What happened
On 23 July 1999, an Aerospatiale AS332L Super Puma, registration G-PUMH, was conducting a public transport flight from Aberdeen to the North Alwyn Platform. Due to higher than anticipated fuel consumption and stronger than forecast winds, the crew decided to divert to Kirkwall Airport in the Orkney Islands for refueling.
Upon arrival at Kirkwall, the commander performed a hover over the runway and proceeded to taxi toward taxiway Delta. The landing was initially perceived as normal, with the crew believing all wheels were firmly on the ground. However, approximately 40 seconds after the aircraft came to a halt, the nose landing gear collapsed, causing the helicopter to pitch forward. As the commander attempted to cushion the descent, the main landing gear also retracted, leaving the aircraft resting upright on the taxiway. There were no fatalities and no injuries among the 2 crew members or 16 passengers on board.
The investigation
The investigation focused on why the landing gear retracted while the aircraft was on the ground. Analysis of the Flight Data Recorder (FDR) revealed that the landing gear selector had been moved to the UP position shortly after takeoff from Aberdeen. However, the aircraft's ground/flight logic had failed to transition to 'flight' mode, meaning the gear remained extended during the flight. This logic error persisted until the aircraft was taxiing at Kirkwall, at which point the system finally registered a change in state. Because the selector was still in the UP position, the gear began to retract while the aircraft was stationary.
During the approach, the co-pilot noted 'three greens' on the landing gear panel but did not verify the physical position of the selector. The commander also observed the green lights but did not check the selector itself. The investigation found that the landing gear selector is black against a black background, making its position difficult to discern at a glance, especially when the crew's attention is naturally drawn to the illuminated indicator lights.
Findings
- A faulty nose landing gear 'centre extended' proximity sensor provided an incorrect 'in air' signal to the Landing Gear Logic PCB.
- This technical failure caused the ground/flight logic to remain in 'ground' mode despite the selector being in the UP position.
- The landing gear selector was left in the UP position following departure from Aberdeen.
- High workload during the diversion to Kirkwall contributed to the crew not noticing the mismatch between the selector position and the indicator lights.
- The physical design of the selector made it difficult to verify its position visually during a cursory check.