Incident
On 15 February 2006, a BAe 146-300 aircraft, registration G-JEBB, was on a positioning flight to Birmingham International Airport. During the final approach at an altitude between 100 and 200 ft above ground level, the aircrew heard a loud bang. Shortly afterward, a hydraulics caption illuminated, and the commander, who was the handling pilot, observed the hydraulic fluid level in the yellow system falling. Believing a mechanical failure had occurred, the commander continued the landing since the aircraft was in landing configuration and only half the roll spoilers were affected. After an uneventful landing, the commander turned off the engine-driven and AC pumps to minimize further damage. An engineer on board visually checked the No 2 engine, where the pump for the yellow system was located, and found no damage. As a precaution, the No 2 engine was shut down, and the aircraft taxied using three engines. The park brake was unavailable, so toe brakes were used until chocks were placed. Post-flight checks determined that the yellow system hydraulic accumulator had burst, and a metal pin from the accumulator had pierced the fuselage pressure hull.
Aircraft and Crew
The BAe 146-300 (year of manufacture 1991) was powered by four Lycoming ALF502R-5 turbofan engines. The commander held an Airline Transport Pilot's Licence, was 35 years old, and had 4,500 flying hours total, with 3,000 on type. Three crew members were on board; no passengers. No crew injuries occurred.
Accumulator Failure
The aircraft had two hydraulic accumulators located under the fuselage floor near the main landing gear, inside the pressure hull. The burst accumulator consisted of a pressure cylinder with a piston; one side contained hydraulic fluid, the other nitrogen at nominally 1,000 psi. This accumulator was assembled in 2001 and installed 10 months before the incident, during which the aircraft made 1,844 landings and accumulated 1,593 flying hours. The cylinder was machined from solid steel bar stock (material S98) with a wall thickness of 2.8 mm. The manufacturer's job card specified fluorescent magnetic particle inspection, but the manufacture, surface treatment, and crack detection were subcontracted. The cylinder manufacturer has since ceased trading.
Metallurgical Examination
The damaged accumulator underwent metallurgical examination, revealing two non-metallic inclusions in the cylinder wall. These were thin strands present in the bar stock prior to machining, located on the outer face running longitudinally. The longer inclusion, 5.7 cm long, was where the cylinder initially burst, followed by failure at the second inclusion. Both inclusions reduced the local wall thickness by approximately two-thirds. The discontinuities then grew due to low cycle, high stress fatigue. Magnetic crack tests detected no other defects.
Safety Actions
The accumulator and aircraft manufacturers were promptly informed of the examination results. They are examining their inspection records and implementing a programme of non-destructive crack detection for components considered at risk. They are also reviewing manufacturing and inspection processes for accumulators. The AAIB did not consider it necessary to make safety recommendations.
Probable Cause
The failure of the hydraulic pressure accumulator was caused by a pre-existing inclusion of non-metallic material, and this defect progressed through low cycle fatigue resulting in the cylinder bursting.