What happened
On 11 March 2005, an Airbus A321-231, registration G-MEDG, operated by British Mediterranean Airways, was performing a night approach to Runway 36 at Khartoum Airport, Sudan. The approach was conducted in weather conditions consistent with a dust storm. The crew was utilizing a Managed Non-Precision Approach (MNPA) using the aircraft's autopilot and Flight Management and Guidance System (FMGC).
During the descent, the handling pilot changed the autopilot mode to a selected descent rate, believing the aircraft was too high. This change resulted in an unstable approach, with the aircraft descending at an excessive rate through 1,000 ft agl. As the aircraft reached its Minimum Descent Altitude, both pilots mistakenly believed the other had visual contact with the runway approach lights.
Upon realizing the lack of visual references, the pilot initiated a go-around at approximately 180 ft agl. However, the aircraft continued to descend to a radio altitude of roughly 125 ft agl, triggering an Enhanced Ground Proximity Warning System (EGPWS) "terrain ahead, pull up" alert. The aircraft's minimum terrain clearance during the recovery maneuver was recorded at 121 ft.
The investigation
The AAIB investigation examined the aircraft's Flight Recorders and the FMGC navigation database. The investigation identified a critical discrepancy between the crew's approach charts and the aircraft's navigation database. While the database contained a recent update from the Sudanese authorities placing the final descent point at 4.4 nm from the threshold, the crew's charts still showed the point at 5 nm.
Furthermore, the investigation looked into the operational procedures of the operator and the regulatory framework provided by the UK CAA regarding MNPA operations. The investigation also assessed the performance of the EGPWS and its ability to provide timely warnings during the late stages of an approach when GPS data integration might be limited.
Findings
- The crew was unaware of a significant mismatch between the approach chart parameters and the navigation database.
- A discrepancy in the descent point location led to the aircraft beginning its descent 0.6 nm later than the pilots anticipated.
- Confusion between the two pilots regarding visual contact with the runway lights contributed to the unstable approach.
- The approach became unstable after the handling pilot manually selected a descent mode to correct a perceived high profile.
- The operator's training and operational procedures for MNPA were found to be incomplete and inconsistent.
- The EGPWS alert occurred late in the maneuver, and the investigation noted that the system's ability to provide sufficient warning was constrained by the lack of a direct GPS data feed to the EGPWS.