What happened
On 28 May 2004, a mid-air conflict occurred in French airspace near the TOSMI waypoint, approximately 88 NM southeast of Geneva. The incident involved an Alitalia Airbus A321, flight AZA 8TB, traveling from Lisbon to Milan, and a private Beechcraft B350, registration HB-GJN, flying from Granges to St-Tropez La Môle.
While the HB-GJN was cruising at flight level 250, the radar controller at Geneva ACC issued a series of rapid descent clearances to the AZA 8TB. Initially cleared to descend to flight level 240 at 2000 feet per minute, the pilot was instructed just one minute later to continue descending to flight level 230 at a much higher rate of at least 3000 feet per minute.
As the Airbus A321 descended through the flight level of the Beechcraft B350, its onboard Traffic Collision Avoidance System (TCAS) issued a "climb" resolution advisory. The pilot of AZA 8TB responded by climbing to flight level 260. At the closest point of approach, the two aircraft were separated by only 3.1 NM laterally and 400 ft vertically.
The investigation
The investigation examined the air traffic control environment and the specific instructions provided during the descent. Investigators found that the radar controller was managing a high workload due to heavy Friday afternoon traffic. The sector capacity had been increased to 40 movements per hour to prevent congestion, which contributed to the high volume of communications.
Technical analysis of the radar tracks confirmed that the rapid succession of descent instructions and the high rates of descent imposed on the Airbus A321 made the loss of separation inevitable. While the radar coordinator had coordinated a lower flight level with the adjacent sector, the specific instruction to descend at 3000 feet per minute placed the aircraft on a collision course with the HB-GJN, which was not equipped with TCAS and maintained its assigned altitude.
Findings
- The primary cause of the incident was the rapidly imposed rate of descent on the AZA 8TB, which led to an unavoidable loss of separation with the HB-GJN.
- The radar controller was operating under a high workload in a heavily congested sector.
- The HB-GJN was not equipped with TCAS, as regulations at the time did not require it for that aircraft type.
- The radar controller sought to resolve a potential conflict quickly by accelerating the descent of the A321, but the speed of the convergence exceeded the ability to maintain separation.