Incident Overview
On 12 July 2001, at 0920 UTC, a Sikorsky S76A (Modified) helicopter, registration G-BMAL, was involved in a ground maneuvering accident at North Denes Aerodrome, Norfolk. The aircraft, manufactured in 1980 and powered by two Turbomeca Arriel engines, was being repositioned on the ground with two crew members on board and no passengers. No injuries were reported.
Sequence of Events
The helicopter had been parked into wind, with its tail pointing toward the area from which passengers would arrive. The commander decided that the co-pilot, who was the handling pilot seated in the left seat, should turn the helicopter 90 degrees to the left to present the left door for boarding. After completing the taxi maneuver, the handling pilot brought G-BMAL to a stop and reached with his right hand over the centre console to apply the 'T'-shaped parking brake. However, he inadvertently pulled the commander's collective lever. The aircraft lifted rapidly, and the co-pilot countered by immediately lowering his own collective lever. The helicopter then landed heavily, tail first.
Damage
The incident resulted in buckled skin and stringers in the lower portion of the tail cone.
Cockpit Voice and Flight Data Recorder
The aircraft was fitted with a GEC-Plessey Avionics Combined Voice and Flight Data Recorder (CVFDR) type 900/D51506. Crew speech and flight data were recovered successfully, though some crackling was present on voice tracks. Prior to the accident, crew transmissions were normal with no indication of malfunction. The flight data showed that about 3 seconds before the recording ended, collective pitch increased, normal acceleration rose to about 1.8 g, and pitch attitude increased from about 0 to 6.5 degrees nose-up. Main rotor speed decayed from about 108% to 100%. Radar altitude indicated a maximum height of about 2.5 feet. Within about 1.5 seconds, collective pitch was restored. The roll angle changed from about 2.5 degrees left to about 1.5 degrees right. Normal acceleration reached a minimum of about 0.3 g. The CVFDR's acceleration cut-off switch activated at about 2 g, precluding recording of the peak acceleration.
Company Report
The operating company conducted a thorough investigation, which confirmed the circumstances. The report noted that it had been common practice for the left-seat pilot, when handling, to operate the parking handle located on the right side of the centre console, adjacent to the right collective lever. The handle was not visible from the left seat without leaning over. The report raised two issues: local base procedures allowed single-pilot ground maneuvering from the left seat, and the aircraft checklist did not contain a procedure for ground repositioning. The company subsequently planned to change procedures to ensure only the right-seat occupant operates the parking handle, and to produce a formal procedure and checklist for ground repositioning.
Additional Comment
The early activation of the g-switch denied knowledge of the peak normal acceleration, which would have helped the operator gauge the extent of inspections and repairs. Safety recommendations (97-32 and 99-24) had been issued regarding the use of g-switches to stop flight recorders during impact. The study of alternative means by EUROCAE Working Group 50 had not yet been published as of the report date.