What happened
On 29 September 2008, a Boeing 737-800, registration D-AHFO, operating as TUIfly flight HLX 2CX, was descending from flight level FL 310 toward Basel-Mulhouse. Simultaneously, a Piper P46T, registration OO-NMU, was climbing through flight level FL 240 on a private flight toward Antwerp.
During the descent of the Boeing aircraft, the air traffic controller in sector KL2 cleared the crew to descend to flight level FL 260 to maintain separation from the climbing Piper. To facilitate a requested route change, the controller initiated an electronic transfer of the Boeing aircraft to the frequency of the INE sector. However, this transfer was executed without proper coordination with the intermediate sector (KL1), effectively bypassing the standard procedure.
As the Boeing aircraft descended, it was cleared to continue its descent to FL 2 and even FL 210. Because the controller in the receiving INE sector was not aware of the specific flight level clearance, a dangerous convergence occurred. The aircraft came within approximately 2.3 NM of the Piper aircraft, with an altitude difference of only 300 feet, triggering a Short-Term Conflict Alert (STCA) and a TCAS Traffic Advisory for the Boeing crew.
The investigation
The investigation established that the air traffic control coordination did not match the actual sector configuration at the time of the incident. While the controller in sector KL1 had proposed flight level FL 260 to avoid the Piper, the transfer of the Boeing aircraft to the INE sector frequency was performed via a method that did not align with the standard "silent transfer" protocols.
Investigators found that the radar display information was inconsistent between the different control sectors. Specifically, the cleared flight level was visible in "hook" mode on some screens but was suppressed during certain manual manipulations on others. Furthermore, the radar coordinators were occupied with telephone coordination with Zurich at the moment the pilot reported his descending flight level, causing them to miss the critical altitude information.
Findings
- The primary cause was a non-standard inter-sector coordination where the aircraft was transferred to a new frequency without ensuring the receiving sector was aware of the specific cleared flight level.
- The transfer procedure used bypassed the intermediate sector (KL1) in a way that violated the established floor level agreements.
- Discrepancies in how radar labels displayed cleared flight levels between UAC and TCG sectors contributed to the controller's lack of situational awareness.
- The automated system failed to trigger an alert for the non-standard electronic coordination between sectors.
Safety action
- The Federal Office of Civil Aviation must ensure that working methods and data presentation on radar screens are standardized across both UAC and TCG sectors.
- The Federal Office of Civil Aviation must require the implementation of automated alerts for instances of non-standard inter-sector electronic coordination.