Incident Overview
On 29 September 2008, a serious incident (AIRPROX) occurred in Geneva-controlled airspace near SOSAL, 20 NM west south-west of Fribourg. The aircraft involved were a Boeing B737-800, registration D-AHFO, operating as TUIfly flight HLX 2CX, and a Piper P46T, registration OO-NMU, operated by Vianatura. The incident resulted from a coordination with a control sector that did not correspond to the sector configuration at the time, leading to an aircraft being transferred to an inappropriate frequency and a subsequent dangerous convergence.
History of the Flights
Flight HLX 2CX was a scheduled IFR commercial flight from Fuerteventura (GCFV) to Basel-Mulhouse (LFSB). The Piper P46T was conducting a private IFR flight from Varese (LILN) to Antwerp (EBAW).
At 10:51:36 UTC, the Piper pilot contacted Geneva sector KL1, reporting a climb to FL240 and en route to waypoint PENDU, requesting FL260 for cruising. At 10:52:12 UTC, the B737 pilot contacted sector KL2, having just passed La Tour du Pin VOR and descending from FL310 to FL300. The sector KL2 radar executive (RE) was a controller undergoing requalification.
Coordination Error
At 10:59:18 UTC, sector KL2 cleared HLX 2CX to descend to FL260. The sector KL2 radar planner then initiated an electronic transfer of communication to sector INE instead of KL1. Consequently, at 10:59:42 UTC, HLX 2CX was transferred to the frequency of sector INI NE rather than KL1. The silent transfer procedure required a descending aircraft to be cleared to the floor level of the receiving sector; KL1's floor level was FL250, not FL260. This mismatch was a key issue.
Events Leading to Conflict
Upon first contact with sector INE, the pilot reported descending to FL260, but this did not catch the radar controller's attention. The controller performed an Assume of Control (AoC) manipulation, but the cleared flight level was not displayed in hook mode due to insufficient time. Radar coordinators were occupied with a telephone coordination and did not hear the reported level.
At 11:01:02 UTC, the INE controller cleared HLX 2CX to continue descent to FL210 with a rate of 2000 ft/min or more. At that time, HLX 2CX was at FL286, and the Piper was at FL250 on a perpendicular route, approximately 15 NM apart.
At 11:02:19 UTC, the Short-Term Conflict Alert (STCA) activated in sectors KL1 and INI NE. HLX 2CX was 4.3 NM from the Piper, passing FL262 in descent. The crew of HLX 2CX received a TCAS Traffic Advisory. At 11:02:23 UTC, with 3.8 NM lateral distance and 1000 ft altitude difference, the pilot queried traffic. The controller replied, "Affirm, turn left…immediately, this traffic is unknown to my sector at level two five zero."
At 11:02:37 UTC, HLX 2CX crossed the Piper's route at FL255, lateral distance 2.5 NM. Six seconds later, lateral distance was 2.3 NM with 300 ft altitude difference. Sector KL1, not in radio contact with HLX 2CX, inquired about the descent level.
Conclusion
The serious incident was caused by a coordination with a control sector that did not correspond to the sector configuration at the time, resulting in an aircraft being transferred to an inappropriate frequency and a dangerous convergence.