What happened
On 29 December 2003, an Aerospatiale AS350B Ecureuil, registration G-EJOC, was preparing for departure from a confined landing site in Houghton, Norfolk. The site was bounded by a house, a lake, and trees. Following a standard engine start, the pilot performed two separate hydraulic system checks. During the second check, which involved the hydraulic isolation switch, the pilot noted that the 'HYD' warning light remained illuminated on the central panel. After cycling the switch several times, the light eventually cleared.
As the pilot lifted the aircraft into a hover, the flight controls suddenly became stiff and difficult to manipulate. The pilot observed that the 'HYD' warning light had returned to an illuminated state, though the warning horn did not sound. Attempting to return to the original landing site to avoid further complications, the pilot maneuvered the aircraft through a restricted area. During this repositioning, the tail rotor struck tree branches, resulting in damage to the tail rotor. The pilot was able to land the aircraft safely, and no injuries were reported.
The investigation
Investigators examined the aircraft's systems and found no mechanical or technical faults with the helicopter. The investigation focused on the sequence of the pre-flight hydraulic checks. It was noted that the pilot had cycled the hydraulic isolation switch rapidly to clear the warning light, which may have prevented the system from fully re-engently.
Furthermore, the investigation looked into why the warning horn failed to sound during the loss of pressure. It was suggested that the horn might have been inadvertently deactivated during the checks. The investigators also considered the visibility of the warning panel, noting that the instrument panel glare-shield could obscure certain lights depending on the pilot's seating position.
Findings
- The primary cause of the control stiffness was the unintentional isolation of the hydraulic system from the flight controls prior to takeoff.
- The pilot likely failed to allow sufficient time for the hydraulic system to stabilize after cycling the isolation switch.
- It is probable that the warning horn was inadvertently switched off, and the hydraulic test button was accidentally engaged, which would explain why the horn remained silent when pressure was lost.
- The pilot's decision to return to the confined landing site, rather than continuing over the lake to a clearer area, contributed to the tail rotor strike.