On Saturday the 8th of August 2020 at 19:40:10 UTC, two flights flying in opposite directions, in the same corridor over India were vertically separated by just 1,000ft. The flights were SQ322 from Singapore to London at FL340 and CV7306 from Luxembourg to Singapore at FL330.

While this is allowed under ICAO Reduced Vertical Separation Minima, given the handful of airplanes in the sky due to the pandemic, is allowing just 1,000ft taking unnecessary risks?

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    $\begingroup$ This has been done all over the civilized world for years & years. What gives you reason to think that 1,000' of vertical separation is an "unnecessary risk?" How much separation would no longer be in that category? $\endgroup$ – Ralph J Aug 9 '20 at 1:30
  • $\begingroup$ There have been several recent incidents where airplanes came so close that pilots had to rely on TCAS to avoid a collision. Accidents never happen due to a single error. At the legal minimum the planes are potentially less than 20 seconds apart should something go amiss. Given that there were no other planes in the sky, it could be argued that choosing the legal minimum may increase the (albeit very low) risk of a collision. $\endgroup$ – alexth Aug 10 '20 at 7:50
  • $\begingroup$ @alexth, TCAS activations are somewhat common, but it is always because the controller made a mistake, the pilots made a mistake, or because the one aircraft is told to level off 1,000 ft below (climbing) or above (descending) another and the vertical speed is more than 1,000 ft/min, so TCAS triggers before they reach it, because it is time-based. I've never heard of one when the levels were assigned correctly, but the aircraft failed to maintain it. $\endgroup$ – Jan Hudec Aug 10 '20 at 8:57
  • $\begingroup$ You should define why you think this is an "unnecessary risk" because as this question is currently worded it feels sort of opinion based. Some might consider going in a pressurised tube into an unbreathable altitude an unnecessary risk. $\endgroup$ – Jamiec Aug 10 '20 at 10:02
  • $\begingroup$ @JanHudec, thanks. Both the 1996 Charkhi Dadri and 2002 Überlingen mid-air collisions are examples where the planned vertical separation of 1,000ft was potentially a contributing factor in that it left little room for error. Neither ATC nor the pilots had enough time to notice and correct their mistakes. $\endgroup$ – alexth Aug 10 '20 at 13:45

Under usual conditions aircraft pass directly above each other at 1,000 ft vertical separation thousands of times a day and it's never been a problem¹. The autoflight systems can maintain altitude to 100 ft reliably, so 1,000 ft is more than enough (hand-flown aircraft must be separated by 2,000 ft from FL290 (29,000 ft) up).

The risk does not become higher because there are fewer aircraft flying around, so there is no reason to change the procedures. Quite contrary, changing procedures would carry an increased risk of human error, so it would be counter-productive.

And in this case, procedure include the typically assigned altitudes for route and direction. Best put the planes to the flight levels the pilots are used to.

¹ All mid-air collisions (under instrument rules) occurred because some of the humans involved selected incorrect altitude, never because the plane would not be maintaining the altitude.


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