Lufthansa B748 and British Airways B744 near Glasgow on Jun 23rd 2013, loss of separation
Last Update: October 22, 2013 / 16:08:55 GMT/Zulu time
A British Airways Boeing 747-400, registration G-BNLM performing flight BA-87 from London Heathrow,EN (UK) to Vancouver,BC (Canada), was enroute at FL340 nearing Glasgow,SC (UK) roughly tracking 330 degrees.
About three minutes prior to the occurrence the Scottish Center Montrose Sector controller cleared LH-418 to climb to FL340. The aircraft climbed to FL340 reaching FL340 when both aircraft were about 24.3nm horizontally apart but closing. The controller noticed the impending conflict about 3 minutes later, at 12:55:22Z the low priority short term conflict alert activated at his control desk a few seconds later when the aircraft were 9.8nm horizontally apart. The controller wanted to issue instructions to ensure separation however was unable due to some other aircraft talking on the frequency. When able to the controller instructed BA-87 to turn to the left onto a heading of 270 degrees stating "avoiding action, Speedbird eight seven, turn left immediately heading two seven zero degrees, traffic in your right one o'clock", which was correctly read back by the crew including using the full callsign, then LH-418 was instructed to turn right onto a heading of 050 (instruction "Lufthansa four one eight, avoiding action, turn right immediately heading zero five zero degrees, traffic in your left eleven o'clock", which also was read back correctly using the flight number only.
40 seconds later both aircraft were showing a track of 340 degrees at same altitude, horizontal separation 6.6nm.
The controller radioed "Speedbird eight seven, avoiding action, turn left immediately two six zero degrees, traffic in your right one O'clock", the pilot acknowledged 260 degrees and traffic in sight.
Another 7 seconds later the high priority short term conflict alert activated as separation had been lost, the aircraft were now 4.9 nm horizontally apart at same flight level, BA-87 seemed to have turned slightly right.
An additional controller arrived at the controller's desk to provide assistance and suggested to issue vertical separation instructions, which the controller so far had been reluctant to issue in accordance with his training in order to not interfere with TCAS instructions. Following the recommendation of the collegue he now issued "Speedbird 87, descend now immediately", immediately after LH-418 reported they were following a TCAS climb, at that time the aircraft were separated 300 feet vertical and 3.5nm horizontally.
Required separation was restored at 12:56:37Z when BA-87 descended through FL332 and LH-418 climbed through FL344 at 2.8nm horizontal distance remaining.
BA-87 was subsequently instructed to maintain FL330 while LH-418 returned to FL340. Both flights continued to their destinations for safe landings.
The British AirProximity Board released their final report concluding the probable cause of the loss of separation was:
The pilots of aircraft on converging tracks flew into conflict because, although they acknowledged timely avoiding action, they did not follow it.
The British AirProximity Board annotated that the tracks of both aircraft did not appear to have changed significantly until being clear of conflict again. In post occurrence interviews the crew of BA-87 recollected that they were given an immediate right hand turn to 050 degrees to avoid other traffic, while the crew of LH-418 recollected they were given an immediate left hand turn to 270 degrees.
The Board analysed that the controller issued the climb instruction to LH-418 at a point, where there was little distance to go until conflict. However, the controller recognized the pending conflict in time and issued instructions in time that were suitable to maintain required separation. Simulation showed that had the instructions been followed the separation would not have been lost.
The Board continued analysis: "It was apparent that both crews had taken each othersâ€™ instructions, and the Board found it hard to determine why this had occurred; unfortunately no Human Factor report was available from either crew. The Board was surprised that all four pilots had misheard or misinterpreted the avoiding action instructions despite at least one of the crews reading them back correctly." A callsign confusion could be ruled out from ATC recordings, all callsigns in instructions and readbacks were clear.
The Board further analysed: "It was possible that the crews may have been distracted because this would have been about the time that they would have been receiving their Oceanic clearances on data-link. Another possibility mooted by an airline-pilot Member was that, having settled into their trans-Atlantic routine, it was unusual for pilots to be issued with avoiding action instructions at that altitude and location. Expecting only routine information to be transmitted at that time, they may have been perplexed by the avoiding action information and instinctively responded without properly assimilating it. He also noted that, during simulation training, avoiding action was only practiced as a result of a TCAS alerts, and not as a result of ATC instructions; this was an important consideration as to their potential familiarity with receiving, assimilating and actioning such RT instructions in a timely manner."
The Board annotated that they were "disappointed" that BA-87 had not reported their TCAS resolution descent.
This article is published under license from Avherald.com. © of text by Avherald.com.
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