What happened
On 31 March 2003, an Airbus A-320-231, registration G-MEDA, was conducting a scheduled flight from Egypt to Addis Abeba, Ethiopia. During the arrival phase, the crew attempted two separate approaches to Runway 25L using the local ADS VOR/DME navigation aids.
During the first approach, the crew encountered issues with the VOR signal and subsequently performed a go-around. On the second approach, while flying in instrument meteorological conditions, the aircraft crossed a ridge of high ground. The aircraft's radio altimeter provided a height callout, alerting the crew to the proximity of the terrain. Simultaneously, the Enhanced Ground Proximity Warning System (EGPWS) issued a 'TOO LOW TERRAIN' alert. The crew immediately initiated a go-around, having passed within only 56 feet of the terrain. The aircraft eventually diverted to land at Djibouti Airport.
The investigation
The investigation focused on why the aircraft's advanced navigation and terrain awareness systems failed to prevent the flight path from intersecting with high ground. Investigators examined the performance of the ADS VOR/DME and the aircraft's Flight Management System (FMS).
It was established that the ADS VOR antenna had experienced water ingress, which caused it to malfunction. This malfunction resulted in the transmission of incorrect navigational data. Because the aircraft's FMS, navigation displays, and EGPWS all relied on this single, erroneous source for position calculation, the error propagated through all primary situational awareness systems. The investigation also reviewed the maintenance records of the ground-based VOR equipment and the certification standards for the aircraft's terrain warning systems.