What happened
On 17 November 2003, a near-miss incident occurred in the Swiss Radar Area West involving two aircraft flying toward Munich. The first aircraft, a Fokker 100 (registration D-AGPF) operated by Germania Fluggesellschaft, was cruising at flight level 340. Simultaneously, a Boeing 737-8 (registration EC-HMK) operated by Futura International Airways for Gambia International Airlines was cruising at flight level 360.
At 18:49 UTC, a trainee radar controller in sector MS5/MA5 instructed the Boeing 737-8 to descend to flight level 300 with a descent rate of at least 2000 feet per minute. This instruction was incorrect, as the previously coordinated exit flight level for the descent was flight level 330. Although the pilot read back the incorrect level of 300, the error went uncorrected by the air traffic controllers.
By 18:51 UTC, the Short Term Conflict Alert (STCA) triggered a proximity alert. The Boeing 737-8 was descending through flight level 327, while the Fokker 100 was maintaining flight level 320. The aircraft passed with a minimum vertical separation of only 100 ft and a lateral distance of 4.2 NM. The controller immediately ordered the Boeing 737-8 to climb back to flight level 330.
The investigation
The investigation established that the trainee controller issued a clearance that deviated from the coordinated plan. While the controller recorded flight level 330 on the control strip, the verbal instruction was for flight level 300. The investigation also noted that the pilot's read-back of the descent rate was inaudible on the recordings.
Investigators found that the two aircraft were on converging headings, with the Boeing 737-8 traveling at a higher speed (435 kt) than the Fokker 100 (405 kt), which closed the gap between them. Although both aircraft were equipped with TCAS, the pilot of the Boeing 737-8 reported seeing traffic on the display, though no specific resolution advisory was issued.
Findings
- The primary cause was the issuance of an incorrect flight level clearance by the trainee radar controller.
- The error was not detected by the supervising coach or the sector coordinator.
- A contributing factor to the lack of error detection was that the coach and coordinator were not wearing headsets, which may have prevented them from overhearing the erroneous instruction.