NAC B744 near New Delhi on Dec 23rd 2018, climb without clearance causes loss of separation with two aircraft
Last Update: October 5, 2020 / 10:40:11 GMT/Zulu time
A KLM Boeing 777-300, registration PH-BVB performing flight KL-875 from Amsterdam (Netherlands) to Bangkok, was enroute at FL330 exactly above N919CA.
An Eva Air Boeing 777-300, registration B-16716 performing flight BR-61 from Bangkok (Thailand) to Vienna (Austria), was enroute at FL320 on the same airway in opposite direction.
When the 747 began the climb an alert activated at the controller's desk, who in response immediately turned the 747 to their left off the airway and as a precaution turned the Eva Air to their left, too. TCAS advisories occurred between the 747 and the two other 777s, the 747 reached a maximum FL330 then began to descend again already turned to their left and was subsequently cleared to climb to FL350.
All three aircraft continued to their destinations for safe landings.
According to ADS-B position data the separation between N8-840 and KL-875 reduced to 0 feet vertical and 2.2nm horizontal at 05:04:43L (23:34:43Z Dec 22nd).
India's DGCA released their final report dated Jan 18th 2020 concluding the probable cause of the serious incident was:
The probable cause of incident was incorrect use of plain language while passing information to aircraft, which led to "Confirmation Bias" and incorrect understanding of the ATC advisory.
Lack of Situational Awareness amongst NCR840 crew was a contributary factor.
The DGCA analysed:
NCR840 flying on route M890, established VHF contact with ACC (East) on 120.9MHz Frequency at 23:27:35 UTC, and was maintaining FL310. KLM875 which was maintaining FL 330 on the same route reported to ACC (East) at 23:28:20 UTC. EVA061 was reciprocal traffic on the route maintaining FL 320.
At 23:32:40 UTC NCR840 requested for climb to FL390 while EVA061 was 40 NM from it and KLM875 was just above at FL330. Controller asked NCR840 to "STANDBY". However, at 23:32:56 UTC controller informed NCR840 "..., LEVEL A VAILABLE THREE FIVE ZERO'. This information was interpreted by NCR840 as clearance for climb to FL 350. NCR840 acknowledged, and read back by saying "ROGER THREE FIVE ZERO FOR NCR...". Thereafter controller instructed "STANDBY FOR HIGHER"
The information about availability of FL350 without asking for intention of NCR840 or a clear call for action left a gap in communication with a chance for misinterpretation. The standard phrase for giving clearance for climb as per MATS would be "CLIMB TO (Level)". Controller did not give clearance using standard phraseology, instead "... LEVEL AVAILABLE THREE FIVE ZERO" in plain language was used to provide information to NCR840, which it had not requested. Due to "Confirmation Bias" NCR840 interpreted this information as availability of FL350 for it and permission to climb. The read-back (In Plain Language) by NCR840, "ROGER THREE FIVE ZERO FOR NCR..." clearly indicates that NCR840 had misunderstood the advice. The controller was required to ascertain that the clearance or instruction has been correctly acknowledged by listening to the read-back. The controller did not correct NCR840 and rather gave instruction "STANDBY FOR HIGHER", which further strengthened the misunderstanding as it was understood by NCR840 to be "STANDBY" instruction for FL390.
The B747-400 aircraft (NCR840) is equipped with TCAS display on the Navigation Display. The TCAS display would have indicated the traffic just 2000 feet above the aircraft at FL330 (KLM875). The crew should have been aware of the proximity of the traffic by monitoring their Navigation Display. As NCR840 started climbing, Predicted Conflict Warning between EVA061 and NCR840 was triggered at 23:33:28 UTC just as NCR840 climbed to FL311. Controller instructed NCR840 to turn left, heading 090°.
The PIC of NCR840 who had swapped seat with Co-Pilot 2, and was seated on observer seat, had meanwhile disconnected the auto throttle and autopilot, while simultaneously reducing thrust and initiating a left descending turn. While the actions of the PIC of NCR 840 may have increased separation between the aircraft, the inability of the crew on seat to carry out the desired actions are not understood. The control intervention from the observer seat and the reported wake turbulence encountered have the potential to result in an unacceptable flight path.
Controller also instructed EVA061 to turn to its left, heading 0270°, but same was not acknowledged by EVA061. At 233339 UTC PCW between KLM840 and NCR840 was also triggered. NCR840 continued to climb and at 23:33:50 UTC, a CCW was triggered between NCR840 and KLM875 while NCR840 was at FL319.
This article is published under license from Avherald.com. © of text by Avherald.com.
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