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I know any pilot flying in VMC, whether under VFR or IFR, must "see and avoid." How often do airline pilots actually "see" and have to "avoid" other traffic at cruise altitude without being told about the traffic by ATC? Can it happen without someone doing something wrong?

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  • $\begingroup$ It's possible if someone departs from a clearance, but that's what TCAS is for. The vertical separation is only 1000 ft at the FLs above 280, and because of GPS, everybody is exactly on the airway centerline, so when someone passes above or below on your airway, they pass precisely overhead or underneath, 1000 ft above/below. They look like they are coming straight at you until they are a couple miles out. $\endgroup$
    – John K
    Commented Feb 15, 2023 at 20:34

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In the U.S., at cruise altitude for airline operations (normally above FL180 and in class A airspace) it should never happen because all aircraft are operating under IFR and being positively separated by ATC.

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    $\begingroup$ In theory. I was on a corporate customer acceptance flt, flying a big circular route while the customer checked out the cabin systems. We made a boo boo in the FMS entries and left out a waypoint in the 12 or so that made up our big circle. Airplane started turning toward the waypoint AFTER the one we were supposed to turn to, making our turn sharper than it would have. Kindly ATC said, "XXXX I see you're heading toward waypoint Y instead of waypoint X. Would like like me to amend your clearance?" We looked at each other; yikes. "Yes we'll take the revised clearance and thank you VERY much". $\endgroup$
    – John K
    Commented Feb 16, 2023 at 2:30
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Above FL180, basically never. Below, just like any other VFR pilot

Expanding on 757toga's answer, in the US there are altitudes above which it is not legal to fly except under IFR. At those altitudes, there should never be anyone. However, that leaves flights over international waters as well as flights below those altitudes.

Below 18000', there is no positive control of the airspace. The FAA politely asks that VFR pilots cruise at altitudes ending in 500', e.g. 3500', 10500, 14500', etc... However, the request is not a requirement, in that not everyone in the sky is is cruising. There are many, many users of the sky, such as gliders, aerobatics, balloons which would not be expected to be in level flight.

So IFR flights below FL180 still need to aggressively practice see-and-avoid. Airlines will fly at those altitudes often enough when coming into and out of airports, so they will need to see-and-avoid just like a normal VFR pilot.

The only time an IFR pilot can be sure there's no one at a sub-FL180 altitude is when flying in a cloud and thus knowing there are no VFR pilots around.

ATC will provide guidance if there are transponder or radar blips, but there is no universal transponder requirement and so many planes are invisible to ATC.

That being said, the sky is a very, very, very empty place. The number of times I've had to see and avoid, as opposed to simply see is... 0? (I'm not counting flying in a gaggle with other glider pilots.) I imagine most pilots are in a similar boat.

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    $\begingroup$ 14 CFR 91.159 is not phrased as a request... $\endgroup$
    – randomhead
    Commented Feb 15, 2023 at 20:18
  • $\begingroup$ The only times that I have had to see and avoid another airplane were within Alert areas for high amount of flight training. Airplanes in those areas usually do a lot more climbing, descending, and turning due to the nature of operations. $\endgroup$ Commented Feb 15, 2023 at 22:42
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    $\begingroup$ "Below 18000', there is no positive control of the airspace." -> What about class B airspace? $\endgroup$
    – Bianfable
    Commented Feb 16, 2023 at 16:41
  • $\begingroup$ @Bianfable that's a fair point, but those airspaces are few and far between. $\endgroup$ Commented Feb 16, 2023 at 21:11
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    $\begingroup$ @downvoters in general, I think you should reconsider your downvote. Pilots who see the airspace as only IFR cruise or VFR cruise are some of the biggest threats to users who are pure VFR. It's galling that there are so many of us who seemingly never look outside our cockpits, because too many of us feel 91.159 and ADSB gives us invincibility. $\endgroup$ Commented Feb 16, 2023 at 21:19

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