Two Boeing 777 aircraft — one belonging to El Al and another to Qatar Airways — came within a minute of collision at 35,000 feet over the Arabian Sea on March 24, in an area managed by Mumbai’s air traffic controllers. On this route, planes are expected to stay at least 10 minutes apart.
The incident came to light after a preliminary probe report was recently published by the Aircraft Accident Investigation Bureau, a division of the Union Ministry of Civil Aviation. It said that no cockpit alerts had been generated to warn the pilots of the two planes. According to sources, at least two of the air traffic controllers on duty during the incident have been suspended and sent for corrective training.
Dangerously close
According to the AAIB’s preliminary report, the breach of standard separation between the two aircraft took place on March 24 at 7:36 a.m. (Indian Standard Time). El Al’s flight ELY-81 was flying from Tel Aviv to Bangkok and the Qatar Airways flight QTR-8E was bound for Male from Doha.
Both the aircraft were on their designated flight route and flying at the same level, that is, at 35,000 feet. The breach of separation took place between the two aircraft at waypoint ‘GOLEM’, which is over Arabian Sea, and falls in the Mumbai flight information region.
The standard lateral distance on this oceanic route is 10 minutes. However, these two aircraft were dangerously close, with the minimum separation between them narrowing to just 9.1 nautical miles, which means that the two planes were just one minute apart, according to the report.
No cockpit alerts
It said that there was no cockpit alert generated to warn pilots of a collision threat by the Traffic Alert and Collision Avoidance System (TCAS), and neither was there any air traffic control (ATC) alert. The investigation is yet to ascertain whether this was because the two aircraft were not close enough to produce the TCAS alert, or if there was any other reason.
“The final report will also be published in due course,” AAIB chief G.V.G. Yugandhar told The Hindu in response to queries about what may have led to the safety breach.
Flight speed unknown
Former airline instructor pilot and aviation safety adviser Captain A. (Mohan) Ranganathan said that the TCAS did not generate traffic advisories or resolution advisories because the system would trigger an alert only if the two aircraft were within close proximity to each other with a threat of a collision. Such a scenario would occur when the separation ranged between 20 and 45 seconds, he said.
As both aircraft were cruising at 35,000 ft and without any traffic advisory, the pilots would have been aware of each other’s planes, with their positions noted on the cockpit primary flight display based on data from the Mode S transponder. Had visibility levels been good, each of the pilots could have seen the other aircraft. Captain Ranganathan said that, to understand the incident, it was important to know the speed at which each aircraft was travelling. This has not been mentioned in the report.
Procedure wise, by applying the 10-minute separation criteria, it was not a pilot error and one would have to look at the actions of the controllers handling the aircraft prior to ‘GOLEM’, he added.
No TCAS activation
Another aviation expert said that, typically, an aircraft would need to be 40 or 50 seconds away from the other for the TCAS to issue traffic advisories, and around 35 seconds from collision for a resolution advisory to be issued. In this incident, instead of there being a 10-minute separation, it was one minute, which would mean that the aircraft were nine or 10 miles apart and probably not on a collision course. This could also explain why the TCAS did not activate, the expert said.
A senior pilot who flies the Boeing 777 aircraft said that the TCAS is usually activated when there is an alert within a range of approximately six nautical miles.
(With inputs from Murali N. Krishnaswamy)
Published – September 28, 2024 07:56 pm IST
