DAVID LEARMOUNT / LONDON
Authority reacts to incident in which British Mediterranean Airways flight to Addis Ababa avoided ridge by 50ft
After a serious incident in which a British Mediterranean Airways Airbus A320 missed a mountain ridge by just over 50ft (15m) descending into Addis Ababa, Ethiopia, the UK Civil Aviation Authority has warned airlines that aircraft should be equipped with satellite navigation as well as terrestrial aids if they fly remote or oceanic routes to isolated destinations.
Addis Ababa was to be the British Airways franchised flight's final destination on a scheduled service via Alexandria, Egypt, but the crew abandoned two poor-visibility approaches because of erroneous signals from the airport's VOR/distance measuring equipment (DME). The A320 diverted to Djibouti.
The UK Air Accidents Investigation Branch (AAIB) says during the aircraft's first approach to runway 25L at Addis Ababa's Bole International airport, the crew noticed a VOR/DME indication fluctuation, despite strong identification and air traffic control's assurance, after the go-around, that it was serviceable.
The aircraft was not fitted with GPS satellite navigation and there was a thunderstorm nearby so the crew avoided using the Bolenon-directional beacon, which left no secondary check for VOR/DME accuracy. The crew reported that, on returning for a visual approach the following day, the VOR showed a radial error of 22° outbound and 30° inbound.
On the second approach the aircraft received a 400ft radio altimeter callout followed by a "too low terrain" alert from the enhanced ground proximity warning system (EGPWS). The AAIB believes that, because the faulty VOR/DME was providing the only position input to the aircraft's flight management guidance system, the aircraft's position and the EGPWS display on the navigation display were wrong. The aircraft was actually 4.8km (2.6nm) right of track, believes the AAIB.
When a GPS-equipped A320 flew a good weather VOR/DME approach to Bole on 10 April, the crew reported that if they had followed the signal the aircraft would have been "well to the north [of track] and very close to high ground". The CAA has recommended that aircraft be fitted with GPS for "routes involving long sectors both over water and terrain that terminate in remote areas served with few navigation aids".
Source: Flight International
