What happened
On 17 February 2002, a Eurocopter EC135T1, registration G-SPAU, was conducting a police air support operation near Muirkirk, Scotland. The flight, involving a pilot and two police observers, had been flying in visual conditions until the aircraft unexpectedly entered thick cloud while transiting a valley.
Seeking to maintain altitude and heading while flying in the cloud, the pilot engaged the 'ALT' and 'HDG' modes of the autopilot. Upon engaging the heading mode, the aircraft began a turn to the right, likely attempting to acquire a heading bug defaulted to north. While attempting to manually correct this rightward roll, the pilot inadvertently pressed the 'AP/SAS DCPL' fast decouple switch on the cyclic control. This action disconnected the autopilot and the analogue stability augmentation systems, causing the actuators to return to their null positions and inducing a sudden, steep nose-down attitude and a 45-degree bank. Despite corrective efforts, the helicopter struck the ground in a field, resulting in the destruction of the aircraft and one serious and two minor injuries.
The investigation
The AAIB examined the wreckage and the flight control systems. While an 'ACTUATION' warning regarding the roll axis had been present since before takeoff, investigators determined that the roll SEMA was defective due to moisture-induced corrosion but was not the cause of the loss of control. Analysis of the engine parameters and flight data confirmed that the engines were functioning normally up to the moment of impact.
Investigators also reviewed the pilot's experience, noting that while the pilot was qualified for night operations, he had limited experience with the specific autopilot systems on this aircraft type. The investigation also looked into the survivability of the occupants, finding that the crashworthiness of the seats and the integrity of the fuel tank contributed significantly to the crew's survival despite the severity of the impact.
Findings
- The pilot inadvertently activated the fast decouple switch while attempting to correct an uncommanded turn.
- The engagement of the heading mode initiated a turn to the right because the heading bug was likely at its default north position.
- The sudden loss of control was caused by the disconnection of the autopilot and stability augmentation systems, which caused the flight control actuators to move to their null positions.
- The pilot's lack of extensive experience with the complex autopilot modes on the EC135 contributed to the error.
- The lack of night vision goggles (NVGs) prevented the pilot from seeing and avoiding the cloud layer during the night flight.