Incident Overview
On 28 April 2000 at 2014 UTC, an air proximity event (airprox) occurred near the DITON intersection in Swiss airspace. The two aircraft involved were MON6481, a Boeing 757 operated by Monarch Airlines with registration G-MONK, and VKG640, an Airbus A320 operated by Premiair (Viking) with registration OY-CNB. Both aircraft were in cruising flight at flight level 350 (FL 350) under instrument flight rules (IFR) in controlled airspace class C. Traffic volume was moderate at the time.
Sequence of Events
MON6481 was en route from SUXAN via DITON and HOC toward France, while VKG640 was traveling from Geneva via KORDES and KUDES toward Germany. The radar executive (RE) controller for sector U2/U3, upon reviewing flight progress strips, identified a potential conflict due to both aircraft planned to arrive almost simultaneously at the crossing point. As a preventive measure, she instructed VKG640 on first contact to navigate directly to TGO, which intended to give the aircraft a slightly westerly course and, in her assessment, reduce conflict risk.
Shortly thereafter, MON6481 made first contact on the same frequency. After reassessing the situation, the controller concluded that lateral separation at the crossing point would be 7 to 10 nautical miles (NM), sufficient to forgo further measures.
Approximately six minutes later, the controller observed an optical short-term conflict alert (STCA) on the radar display for both aircraft. Recognizing that the expected lateral separation would not be achieved, she ordered MON6481 to turn right by 15 degrees. This maneuver was immediately identified by the radar planner as counterproductive and actually worsened the conflict. She then instructed VKG640 to turn left to a heading of 350 degrees and issued traffic information about MON6481. Realizing that further heading changes could not resolve the situation, she ordered MON6481 to climb to FL 370 and issued traffic information about VKG640.
Twenty seconds later, MON6481 reported a TCAS descent (Traffic Collision Avoidance System resolution advisory) and initiated a rapid descent to FL 340. About 40 seconds after that, the two aircraft crossed within half a nautical mile laterally and with a minimum vertical separation of 900 feet.
Findings
- Both aircraft were in controlled airspace class C and operating under IFR with continuous radio contact with ATC.
- The conflict potential was clearly indicated on the flight progress strips.
- On first contact, the controller directed VKG640 to proceed direct to TGO.
- Approximately six minutes after both aircraft were in contact, an STCA alert appeared when lateral separation was 19.5 NM and their routes converged.
- About 10 seconds after the STCA alert, the controller ordered MON6481 a 15-degree right turn.
- About 30 seconds later, she ordered VKG640 a left turn to heading 350 and issued traffic information; separation had reduced to 11 NM.
- About 10 seconds after that, she ordered MON6481 to climb to FL 370 and issued traffic information; separation was then 7 NM.
- At 5.8 NM lateral distance, MON6481 reported vague visual contact with the other aircraft.
- Seconds later, at request, VKG640 also reported visual contact.
- At approximately 4.5 NM lateral distance, MON6481 initiated a TCAS descent.
- About 40 seconds later, the aircraft crossed with 900 feet vertical separation.
Cause
The incident resulted from the radar controller of sector U2/U3 misjudging the conflict potential of two aircraft crossing at the same flight level and taking inappropriate last-minute measures to avoid a possible collision.