A Sun Express Germany Boeing 737-800, registration D-ASXP performing flight XG-3208 from Burgas (Bulgaria) to Frankfurt/Main (Germany), was climbing out of Burgas and had been cleared to climb to and maintain FL220 while tracking ATS one way route T226 between waypoints ETUBA and NAVOD expecting to climb to FL380.
A Go2Sky Boeing 737-800 on behalf of Smartwings Slovakia, registration OM-GTF performing flight 6D-6129 from Antalya (Turkey) to Bratislava (Slovakia), was enroute at FL240 between waypoints ETUBA and NAVOD tracking ATS route T226 and did not want to change their flightlevel.
In preparation for the climb ATC instructed XG-3208 to turn left onto a heading 270. The crew however read back "Climb FL270", which was not picked up by ATC. The crew selected FL270 into their Selected Altitude, which was duely transmitted by Mode-S to the controller's desk, where a disagree message appeared between cleared and assigned altitude. The aircraft began to climb, about 15 seconds later the radar issued a short term conflict alert (STCA) between XG-3208 and 6D-6129 while XG-3208 was climbing through FL223.
In reaction to the STCA ATC instructed XG-3208 to descend back to FL220 immediately, 4 seconds later the crew read that instruction back. In the meantime the aircraft already climbed through FL236 at 2900 fpm. The separation reduced to 0 feet vertically and 2.6nm horizontally and the aircraft continued to climb reaching FL250 re-establishing vertical separation. ATC thus instructed XG-3208 to continue the climb to FL260.
6D-6129 complained: "We have a traffic very close to us, now 5 miles, I suppose. This was well below the minimum". ATC confirmed the minimum horizontal distance had been 2.5nm.
Both aircraft continued to their destinations without further incident.
On Sep 8th 2020 Bulgaria's AAIU released their final report concluding the probable causes of the serious incident were (EXE: executive radar controller, PLN: planning air traffic controller):
- Misunderstanding by the flight crew of SXD7BY of the ATC clearance issued by EXE air traffic controller, that lead to climb to FL 270 instead to turn left onto heading 270
- EXE and PLN air traffic controller did not detect the discrepancy between the received readback and the issued instructions.
- EXE and PLN air traffic controller did not detect and interpret the deviation message in the track label of SXD7BY for the difference between the Selected Altitude downlinked Mode S parameter and the cleared flight level.
The AAIU reported there had been no imminent risk of a collision, neither crew received a TCAS traffic or resolution advisory.
The AAIU analysed:
The flight crew (XG-3208) reported that they are flying at FL200 and willing to climb. Initially, EXE ATCO issued climb instruction to FL220. At this point, if the EXE ATCO was issued the instruction for climbing to a higher FL (FL380 in accordance with the flight plan), a potential conflict with TVQ6129 will occur, flying on FL240 in parallel heading in a horizontal distance of less than 5 NM.
The EXE ATCO, after agreeing with the PLN ATCO, changed the plan to avoid potential conflict by issuing an instruction for turning left to heading 270 SXD7BY turn left heading 270. The flight crew wrongly repeated the instruction Climb FL270, SXD7BY. The EXE ATCO and PLN ATCO did not react to the wrongly repeated instruction by the flight crew, after that the aircraft started a climb.
Immediately after that, SATCAS visualized a deviation massage in the track label of SXD7BY for an existing difference between the CFL introduced by the ATCO and the SAL by flight crew, this was not noticed and for this reason, there was no reaction by EXE ATCO and PLN ATCO. After 21s, SATCAS generated a STCA between SXD7BY and TVQ6129. The EXE ATCO reacted to the alert and issued instructions to SXD7BY to avoid the conflict and avoid the separation infringement. It should be noted that ATCO had twice instructed the crew of SXD7BY to descent immediately to FL220, and once to fly on heading 270 for preserving the separation and the crew correctly replied but did not performed the instructions at all. The aircraft continued to climb, crossing FL236 with a vertical speed of 2900 ft/min and reached FL250. In view of the provided vertical separation between the two aircraft of 1000ft, the EXE ATCO correctly adjusted the instruction to SXD7BY to continue a climb to FL260.
In accordance with the third aspect of the possible inefficient implementation of procedures by ATCO when conducting radiotelephone communication, it could be concluded that the EXE ATCO and PLN ATCO did not correct the flight crew of SXD7BY for the wrongly understood instruction and did not detect and interpret the deviation massage in the track label of SXD7BY, thus creating conditions for a dangerous convergence.