What happened
On 10 January 2006, a Boeing 747-412, registration 4X-ELS, was conducting a public transport flight approaching London Heathrow Airport. The aircraft was cleared for an ILS approach to Runway 2/7R. While the crew was established on the localiser at 4,000 ft, the autopilot captured the glideslope, initiating a descent.
As the aircraft progressed toward the runway, the glideslope indications began providing increasingly aggressive "fly down" commands. This caused the autopilot to pitch the aircraft's nose down, leading to a rapid descent rate of approximately 1,800 ft/min. The aircraft reached a minimum altitude of roughly 1,200 ft at a distance of 8 nm from the threshold. During this period, a glideslope failure indication appeared, and an EICAS caution message of "no autoland" was displayed.
The pilot in command disconnected the autopilot and manually climbed the aircraft back to 1,800 ft. Once the glideslope indications stabilized, the pilot re-engaged the autopilot to capture the glidepath. The aircraft subsequently completed a successful manual landing.
The investigation
An investigation by the AAIB examined the aircraft's central maintenance computer, which revealed a fault code relating to an ILS beam error. This specific error indicated that both the left and right ILS receivers had detected a loss of the external glideslope signal.
Engineers also reviewed ground equipment logs, which showed no malfunctions in the Heathrow navigation aids during the time of the incident. Furthermore, the investigation noted that the flight data and cockpit voice recorders had been overwritten by the time the investigation commenced, limiting the ability to analyze the exact flight dynamics.
Findings
- The primary cause of the descent was an error in the glideslope signal sensed by the aircraft's flight control computers after the autopilot had captured the path.
- The source of this signal error could not be definitively identified, as it was either an external signal interference or an unidentified internal aircraft fault.
- A communication breakdown between the flight crew and Air Traffic Control prevented the crew from being immediately informed of the glideslope's status, as the controller initially believed the signal was serviceable.
- The risk of a controlled flight into terrain was mitigated by the high visibility and the fact that the crew was able to regain manual control before the altitude became critical.