What happened
On 7 April 2004, a US Navy McDonnell Douglas DC9-30, operating flight JM 737, was transiting Swiss airspace from Ramstein to Sigonella. Simultaneously, a Portugalia Airlines Embraer E145, registration CS-TPL, was flying from Lisbon to Basel-Mulhouse.
At approximately 13:32 UTC, air traffic controllers in the Geneva sector identified that the CS-TPL was at flight level FL 340, which violated a local agreement requiring aircraft bound for Basel-Mulhouse to be at or below FL 300 before entering the Geneva control area. To resolve the potential conflict with the US Navy aircraft, the Geneva controller requested that the Marseille controller instruct the Embraer to descend immediately.
However, the US Navy aircraft was simultaneously being cleared to climb to FL 350. The subsequent instructions led to both aircraft on converging trajectories. The Marseille controller instructed the Embra 145 to descend, but did not enforce the requested high rate of descent. As the aircraft approached the crossing point, the Geneva radar Short Term Conflict Alert (STCA) was triggered. The aircraft eventually crossed paths with a lateral distance of 2 NM and a vertical separation of only 900 feet. The pilot of the CS-TPL noted that they had TCAS information during the encounter.
The investigation
The investigation examined the communications between the Geneva and Marseille radar sectors, as well as the flight management of both crews. Investigators analyzed radar recordings which showed that the vertical trajectories of the two aircraft were divergent at the point of crossing, but the separation was significantly reduced during the approach.
The investigation also looked into the workload of the Geneva sector L3/K3 controllers. It was noted that the radar controller had been working an extended shift due to personnel shortages and was performing duties in a high-traffic environment. The investigation also reviewed the instructions given to the US Navy crew, noting that a request for a specific rate of climb was made using an inappropriate phrasing that the pilot did not explicitly read back.
Findings
- The primary cause of the incident was the simultaneous change in flight levels for both aircraft on converging routes, which was managed across different control sectors and frequencies.
- The Geneva controller's strategy to resolve the conflict involved requesting a rapid descent for the Embraer, but the implementation of this descent was delayed and lacked the necessary rate of descent.
- The US Navy pilot was instructed to climb at a specific rate, but the phrasing of the instruction was not optimal for ensuring the rate was understood and read back.
- High workload and personnel shortages in the Geneva sector contributed to the difficulty in managing the complex traffic situation.
- The Marseille controller failed to impose the specific 2500 ft/min descent rate requested by the Geneva coordinator, leading to a slower descent by the CS-TPL.