What happened
On 4 January 1999, a Boeing 707-351C, registration 5X-JET, was conducting a cargo positioning flight from Lagos, Nigeria, to Manston, UK. The flight was intended to facilitate maintenance work, including the installation of a new collision avoidance system. During the initial departure from Lagos, the crew discovered that the landing gear would not retract normally. Following a brief return to Lagos to address the issue, the aircraft departed again at 1552 hrs. On this second departure, the right main landing gear remained in the extended position.
To compensate for the aerodynamic drag caused by the gear, the commander departed with a significantly higher fuel load than required by the flight plan. However, the aircraft's progress began to lag behind the schedule almost immediately. By three hours into the flight, the aircraft was approximately 28 minutes behind its planned progress. As the flight continued through French airspace, the crew requested a diversion to London Gatwick due to deteriorating fuel levels.
Upon contacting London Control, the crew declared a MAYDAY due to a low fuel state, informing controllers that the aircraft was operating with the landing gear down. At one point, the crew reported having only 15 minutes of fuel endurance remaining. Despite the high-stress situation, the aircraft followed a continuous descent clearance and performed a normal landing at Gatwick at 2243 hrs. There were no fatalities and no injuries to the crew or passengers.
The investigation
The AAIB examined the aircraft's technical logs, flight logs, and fuel consumption data. The investigation focused on why the excess fuel carried at departure was insufficient to cover the flight duration. Investigators compared the planned fuel consumption against the actual fuel flow recorded during the flight. The investigation also looked into the maintenance history regarding the landing gear deboost valve lever, which had been identified as the cause of the initial retraction failure.
Findings
- The aircraft's flight progress was significantly delayed, which increased the total flight time beyond the original estimates.
- The increased fuel flow caused by the extended landing gear resulted in the consumption of the intended fuel margin.
- While the crew had departed with a fuel load well in excess of the minimum requirements, the drag from the gear meant the excess was depleted before arrival.
- The flight plan used for the mission had been prepared for a different aircraft in the fleet and did not account for the specific aerodynamic penalties of flying with the gear down.