What happened
On November 17, 1999, at approximately 14:30 UTC, a separation incident occurred near the boundary of the Geneva CTR and TMA, specifically 2 NM northwest of the PAS VOR/DME. The incident involved a Fokker 50 (VLM506) operating a commercial flight from Geneva to Antwerp and a Falcon 900 (registration PT-OEX) operating a non-commercial flight from Geneva to Zurich.
Following the departure of the Fokker 50, the Falcon 90 and PT-OEX took off two minutes later. During the departure sequence, the Geneva Tower controller instructed the pilot of PT-OEX to "start right turn." The pilot responded with "stop right turn." The controller did not identify this incorrect read-back and subsequently transferred the aircraft to Geneva Departure frequency.
As both aircraft climbed, the separation between them rapidly decreased. The Fokker 50 was climbing through 6,800 feet when the Geneva Departure controller identified the conflict, noting the Falcon 900 was passing at flight level 85. The controller issued an immediate corrective instruction to the Falcon 900, ordering a heading change to 020. Radar records indicated that the minimum separation between the two aircraft dropped to approximately 0.7 NM and 300 feet.
The investigation
The investigation focused on the radio communications between the Tower and Departure controllers, as well as the radar data. Investigators examined the transcript of the telephony recordings, which revealed the specific wording used during the instruction and the subsequent read-back. The investigation also analyzed the radar tracks, which showed a discrepancy between the perceived aircraft path and the actual flight path.
Findings
- The primary cause of the incident was the use of improper phraseology by the controller, specifically the phrase "start right turn," which is not part of standard aviation terminology.
- The incident was compounded by a faulty read-back from the pilot of PT-OEX ("stop right turn") that went uncorrected by the controller.
- Radar data suggested a potential issue with radar precision, as the tracks gave the impression that the Falcon 900 was turning left when it was actually turning right, which may have misled the controller.
- Neither the pilot of the Fokker 50 nor the pilot of PT-OEX chose to file a formal airprox report following the event.
Safety action
To prevent future misunderstandings, Swisscontrol committed to instructing air traffic controllers to use only official standard phraseology, specifically using "turn XXX now" instead of "start XXX turn now."