What happened
On 15 February 2006, a BAe 146-300, registration G-JEBB, was conducting a non-revenue positioning flight to Birmingham International Airport. While the aircraft was on final approach, at an altitude between 100 and 200 feet, the crew heard a loud bang. This was immediately followed by a warning light indicating a drop in the yellow hydraulic system fluid level.
The pilot suspected a mechanical failure in the hydraulic bay or the No 2 engine, where the yellow system's engine-driven pump is located. To mitigate further risk, the commander continued the landing but turned off both the engine-driven and AC pumps once the aircraft was at a safe speed. Following the landing, an engineer performed a visual inspection of the No 2 engine. Although no external leaks were visible, the engine was shut down as a precaution. Due to the failure, the parking brake was unavailable, requiring the use of toe brakes to hold the aircraft until it was chocked.
The investigation
An investigation into the incident revealed that a hydraulic accumulator had ruptured. This failure not only caused the loss of functionality in the yellow hydraulic system but also resulted in a metal pin from the accumulator piercing the aircraft's fuselage pressure hull.
Metallurgical analysis of the damaged component showed that the accumulator, which had been installed 10 months prior, contained two thin strands of non-metallic material within the cylinder wall. These inclusions were present in the original steel bar stock before the part was even machined. The presence of these inclusions had locally reduced the wall thickness by approximately two-thirds. Over time, the stress from low-cycle fatigue caused these defects to grow, eventually leading to the rupture.
Findings
- The primary cause of the failure was material defects in the form of non-metallic inclusions within the accumulator cylinder wall.
- These inclusions were present in the raw steel bar stock prior to the machining process.
- The structural integrity of the cylinder wall was compromised by a reduction in thickness at the site of the inclusions.
- The failure progressed due to low-cycle, high-stress fatigue.
- The burst was severe enough to cause a small hole in the aircraft's pressure hull.