What happened
On 4 April 2006, an Airbus A320-A1, registration EC-GRF, was preparing for a scheduled passenger flight from London Heathrow Airport to Madrid. During the initial taxi phase, the crew received a low fluid level warning for the Yellow hydraulic system on the ECAM. The pilot stopped the aircraft to investigate, noting a large pool of liquid on the taxiway.
After performing the required checklists, the flight was cancelled, and the aircraft was directed back to the terminal. Upon arriving at the stand, the crew applied the parking brake. However, the aircraft began to move forward unexpectedly. Despite attempts by the crew to stop the movement using the brake pedals, the aircraft continued rolling until the left engine struck the airbridge railings. The impact caused damage to the engine inlet cowl and the airbridge protective structures. There were no injuries to the 110 passengers or 6 crew members on board.
The investigation
Investigators examined the aircraft's hydraulic systems and flight data. The Quick Access Recorder (QAR) revealed that while Yellow hydraulic pressure was initially available, it dropped significantly before eventually falling to zero. The investigation focused on the Brake Dual Distribution Valve (BDDV) and its associated components.
Metallurgical analysis of a failed hydraulic reducer union, made of aluminium alloy, showed that a fatigue crack had initiated at the thread root. This crack propagated under low-magnitude cyclic stress, likely exacerbated by periods of vibration or high static loads. The examination found no evidence of corrosion, manufacturing defects, or improper installation of the fitting.
Findings
- A fracture in a hydraulic union on the BDDV caused a leak in the Yellow hydraulic system.
- The loss of fluid led to the depletion of the Yellow system pressure, rendering the parking brake inoperative.
- The crew's decision to use the brake pedals to stop the moving aircraft was ineffective because the parking brake is operated by the Yellow system, and the loss of pressure prevented the application of the alternate braking mode.
- A significant contributing factor was that the crew did not select the parking brake to the OFF position to re-activate the normal braking system (powered by the Green system) when the aircraft began to move.