What happened
On 12 December 2004, a Boeing 747-2D7B, registration N523MC, was conducting a cargo flight from Chicago to London Stansted. The crew intended to perform a practice Category II autoland on Runway 05. During the approach, the aircraft was cleared to descend from 3,000 feet to 2,000 feet amsl. However, the aircraft entered a steep descent, averaging 1,570 feet per minute, which significantly exceeded the commander's intended vertical speed.
As the aircraft descended, the co-pilot observed 'flags' on the instruments, indicating that the localiser and glideslope signals were not being received. This technical anomaly caused all three crew members to focus on troubleshooting the equipment. During this period of distraction, the aircraft's descent continued unchecked until it broke through the cloud layer at approximately 900 feet amsl. Upon gaining visual contact with the ground and the PAPI lights, the commander disconnected the autopilots and manually leveled the aircraft to intercept the glideslope.
Despite the approach being unstabilised and the aircraft not yet being in the proper landing configuration, the commander decided to continue the approach because the unresolved instrument issue made him reluctant to climb back into the clouds. The aircraft eventually captured the glideslope at 3 nm from the threshold and landed uneventfully.
The investigation
The AAIB investigation examined radar recordings, ATC communications, and crew interviews. Radar data confirmed the aircraft's descent path deviated sharply from the normal glideslope. The investigation also reviewed the aircraft's technical logs and the performance of the Ground Proximity Warning System (GPWS), which had not activated because the commander's manual intervention pre-empted the warning.
Investigators also looked into the reporting procedures. While the co-pilot and flight engineer used confidential reporting systems, the commander did not report the incident to the company or the airport authority. The airport only became aware of the event following noise complaints and subsequent radar reviews. Furthermore, the investigation found that the radar controller had omitted the required task of monitoring the aircraft's altitude after it was transferred to the Tower controller.
Findings
- The primary cause of the incident was a serious breakdown in crew effectiveness, as all crew members became preoccupied with troubleshooting instrument flags, leaving the flight path unmonitored.
- The crew failed to maintain a stabilised approach, as the aircraft was not in the correct configuration and was descending at an excessive rate.
- The commander's decision to continue the approach was influenced by a reluctance to re-enter cloud cover due to the ongoing technical uncertainty.
- There was a failure in the reporting chain, as the incident was not formally reported to the airport or the operating company by the commander.