What happened
On 21 October 2009, an ATR 72-201, registration EI-REH, arrived at Manchester Airport Stand 7 following a scheduled flight from Galway. After the aircraft reached its designated stopping position, the flight crew applied the parking brake and feathered both propellers. However, the aircraft unexpectedly began moving forward again.
Despite the crew's attempts to stop the movement using the toe brakes and subsequently the parking/emergency brake lever, the aircraft continued to roll. The movement resulted in the No 2 engine propeller striking a stand guidance mirror assembly. The impact caused significant damage to the propeller blades and the mirror infrastructure, with one blade becoming lodged in the assembly. The crew promptly shut down the engines and requested emergency services.
The investigation
Investigators examined the aircraft's hydraulic systems and discovered a trail of hydraulic fluid beneath the fuselage. The investigation revealed that the Blue hydraulic system reservoir was empty. Upon testing the system, it was found that pressure in the Blue system decayed rapidly due to a leak at a hydraulic fuse located in the parking/emergency brake line.
Detailed metallurgical analysis of the failed fuse showed a crack in the valve body. This crack was found to have originated from multiple points within the casting, likely influenced by shrinkage porosity. The crack eventually led to a brittle overload failure when the aircraft arrived at the stand, causing the rapid depletion of the hydraulic accumulator pressure.
Findings
- The primary cause of the uncontrolled movement was the failure of a hydraulic fuse in the Blue hydraulic system, which led to a loss of pressure in the parking/emergency brake accumulator.
- The fatigue crack in the fuse was likely driven by repetitive pressure cycles and influenced by manufacturing defects (shrinkage porosity) in the cast aluminium alloy.
- The ground crew did not place chocks under the wheels before attaching the fixed electrical power cable, a deviation from standard procedures that left them in a hazardous position when the aircraft moved.
- The crew's decision to use the parking/emergency brake lever, while attempting to stop the aircraft, inadvertently contributed to the rapid discharge of the remaining hydraulic fluid through the failed component.