What happened
On 18 October 2004, an Eurocopter AS350B3 helicopter, registration G-BZVG, was conducting a training flight at Oxford Kidlington Airport. The flight involved an instructor and a student performing a simulated hydraulic failure approach and landing.
During the exercise, the student used the hydraulic test pushbutton to simulate a loss of hydraulic power. Following the established procedure, the instructor then switched the hydraulic cut-off lever to the 'off' position. However, the instructor failed to reset the hydraulic test pushbutton to the 'on' position before the final phase of the landing.
As the helicopter neared the ground, the aircraft began to yaw to the left and the nose rose. The instructor took control and attempted to correct the developing roll and yaw by applying significant cyclic and pedal inputs. Fearing that re-engaging the hydraulics at such a low altitude might cause an over-control situation, the instructor opted to maintain manual control. The physical effort required to counteract the forces proved too great; the helicopter rolled to the left and struck the grass surface, resulting in extensive damage to the fuselage and main rotors. One crew member sustained serious injuries.
The investigation
The AAIB examined the wreckage and the aircraft's control systems. No evidence of mechanical failure prior to the impact was discovered. The investigation focused on the control forces experienced during the manual flight phase.
It was established that the hydraulic test pushbutton had remained in the depressed 'test' position throughout the landing. This specific setting had two critical consequences: it depressurised the tail rotor load compensator, significantly increasing the required right pedal force, and it prevented the restoration of hydraulic pressure to the system via the cut-off switch.
Furthermore, the investigation looked into the distribution of safety information. While the manufacturer had issued cautionary messages (TELEX) regarding the high control forces and correct switch usage, these updates had not yet been integrated into the UK-specific Flight Manual Supplement. The training organization had not received or implemented these specific instructions prior to the accident.
Findings
- The accident was caused by the instructor not following the correct sequence of hydraulic switch selections, specifically failing to reset the hydraulic test pushbutton.
- The failure to reset the pushbutton left the tail rotor load compensator depressurised, which increased the lateral control forces required to maintain heading.
- The instructor's decision not to re-engage the hydraulics due to the low altitude prevented the restoration of hydraulic assistance that could have mitigated the loss of control.
- Safety-related information regarding the magnitude of control forces was not readily available to the crew because the updated Flight Manual Supplement had not yet been circulated to UK operators.