16th of February 2006
A British Aerospace BAe 146-300 was on a positioning flight to Birmingham International Airport on this day in 2006. Onboard a crew of three. While fully configured for the landing at Birmingham, at 100 - 200 feet / 30 - 60 meters AGL (Above Ground Level) a loud bang was heard by the crew. Shortly afterward a hydraulics caution illuminated on the MWP (Master Warning Panel).
The failed accumulator as it was removed from the aircraft (source aaib.gov.uk)
A check by the crew revealed a steady decline in Yellow hydraulic system quantity indication on the Hydraulic panel, located on the overhead panel in the flight deck.
The Captain concluded the loud bang and the drop in Yellow system hydraulic quantity were related, either a failure in the Hydraulic Bay or on the number 2 engine (where the Yellow hydraulic system Engine Driven Pump is located). As they were stabilised and the aircraft handled normally, the Captain decided to continue the landing. With the aircraft fully configured loss of the Yellow hydraulic system only meant half the roll spoilers were not available. (Normal brakes are on the Green hydraulic system).
The landing was uneventful and with the aircraft slowed to a safe speed the crew switched the Yellow hydraulic system Engine Driven Pump and the AC Pump off to reduce the risk of further damage. The third crewmember was one of the company engineers, after the landing, he made a visual check of the #2 engine, no damage or leaks were apparent, as a precautionary measure the engine was shut down. With the Yellow system not available the parking brake was not available, so when the aircraft reached its parking position the aircraft was held in position by Hydraulic system overview
application of the footbrake until chocks were in place.
Close up of the failure area (source aaib.gov.uk)
After the relevant post-flight checks were completed the source of the loud bang was found to originate from the Hydraulic bay. The Yellow hydraulic system accumulator was found to have structurally failed and was cracked wide open, resulting in an immediate loss of the Yellow hydraulic system. A part of the accumulator had been ejected from the accumulator and caused a puncture hole in the fuselage pressure hull.
There are two hydraulic accumulators located under the BAe 146 fuselage floor close to the main landing gear installation and inside the pressure hull, and these are fitted so that the hydraulic system can cope with fluctuations in demand. The accumulator consists of a pressure cylinder with a piston inside. On one side of the piston is hydraulic fluid and on the other is nitrogen, nominally at 1,000 psi. The failed accumulator was metallurgically examined. Two (non-metalic) inclusions were found in the cylinder wall, causing a reduction in wall thickness of approximately 66%. This lead to a low cycle, high-stress fatigue failure of the accumulator cylinder wall. The full Air Accident Investigation Branch report is available by clicking here.
Detailed picture of the accumulator cylinder wall (source aaib.gov.uk)