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This morning I read that mulitple people got (light) physical injuries after a passenger flight had to forcefully drop its altitude quickly in order to avoid collision with a fighter jet. The news article stated that it was flying at a "safe" distance of 1,000 metres for a visual inspection, and that there was no danger at all. Although some other sources state that the distance was only down to a hundred meters

To me (someone who knows next to nothing about flying) a distance of one kilometer sounds pretty close for two planes to fly at (seeing the speeds planes travel at). The article states that this is "in accordance with international standards", but from this answer I understood that 3 to 5 miles (4.8 to 8km) is the general distance. However that answer also states that in a "Military Operations Area" this may not be the case (which this area might have been in, but then I feel like it wouldn't have mattered that it was "in accordance with international standards", as those don't apply then anyway).

That said, are there any alarms or warnings that start going off when another aircraft gets so close to warn the pilot of possible collision, or is a pilot trained to undertake action based on what his/her visuals/flight data are telling him/her at the moment? (I have read before that measuring a distance by visuals is very hard in open air due to there being no reference points, so I think it is unlikely that a pilot would act up on that alone). What is the protocol for a pilot suspecting the possibility of a mid-air collision?


It seems that this incident is also a bit of a political issue. I am not trying to discuss if everyone took the right actions here, and what the intentions of the parties were. Just interested in the protocol behind making such a drop.

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    $\begingroup$ The referenced incident: avherald.com/h?article=4da54d79&opt=0 $\endgroup$ Jul 24, 2020 at 11:58
  • $\begingroup$ @expeditedescent thank you for linking that, I witheld from posting a news article about it myself as I didn't want to bring any bias or standpoints to the post. But that article seems to take an excellent neutral technicals only approach! $\endgroup$
    – Remy
    Jul 24, 2020 at 12:01
  • $\begingroup$ It's a great site for information about aviation incidents and accidents. As long as you ignore the comments section, which is generally not very constructive. $\endgroup$ Jul 24, 2020 at 12:07
  • $\begingroup$ It depends. In the case of a fighter jet doing an interception, a distance of a few meters would be reasonably safe. It's standard for fighters to fly in formations, or do mid-air refueling, with separations of a few tens of meters. For aircraft going in the same direction at roughly the same speed, the 1000 m separation is perfectly safe. $\endgroup$
    – jamesqf
    Jul 25, 2020 at 4:53

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Most modern aircraft are equipped with TCAS (Traffic Collision Avoidance System) which is a last resort to avoid collision if all other safety nets have failed and two aircraft have gotten too close.

A TCAS can generate an RA (Resolution Advisory) which is a distinct message instructing the pilots what to do - typically either "CLIMB" or "DESCEND". When an RA is generated, the pilots have to comply with it immediately, regardless of all other instructions, unless it directly puts the aircraft in danger. In case of two conflicting aircraft, the TCAS system of each aircraft will communicate with the other, so for example, for two flights on converging courses, one will get a "CLIMB" instruction and the other a "DESCEND".

As for timing:

The maximum generation time for an [...] RA is 35 seconds. The time scales are shorter at lower altitudes (where aircraft typically fly slower). Unexpected or rapid aircraft manoeuvre may cause an RA to be generated with much less lead time. [...] The effectiveness of an RA is evaluated by the ACAS equipment every second and, if necessary, the RA may be strengthened, weakened, reversed, or terminated.

From SKYbrary

Essentially, this means that an RA will only be generated if two aircraft are calculated as being very close to colliding (within 30 or so seconds of collision). Note that the actual distance between the two aircraft does not matter in this regard - calculations are based on "time until impact". Two aircraft on slightly converging courses, closing slowly, can get much closer before an RA is triggered compared to two fast aircraft meeting head on.


As for the specific incident you are talking about, it is unclear if a TCAS RA was generated. Normally, fighters intercepting an aircraft will turn off their transponder (at least partly) to disable TCAS, with the specific purpose of avoiding such alerts (since they intentionally want to fly close to the other aircraft).

Even if we consider the "1000 metres" claim to be correct, it is not possible to say if that is "too close" or not. If we are talking 1000 metres vertically, that is more than 3 times the normal required separation of 1000 feet. However, 1000 metres horizontally would, in many (but not all) cases, be too close.

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    $\begingroup$ Amazing! Thinking about it having "too close" be related to "time until impact" rather than distance in meter/feet makes a lot of sense! Just making sure since I'm not too familiar with Aviation.se, is there a usual grace period here for waiting to accept an answer? $\endgroup$
    – Remy
    Jul 24, 2020 at 12:27
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    $\begingroup$ I would accept an answer coming from the guy who has part of the question as his user name! :) $\endgroup$ Jul 24, 2020 at 14:15
  • $\begingroup$ @Remy Happy you found it useful :) Some people suggest waiting 24 hours in case someone in a different timezone comes up with a better answer $\endgroup$ Jul 24, 2020 at 18:12
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    $\begingroup$ @Remy Note that under normal circumstances, "too close" is indeed related to distance and altitude. It's just in the rare case where TCAS takes over (a real risk of collision) that calculations are based on time instead $\endgroup$ Jul 24, 2020 at 18:17
  • $\begingroup$ I had an interesting experience once. We were doing a pre-delivery test flight on a corporate jet at a completion center near St Louis, and were flying VFR in the pattern on downwind after completing an Air Driven Generator drop test, when I got a TCAS TA from a light plane about 10:00 and a couple miles roughly on our level (the tower had said to watch for him; I believe he was heading to join the pattern for a smaller GA runway on the same airport).We spotted him just as the TA turned into an RA,observed he was going to pass well clear behind us, so I was able to ignore the RA and carry on. $\endgroup$
    – John K
    Jul 25, 2020 at 13:49
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So, for instrument flight rules (IFR) flghts, so every airline flight, the minimum vertical distance is 1000ft, so about 300meters. Ofcourse we can't know if the fighter was on an IFR plan or not, or what it was doing up there

That said, are there any alarms or warnings that start going off when another aircraft gets so close to warn the pilot of possible collision

Yes, if both the planes have their transponders on there's a system, called TCAS that, if the planes are in a collision route will thell the pilots what to do to avoid crashing into each other, the pilots MUST do what the TCAS tells them to. But most of the time for IFR flights ATC is responsible for separation between planes

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  • $\begingroup$ Very interesting! What would a "command" from this TCAS system look like, does it state something like (simplified) "Drop x Altitude within n time" ? $\endgroup$
    – Remy
    Jul 24, 2020 at 12:04
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    $\begingroup$ @Remy it is usually something like "DESCEND, DESCEND NOW!". Wikipedia has a list of all TCAS RA instructions. $\endgroup$
    – Bianfable
    Jul 24, 2020 at 12:53

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