What happened
On 7 October 2005, a DHC-8-311 Dash 8, registration G-BRYW, was parked at Stand 8 of Aberdeen Airport for passenger boarding. The aircraft engines had been started, and the Ground Power Unit (GPU) cables had recently been disconnected. At the time of the incident, the aircraft was carrying 50 passengers and 4 crew members.
While the aircraft crew was performing their post-start checklists, the GPU, which was positioned facing the aircraft, began to move forward without anyone in the driver's cab. The vehicle struck the rotating right engine propeller, damaging all four blades, before colliding with the aircraft fuselage under the right wing. The impact caused the engine to come to an abrupt halt and resulted in visible fuel leakage. The commander immediately shut down the engines and initiated emergency procedures, including the discharge of both fire bottles. All passengers were evacuated via the main door and safely reached the terminal.
The investigation
The AAIB investigation focused on the mechanical state of the Houchin C762 GPU. While the aircraft itself showed no serviceability issues, the investigation revealed significant mechanical failures within the ground vehicle.
Inspectors found that the GPU's engine speed was running at a much higher rate than normal idle. This was caused by a detached governor rod, which had disconnected from the fuel pump due to a worn ball joint and a worn spring. Furthermore, the investigation examined the vehicle's transmission controls, finding that the mechanical gate for the forward-neutral-reverse selector was worn, meaning the button required to lock the lever in place no longer functioned effectively.
Findings
- The primary cause of the accident was the mechanical failure of the GPU's engine control components, specifically a worn ball joint and a worn fuel pump spring that allowed the governor rod to detach.
- A worn mechanical gate in the forward-neutral-reverse selector allowed the vehicle to be shifted into forward mode without the required button being depressed.
- The operator's maintenance system was inadequate, as it failed to identify or report these developing defects.
- The manufacturer's recommended servicing schedule lacked specific checks for the governor rods, fuel pump springs, and the selector gate.
- Local practice at the airport involved positioning the GPU facing the aircraft to manage exhaust fumes, a practice that deviated from company procedures but was not considered the primary cause of the collision.