Introduction
On 12 August 2018, a serious incident involving two Boeing 737-800 aircraft occurred in the controlled airspace of the Republic of Bulgaria. The aircraft, with registrations D-ASXP (callsign SXD7BY) operated by SunExpress Deutschland GmbH and OM-GTF (callsign TVQ6129) operated by Go2Sky, spol. s r. o., were under the control of the Sofia Area Control Centre (ACC).
Flight History
At 06:29:48 UTC, TVQ6129, performing a charter flight from Antalya (LTAI) to Bratislava (LZIB), reported to the Sofia ACC East Sector air traffic controller (ATCO) that it was at flight level (FL) 240 and did not wish to change its level. At 06:38:18, SXD7BY, operating a charter flight from Burgas (LBBG) to Frankfurt (EDDF), reported climbing to FL200 and requested a climb to FL380. The ATCO initially cleared SXD7BY to climb to FL220.
At 06:41:48 UTC, the ATCO instructed SXD7BY, then at FL220, to turn left to a heading of 270°. Instead of executing this instruction, the flight crew of SXD7BY initiated a climb to FL270. This resulted in a reduction of horizontal separation between SXD7BY and TVQ6129, which was flying at FL240, to 2.5 nautical miles at the closest point of approach.
Personnel Information
The pilot-in-command and first officer of SXD7BY held valid licenses and medical certificates. The executive (EXE) and planning (PLN) ATCOs on duty at the Sofia ACC were also appropriately qualified.
Aircraft Information
Both aircraft were Boeing 737-800 models. SXD7BY (D-ASXP) was manufactured by Boeing with serial number not specified. TVQ6129 (OM-GTF) was also a Boeing 737-800. Both were equipped with Traffic Alert and Collision Avoidance System (TCAS).
Analysis
The investigation determined that the flight crew of SXD7BY misunderstood the ATC clearance. The instruction to turn left onto heading 270° was interpreted as a clearance to climb to FL270. Furthermore, the EXE and PLN ATCOs did not detect the discrepancy between the read-back provided by the crew and the issued instruction. Additionally, the ATCOs failed to notice the deviation message in the track label of SXD7BY indicating a difference between the selected altitude downlinked via Mode S and the cleared flight level.
Conclusion
The Commission for Investigation concluded that the serious incident was caused by the misunderstanding of the ATC clearance by the SXD7BY flight crew, the failure of the ATCOs to identify the read-back error, and their failure to detect the altitude deviation indication in the radar label.
Safety Recommendations
No specific safety recommendations were issued in this report.