What happened
On April 28, 2000, at 20:14 UTC, a potential collision occurred in Swiss airspace near the DITON intersection. Two aircraft, a Boeing B757 (registration G-MONK) operated by Monarch Airlines and an Airbus A3FR (registration OY-CNB) operated by Premiair, were both cruising at flight level 350.
The aircraft were on intersecting flight paths: the G-MONK was traveling from the direction of SUXAN toward France, while the OY-CNB was traveling from Geneva toward Germany. Initially, the radar controller implemented a minor course change for the OY-CNB to mitigate a perceived conflict. However, as the aircraft approached the crossing point, a Short Term Conflict Alert (STCA) was triggered on the radar monitor.
In an attempt to resolve the situation, the controller issued a series of rapid-fire instructions, including a right turn for the G-MONK and a climb to flight level 370. These maneuvers failed to resolve the conflict and, in the case of the turn, actually worsened the situation. The situation was only resolved when the crew of the G-MONK initiated a rapid descent to flight level 340 following a TCAS Resolution Advisory (RA). The two aircraft eventually crossed paths with a vertical separation of approximately 900 feet.
The investigation
SUST examined the radar data, controller communications, and the flight performance of both aircraft. The investigation focused on the sequence of instructions issued by the radar executive in the U2/U3 sector. Investigators reviewed the controller's initial assessment of the lateral separation, which was predicted to be between 7 and 10 nautical miles, and compared this against the actual STCA alarm that occurred roughly six minutes later.
Furthermore, the investigation analyzed the effectiveness of the controller's instructions, specifically the command for the G-MONK to climb to flight level 370. Data from the aircraft's Mode C transponder indicated that the climb had not yet significantly altered the aircraft's altitude before the TCAS-initiated descent took place.
Findings
- The radar controller misjudged the conflict potential of the two aircraft crossing at the same flight level.
- The controller's initial preventive measure—a simple course change for the OY-CNB—was insufficient to ensure safe separation.
- The instructions issued during the final moments of the conflict were inappropriate and ineffective.
- The command for the G-MONK to climb was particularly problematic, as the climb performance of a large jet at that altitude is limited, and the maneuver did not provide the necessary separation.
- The collision was ultimately averted by the TCAS-descent maneuver performed by the crew of the G-MONK.