What happened
On 31 March 2003, an Airbus A320-231, registration G-MEDA, was performing an approach to Runway 25L at Addis Abeba Airport, Ethiopia. During the ADS2 VOR/DME approach procedure, the aircraft's safety systems failed to prevent a situation that placed the flight at risk of controlled flight into terrain (CFIT). While the onboard systems functioned according to their design specifications, they were unable to effectively alert the crew to the navigational discrepancy.
The investigation
The investigation established that the Addis Abeba VHF Omni-Directional Radio Range (VOR) beacon was transmitting incorrect bearing information. Although the aircraft's Flight Management System (FMS) and Terrain Awareness Warning System (TAWS) possessed sufficient data to identify that the ADS VOR was providing erroneous position information, there was no mechanism in place to communicate this specific error to the flight crew. The investigation highlighted that while modern TAWS capabilities have improved, there remains a lack of requirement to communicate data source inaccuracies to pilots, leaving aircraft equipped with non-GPS systems vulnerable to navigation errors.
Findings
- The primary cause of the incident was the incorrect bearing information being radiated by the ADS VOR ground station.
- The onboard safety systems, including the TAWS, were ineffective at preventing the incident because they lacked a method to alert the crew to the faulty data source.
- The safety of the approach was compromised because the aircraft's systems could not effectively communicate the discrepancy between the FMS-derived position and the actual ground station output.