What happened
On 28 August 1998, a Boeing 737-448, registration EI-BXB, was operating a passenger flight from Cork to London Heathrow. While approaching the Ockham VOR, the crew noticed the Master Caution and Hydraulic lights flickering. The first officer reported that the System B quantity had dropped to 39%. The crew requested a holding pattern to assess the situation and performed the necessary checklists.
To manage the technical issue, the crew selected Flap 5 and reduced the approach speed to 180 knots. Although the situation was treated as an abnormal event rather than an emergency, the crew briefed the cabin crew that emergency services would be present upon landing. During the approach, the crew requested a 10-mile join point.
After landing on Runway 27L, the aircraft came to a stop slightly beyond the normal turn-off point using manual braking and reverse thrust. While the engines were shut down for an inspection by the Rescue and Fire Fighting Services (RFSS), personnel reported seeing smoke emanating from both the left and right sides of the aircraft. This prompted the commander to order an immediate evacuation. During the evacuation, passengers using the overwing exits encountered a significant drop because the aircraft was not in full flap configuration.
During the egress, one infant briefly stopped breathing but was successfully resuscitated by an airport fire officer. One passenger sustained minor back injuries. All passengers were eventually moved to the terminal via buses.
The investigation
The investigation focused on two primary issues: the cause of the hydraulic leak and the malfunction of an escape slide.
Technical analysis of the hydraulic 'B' system pump revealed a failed 'O' ring seal. Laboratory testing of various seals from the same batch indicated that the failure was linked to manufacturing defects, specifically related to the removal of excess flash through buffing, which may have compromised the seal's integrity.
Regarding the evacuation, investigators examined the door 1 Right slide, which had deployed at an angle that rendered it unusable. Testing showed that the slide could function correctly when operated properly, suggesting that the manual inflation procedure or the speed of door operation might impact deployment.
Findings
- The primary cause of the hydraulic failure was a defective 'O' ring seal resulting from manufacturing errors or improper assembly.
- The hydraulic 'B' system pump failed due to a damaged plug and a flattened 'O' ring.
- The deployment issue of the 1R slide was linked to the specific manner in which the door was opened and the inflation sequence.
- The use of Flap 15 instead of full flaps, necessitated by the hydraulic failure, created a larger vertical drop for passengers exiting via overwing routes.