes from a main rotor strike on the tail boom suggested aft cyclic control. A further scar was made by the vertical stabiliser. The aircraft skipped 16 metres downhill where a substantial impact occurred, with engine parts and cockpit glazing fragments found. The aircraft became airborne again and landed 10 metres further in soft, marshy ground, then rolled onto its right side. Main rotor blades were intact, at high flapping angle, indicating low rotational energy consistent with engine failure.
Detailed Examination of Wreckage
The investigation focused on the mixture control cable, which was found disconnected from the mixture lever on the fuel injector servo. The cable attachment had been made using a stud, cup, washers, nut, and split pin. The Maintenance Manual showed a stud with a head wider than the shank; however, a design change in 1995 introduced a stepless shank stud with a head same diameter as the shank. In the latter design, cable failure or pull-through would allow the stud assembly to separate. The stud assembly was not recovered. Examination of the mixture cable end showed evidence of low cycle fatigue and overload, consistent with being cut by wire cutters during attachment. The cable was cut so close to the lever that the joggle contributed no strength. A test showed the mixture cable required only 5 lbs force to pull through, compared to 25 lbs for a correctly assembled fuel shut-off cable.