What happened
On 12 August 1998, an Avro 146-RJ10 and registration G-BXAS arrived at London Gatwick Airport from Amsterdam. Following standard company procedure for parking at stands with fixed jetties, the crew intended to shut down engines 1 and 2 prior to arrival. Because the aircraft's APU was unserviceable, the crew could not rely on it for secondary electrical power.
During the taxiing process, the first officer configured the aircraft for engine shutdown. While the commander confirmed the procedure, the AC pump switch was inadvertently left in the OFF position rather than the required ON position. As the aircraft turned onto Taxiway 2, the Yellow Hydraulic System Low Pressure indicator dropped to zero, and the N1 speeds for both engines 1 and 2 subsequently decreased to zero.
As the aircraft approached Stand 10, the commander applied the parking brake. However, due to the lack of hydraulic pressure, the brakes were ineffective. The aircraft continued to move forward, eventually striking the hardware of the airbridge. The impact caused severe damage to the No 2 engine intake structure. The crew managed to stop the aircraft by applying emergency Yellow brakes, but the collision had already occurred. There were no injuries to the 6 crew members or 32 passengers on board.
The investigation
AAIB investigators examined the aircraft's Flight Data Recorder (FDR) and Cockpit Voice Recorder (CVR). The FDR confirmed that the Yellow Hydraulic System Low Pressure discrete had remained at zero from the moment the switch error occurred. The investigation also included a detailed examination of the engine pylon and wing attachments, which showed no structural damage.
Technical testing was performed on the hydraulic pump and the AC pump operating switch. The hydraulic pump was dismantled and tested at the manufacturer's facility, and the switch was subjected to extensive laboratory vibration testing. No mechanical or electrical faults were found in the pump or the switch that could have caused an unintended loss of pressure.
Findings
- The primary cause of the incident was that the AC pump switch was inadvertently selected to the OFF position during the engine shutdown procedure.
- The crew failed to notice the resulting loss of hydraulic system pressure and the associated Master Warning System indications.
- The lack of hydraulic pressure rendered the primary braking system ineffective, leading to the collision with the airbridge.