|Title:||Descent beneath glide slope, Boeing 747-412, 4X-ELS, January 10, 2006|
|Micro summary:||This Boeing 747 inexplicably descended far beneath the glide slope on approach.|
|Event Time:||2006-01-10 at 1200 UTC|
|Publishing Agency:||Aircraft Accident Investigation Board (AAIB)|
|Publishing Country:||United Kingdom|
|Site of event:||10 miles east of London Heathrow Airport|
|Destination:||London Heathrow Airport, London, England, United Kingdom|
|Airplane Type(s):||Boeing 747-412|
|Type of flight:||Revenue|
|Executive Summary:||During an ILS approach to Runway 27R with the autopilot engaged, the aircraft descended to 1,200 ft altitude at about 8 nm from the runway threshold. The flight crew recovered the aircraft to the ILS glidepath manually and landed normally. Investigations revealed no fault, either on the aircraft or in the ground equipment, to explain the incident.|
The available evidence suggested that an error in the glideslope signal arriving at the aircraft was sensed by both FCCs after the autopilot captured the glidepath. However, monitoring equipment on the ground showed no fault and no cause could be found for the error recorded on board the aircraft.
The AAIB is not aware of any similar incidents immediately before or after this event. Consequently, based on the available evidence, the problem was either external to the aircraft but experienced only by 4X?ELS, or an unidentified internal fault within the aircraft. However, the lack of recorded flight data and the inability to evaluate the aircraft soon after the incident rendered further investigation impracticable.
In this incident, the risk was minimal because visibility below the 1,500 ft cloud base would have permitted the flight crew to gain visual contact with terrain in good time to avoid any Controlled Flight Into Terrain (CFIT) hazard. Had the cloud base been lower, the aircraft’s GPWS should also have provided a timely warning of proximity to the ground.
|Learning Keywords:||Operations - Airspace - Air Traffic Control|
|Operations - Airspace - GPWS|
|Operations - Altitude Excursion|
|Operations - Navigation error|
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