Atlasjet A321 over Bulgaria on Sep 8th 2016, climb instead of descent results in near collision
Last Update: January 5, 2018 / 20:22:52 GMT/Zulu time
A THY Turkish Airlines Boeing 737-800, registration TC-JVS performing flight TK-1966 from London Gatwick,EN (UK) to Istanbul (Turkey), was enroute at FL370 about 140nm southeast of Sofia and about 130nm northwest of Istanbul at the same time, when Sofia Center cleared the flight to descend to FL330.
Bulgaria's AAIU reported that as result the separation between the aircraft eroded to 0 feet vertical and 1.2nm horizontal at FL363, Sofia Center issued vectors to both aircraft turning the A321 left and the B738 right thus establishing diverging flight trajectories and resolving the conflict. The A321 subsequently descended to the cleared FL310, the B738 to the cleared FL330, both aircraft continued to Istanbul for safe landings. The occurrence was rated a serious incident and is being investigated by the Bulgaria's AAIU.
On Jan 5th 2018 the AAIU released their final report concluding the probable causes of the serious incidents were:
Violation of the autopilot vertical speed selection process technology of A321-211 aircraft resulted in climbing of the aircraft instead of executing the clearance issued to KKK8JY for descent.
A state of Expectation Bias of EXE ATCO that led to issuing of clearance to THY4AV for descent during the time when the KKK8JY started to climb in contrary to the previously issued and confirmed by the crew clearance for descent and the presence of Mode S indication displayed on the ATCAS screen for selected FL 310 by the crew of KKK8JY.
The AAIU reported that following the instruction to descend to FL310 the crew of the Atlasjet A321 correctly read back the clearance, selected FL310 as new altitude target into the flight control unit (FCU), which the controller could also see on his radar screen via the Mode-S data downlink. The controller therefore developed an expectation bias and cleared the THY B738 to descend. However, the crew of the Atlasjet selected the FCU's vertical speed to +1500 fpm and the aircraft began to climb instead of descend.
The AAIU analysed:
In view of the above, it is concluded that the serious incident under investigation is most probably a result of the following dominant factors related to the third and fourth hypotheses:
1. Violation of the autopilot vertical speed selection process technology of A321-211 aircraft resulted in climbing of the aircraft instead of executing the clearance issued to KKK8JY for descent.
2. A state of Expectation Bias of EXE ATCO that led to issuing of clearance to THY4AV for descent during the time when the KKK8JY started to climb in contrary to the previously issued and confirmed by the crew clearance for descent and the presence of Mode S indication displayed on the ATCAS screen for selected FL 310 by the crew of KKK8JY.
This article is published under license from Avherald.com. © of text by Avherald.com.
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