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I was reading how-much-is-the-minimum-safe-distance-between-two-planes-in-flight and realized part of it confused me (or maybe I'm just tired.)

In a low- or no-visibility environment (i.e. under IFR rules AND conditions), who is responsible for separation of planes in flight?

§91.111 Operating near other aircraft and §91.13 Careless or reckless operation are both shown in that question/answer, but if a flight is under ATC guidance, are they (ATC) or the Pilot-In-Command (PIC) responsible?

I'm most interested in the U.S. (FAA Regs), but discussions of other regulatory bodies would be fine.

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ATC is always responsible for IFR-IFR separation, regardless of weather conditions. That is their primary mandate, and all other services are provided on a workload-permitting basis.

In visual meteorological conditions, pilots of IFR aircraft are also responsible for seeing and avoiding other aircraft. But for IFR traffic of course this is not always possible, since the aircraft may be going in and out of the clouds.

If a loss of separation (at worst, a collision) occurs between two IFR aircraft, ATC would be held responsible (assuming the pilots followed ATC's instructions). If analysis indicates the pilots could have seen each other, they would also be held responsible.

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    $\begingroup$ Pilots could still be blamed if ATC gave correct instructions, but the pilots didn't follow them. $\endgroup$
    – Jan Hudec
    May 11, 2014 at 15:26
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    $\begingroup$ @JanHudec - Correct and decipherable. The worst airline accident in history was the result of a tower controller giving misleading radio cues to a plane requesting departure while another was taxiing on the runway $\endgroup$
    – KeithS
    Jul 10, 2015 at 19:22
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    $\begingroup$ @KeithS Well, the PIC of the KLM shared more responsibility I think. He was anxious, and started to takeoff without proper clearance, or proper visibility. $\endgroup$
    – SnakeDoc
    Jul 11, 2016 at 21:12
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    $\begingroup$ @J.Hougaard Granted about F and G and this might be worth an edit to add a footnote to that effect. You're wrong about paragraph 2. When IFR-in-VMC in controlled airspace, even though ATC provides IFR-IFR separation, FAR 91.113(b) explicitly requires pilots to see-and-avoid ("When weather conditions permit, regardless of whether an operation is conducted under instrument flight rules or visual flight rules, ..."). Your third point ("human error is never the cause of an incident") is just, with all due respect, nonsense. $\endgroup$
    – TypeIA
    Oct 19, 2016 at 18:08
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    $\begingroup$ @J.Hougaard You're entitled to your opinion, even if it's nonsense. That isn't my "central belief" or that of anyone I'd climb into a cockpit with. Pinnacle 3701 comes to mind. Those guys erred, died, and were single-handedly and solely responsible for the accident. Safe pilots take direct personal responsibility for their actions and do not try to blame "human factors." $\endgroup$
    – TypeIA
    Oct 19, 2016 at 18:18
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ATC provides separation between flights depending on the airspace class in which they are flying. The following rules apply:

  • Class A: Separation provided between all IFR flights (VFR not allowed)
  • Class B: Separation provided between all flights
  • Class C: Separation provided between IFR flights and all other flights. No separation provided between two VFR flights. Traffic information is given to VFR flights about other VFR traffic.
  • Class D: Separation provided between two IFR flights, but not between IFR and VFR flights. No separation provided between VFR flights. Traffic information is provided to all flights.
  • Class E: Same as D, but VFR is not controlled and as such not all VFR flights are known to the controller. Traffic information given as far as practical.
  • Class F: Advisory service provided to IFR flights as far as practical. Traffic information provided to VFR flights as far as practical.
  • Class G: Traffic information provided to all flights as far as practical.

So, in airspace classes A-C, ATC provides separation for IFR flights. In D-E, separation between two IFR flights is provided, but not between VFR flights. In F and G, only advice/information is given, and it is up to the participating pilots to maintain separation based on this information, and a visual lookout.

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