What happened
On 29 December 2014, a Boeing 747-443, registration G-VROM, departed London Gatwick Airport for a scheduled passenger flight to Las Vegas. Shortly after takeoff, while climbing through 6,000 feet, the crew received warnings of low quantity and low pressure in hydraulic system 4. The flight crew determined that a hydraulic fluid leak was causing the rapid depletion of the system.
Following consultation with the operator's maintenance control, the decision was made to return to London Gatwick. During the approach, the crew initiated the alternate gear extension procedure. However, the right wing landing gear failed to lock in the down position. The crew performed various maneuvers, including climbs, descents, and turns, in an attempt to force the gear to lock, but these were unsuccessful. After a period of troubleshooting and fuel jettisoning, the aircraft performed a non-normal landing at 15:45 hrs. The aircraft came to a stop with a right-wing low attitude, and while no injuries were reported among the 447 passengers and 18 crew, the right wing landing gear door was damaged and a strike board was detached from the aircraft.
The investigation
The AAIB investigation focused on the cause of the hydraulic leak and the subsequent gear malfunction. Maintenance records revealed that the wing landing gear actuator had been replaced during the night shift prior to the flight. The investigation found that the maintenance team had struggled to locate the required specialist hoist and instead manually maneuvered the 85 kg actuator into position.
Detailed inspection of the replacement actuator revealed that it had been installed 180° out of alignment. This orientation caused the hydraulic port to strike the landing gear bay structure during gear retraction, leading to the fluid leak. Furthermore, the investigation traced a piece of the aircraft's strike board to a field near Tonbridge, Kent, confirming it had been released during the gear extension process.
Findings
- The primary cause of the hydraulic leak was the incorrect orientation of the landing gear actuator during installation.
- Maintenance technicians, facing difficulties with heavy lifting and a lack of appropriate specialized tooling, manually positioned the actuator, which led to the installation error.
- The design of the actuator lacked distinguishing features to prevent incorrect orientation, and the labeling of the hydraulic ports ('UP' and 'DN') was prone to misinterpretation.
- The loss of hydraulic fluid in system 4 meant the alternate gear extension was undamped, causing the gear leg to accelerate under gravity and strike the gear door.
- The aircraft's strike board was released from the wing gear door due to the mechanical stress of the gear leg becoming 'hung-up' during the undamped extension.