What happened
On 22 November 2006, a Boeing 747-436, registration G-BNLE, was parked at Stand 430, east of Terminal 4 at London Heathrow Airport, following a scheduled passenger flight from San Paulo. During preparations to disembark passengers, a self-propelled passenger stair vehicle was positioned at the number 2 left aircraft door, with the stair head placed against the door sill.
While the vehicle operator was extending the hydraulic stabilization jacks, a mechanical malfunction occurred. As the hydraulic jacks extended, the two jacks responsible for maintaining stair height retracted. This caused the lower forward edge of the stair head to descend, striking and damaging the aircraft's fuselage-to-wing fairing. There were no injuries to crew or passengers, and the damage was limited to a hole in the honeycomb structure of the fairing.
The investigation
An investigation conducted by the AAIB and an engineering examination performed by both the vehicle operator and the manufacturer were undertaken. The technical inspection revealed that the incident was caused by an electrical component failure within the vehicle's hydraulic system.
Findings
- The primary cause of the incident was an electrical component failure within the self-propelled passenger stair vehicle.
- This failure caused the simultaneous retraction of the stair height hydraulic jacks during the extension of the stabilization jacks.
- The resulting descent of the stair head led to the impact with the Boeing 747-436 fuselage.
Safety action
Following the investigation, the manufacturer implemented a modification to the vehicle's design to prevent a recurrence of this specific failure. The equipment operator has also developed a plan to apply this modification across its remaining fleet of stair vehicles.