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ATC will sometimes give pilots the instruction to maintain visual separation from nearby traffic.

I have a couple questions about this:

  • There is a related question here about traffic separation.
    Is the required separation in this case different, and how can they judge this? (I would like a bit more detail for this specific case than was provided on the answer there)
  • Normally pilots will follow their flight plan and ATC instructions. What actions should pilots take to maintain separation? I mean, obviously whatever is necessary, but is there any standard method? Should they anticipate an issue and ask ATC if they need to change course?

This is mostly focused on IFR traffic, but how different would VFR be? The situation could be that ATC clears you for a certain maneuver, and other traffic interferes.

It is my understanding that for IFR traffic the controller would expect that there would be no conflict, but things don't always go as planned.

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2 Answers 2

up vote 13 down vote accepted

Visual separation is addressed specifically in the AIM (see below). It is an ATC instruction sometimes given when the pilot reports another aircraft in sight. ATC is then able to instruct the pilot to "maintain visual separation" from the other aircraft, and it doesn't matter if they are IFR or VFR.

Then it becomes quite simple:

  • Don't hit the other airplane!
    • Do whatever you have to do in order to avoid them. (Usually this is nothing since you just get closer than ATC is allowed to let you otherwise.)
    • If you are on a specific ATC clearance, then notify ATC immediately if you have to do something contrary to that clearance.
  • Keep the other aircraft in sight.
    • Immediately report to ATC if you lose sight of them.
  • Avoid the wake turbulence of the other aircraft.

Standard radar separation no longer applies since the pilot has accepted separation responsibility from the one specific aircraft that the clearance applies to. This allows ATC to let aircraft fly more efficient routes (not having to turn to avoid the other aircraft) and also lets more aircraft land in a given amount of time because they can be closer together on final.

Pilots have to use good judgement as to how close they get to the other aircraft, but it is done visually. They can use any tools at their disposal to help (TCAS, asking ATC how far away they are, etc.) but it all comes down to "that looks good enough for me".

Section 5. Pilot/Controller Roles and Responsibilities

5-5-12. Visual Separation

a. Pilot.

1. Acceptance of instructions to follow another aircraft or to provide visual separation from it is an acknowledgment that the pilot will maneuver the aircraft as necessary to avoid the other aircraft or to maintain in‐trail separation. Pilots are responsible to maintain visual separation until flight paths (altitudes and/or courses) diverge.

2. If instructed by ATC to follow another aircraft or to provide visual separation from it, promptly notify the controller if you lose sight of that aircraft, are unable to maintain continued visual contact with it, or cannot accept the responsibility for your own separation for any reason.

3. The pilot also accepts responsibility for wake turbulence separation under these conditions.

b. Controller. Applies visual separation only:

1. Within the terminal area when a controller has both aircraft in sight or by instructing a pilot who sees the other aircraft to maintain visual separation from it.

2. Pilots are responsible to maintain visual separation until flight paths (altitudes and/or courses) diverge.

3. Within en route airspace when aircraft are on opposite courses and one pilot reports having seen the other aircraft and that the aircraft have passed each other.

More info is here:

4-4-14. Visual Separation

a. Visual separation is a means employed by ATC to separate aircraft in terminal areas and en route airspace in the NAS. There are two methods employed to effect this separation:

1. The tower controller sees the aircraft involved and issues instructions, as necessary, to ensure that the aircraft avoid each other.

2. A pilot sees the other aircraft involved and upon instructions from the controller provides separation by maneuvering the aircraft to avoid it. When pilots accept responsibility to maintain visual separation, they must maintain constant visual surveillance and not pass the other aircraft until it is no longer a factor.

NOTE- Traffic is no longer a factor when during approach phase the other aircraft is in the landing phase of flight or executes a missed approach; and during departure or en route, when the other aircraft turns away or is on a diverging course.

b. A pilot's acceptance of instructions to follow another aircraft or provide visual separation from it is an acknowledgment that the pilot will maneuver the aircraft as necessary to avoid the other aircraft or to maintain in-trail separation. In operations conducted behind heavy jet aircraft, it is also an acknowledgment that the pilot accepts the responsibility for wake turbulence separation.

NOTE- When a pilot has been told to follow another aircraft or to provide visual separation from it, the pilot should promptly notify the controller if visual contact with the other aircraft is lost or cannot be maintained or if the pilot cannot accept the responsibility for the separation for any reason.

c. Scanning the sky for other aircraft is a key factor in collision avoidance. Pilots and copilots (or the right seat passenger) should continuously scan to cover all areas of the sky visible from the cockpit. Pilots must develop an effective scanning technique which maximizes one's visual capabilities. Spotting a potential collision threat increases directly as more time is spent looking outside the aircraft. One must use timesharing techniques to effectively scan the surrounding airspace while monitoring instruments as well.

d. Since the eye can focus only on a narrow viewing area, effective scanning is accomplished with a series of short, regularly spaced eye movements that bring successive areas of the sky into the central visual field. Each movement should not exceed ten degrees, and each area should be observed for at least one second to enable collision detection. Although many pilots seem to prefer the method of horizontal back-and-forth scanning every pilot should develop a scanning pattern that is not only comfortable but assures optimum effectiveness. Pilots should remember, however, that they have a regulatory responsibility (14 CFR Section 91.113(a)) to see and avoid other aircraft when weather conditions permit.

For even more information, there is an entire section of the Air Traffic Control job aid which the controllers use about visual separation.

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Ad "asking ATC how far they are", the radar scope has limited resolution. One of the "Say Again?" articles mentioned the "blip" is about a mile across so at that range the controller won't even know which aircraft is on which side. –  Jan Hudec Apr 1 at 18:54
    
@JanHudec I tend to do that more with approach control while on final, so they usually have better resolution than that. –  Lnafziger Apr 1 at 19:19

ATC will use radar to judge how far apart planes are, in absence of ATC (uncontrolled airspace) it is purely scanning the sky and paying attention this is called See and Avoid and is always in force even during IFR.

If a pilot notices he is on a collision course with another plane he may do anything in his power to avoid said collision. For many planes there is TCAS an automated system to help pilots avoid collisions by issuing a climb or descend command to the pilot if collision is near. If such a command happened then the pilot MUST follow it to minimize chance of collision and notify ATC.

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