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When ATC asks a pilot to fly a holding pattern, he expects the airplane to stay at the required altitude in the sector of the holding pattern.

For the vertical direction, the question is straightforward: how much can the aircraft deviate from its assigned altitude?

For the horizontal direction, the question is more tricky: can the pilot fly a larger than standard holding pattern (lower turn rate, straight legs twice as long as expected)? What happens if on the contrary the pilot flies the pattern in less than 2 minutes?

Bonus question: what prevents the pilot flying an 8-shape instead of an oval one if he stays inside the holding pattern area?

EDIT: my question is really about the limits. It is obvious that a deviation of few millimeter or less than 10 seconds is OK, but at what point does it start to be not allowed (in other word, what is the precision required)?

To better understand the problem I want to address, my comprehension is that as long as you stay in the assigned sector, separation is ensured without any ATC intervention. Thus, I see no added value to be as precise as possible as long as you stay in the assigned sector. You may circle closer to the holding point, the separation will still be assured. But what happens if you fly farther away but still in the assigned sector? And what happens if you fly a non standard shape but still inside the assigned the assigned sector?

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  • $\begingroup$ Since most holding patterns/procedures are designed to align the inbound leg with a specific direction (the Intermediate Leg of an IAP for example) so that the plane is set up to continue on a desired course after the hold, what purpose would flying a figure eight achieve? $\endgroup$ – Dean F. Feb 27 at 13:38
  • $\begingroup$ A related question is what happens when there are several planes circling, and they are incapable of maintaining the same speed. Do slower planes use a tighter circle while maintaining their relative position behind the plane ahead? $\endgroup$ – Ray Butterworth Feb 27 at 14:23
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    $\begingroup$ @RayButterworth The sky is three-dimensional. Vertical separation is always used in holding patterns. It is not possible to have two aircraft in the same holding at the same altitude. $\endgroup$ – expeditedescent Feb 27 at 15:38
  • $\begingroup$ @RayButterworth - a good example of what J. Hougaard is talking about is "The Stack" in Arizona over Stanfield VOR. The same holding pattern is used by each airplane. Just at different, assigned altitudes. Also check out AIM 5–4–10 Timed Approaches from a Holding Fix for another example. $\endgroup$ – Dean F. Feb 27 at 16:50
  • $\begingroup$ To better understand your question, could you explain what you fear would happen if a pilot crosses this hypothetical precision threshold, i.e. flies the holding too imprecisely? $\endgroup$ – expeditedescent Feb 27 at 18:13
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Flying holds does not have to be rocket science and the holds do not have to be perfect ovals. In fact when applying proper wind correction the pattern will not be a perfect oval. There are two main parameters you need to meet to be safe and keep ATC happy when flying a holding pattern:

  1. Maintain your altitude: Sometimes an ATC controller will assign multiple aircraft to a published hold but assign different altitudes. Any deviation of altitude could reduce separation to unsafe levels.
  2. Stay on the cleared side of the hold as much as possible: While there is some leeway if you cross over to the uncleared side (parallel entries typically do cross over to the uncleared side) you want to stay on the cleared side as much as possible. This not only helps with separation of traffic and terrain but also helps keep you from drifting away from the fix especially if you are encountering winds aloft.

As far as your question concerning the time it takes to fly a holding pattern that is not that important as long as you are not drifting away from the fix. Lets say you have a tailwind when flying towards the holding fix which will turn into a headwind when turning outbound. The goal you want to obtain is 1 minute inbound legs. But to maintain that you will need to increase you time on the outbound leg since you are flying into a headwind and losing ground speed. That means your holding pattern will take longer to complete but ATC will not care about that. ATC wants you to keep you altitude and not drift away from the fix. Whatever you have to do to safely make that happen is fine with them.

Question about 8 shaped oval: I assume you are talking about if you accidentally turn into the uncleared side of the holding pattern and fly a figure 8 to get back on course and how to avoid that. The rule of thumb is apply two or three times the amount of wind correction on the inbound course to the outbound course. The reason this is done to account for the difference in turning radius when turning into the wind versus turning away from the wind. The resulting pattern will not be a perfect oval but will look like the pattern below. (Image borrowed from this math stack exchange post).

enter image description here

If you would like more info on what airspace ATC actually expects you to use for a hold click here and see wbeard52's answer.

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  • $\begingroup$ For the 8 shape, I really meant "fly any pattern that os not an ovale but still stay inside the published ovale". $\endgroup$ – Manu H Mar 2 at 14:54
  • $\begingroup$ @Manu H: It would be hard to maintain the hold pattern if the holding shape is not an oval or a wind corrected oval. If you zigzag all over the place you are likely to drift away from the holding fix of cross over to the non-cleared side of the holding pattern. If you want to know how much ATC is watching you in the hold, probably not too much as they would likely be busy. However, you always want to fly the hold as accurately as possible. $\endgroup$ – DLH Mar 2 at 17:13
  • $\begingroup$ Concerning the altitude, what deviation is acceptable? I imagine this is not 1mm (absurdly precise) nor 200ft (too much risk of collision) but somewhere in between, but where? $\endgroup$ – Manu H Apr 27 at 8:04
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Holding procedures, like all procedures, are designed with safety margins to ensure terrain clearance and separation from other traffic/procedures. That does not mean that a pilot is free to deviate from the published procedure within those margins.

For the vertical direction, the question is straight forward: how much can the aircraft deviate for its assigned altitude?

It can't. The pilot must continuously strive to maintain the exact assigned level. If you find yourself 100ft lower than cleared, you can't just keep flying at the level - you need to take action to climb back.

For the horizontal direction, the question is more tricky: can the pilot fly a larger than standard holding pattern (lower turn rate, straight legs twice as long as expected)? What happens if on the contrary the pilot fly its pattern in less than 2minutes?

No, he can't. He wouldn't be following the published procedure.

Bonus question: What do prevent the pilot to fly 8-shape instead of oval one if he stay inside this excepted ovale?

The fact that that's not how a holding pattern is defined, and pilots generally like to follow the rules, since not doing so can have catastrophic (and not always immediately obvious) consequences.

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  • $\begingroup$ I'm sure a procedure designer could get into the actual margins (vertical and horizontal) used when designing a holding procedure. But this question seems to be about pilot procedures, not procedure design. $\endgroup$ – expeditedescent Feb 27 at 12:49
  • $\begingroup$ I think it would be worth making the point that if a pilot does significantly deviate from the expected procedure, ATC will report him/her for pilot deviation. $\endgroup$ – Fiddlesticks Feb 27 at 12:53
  • $\begingroup$ @Fiddlesticks IMO that doesn't really have anything to do with the question. $\endgroup$ – expeditedescent Feb 27 at 13:16
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    $\begingroup$ @Fiddlesticks hoe much is "significant"? $\endgroup$ – Manu H Feb 27 at 16:56
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    $\begingroup$ @ManuH I'm not sure what you mean. Obviously it wouldn't make a difference if your legs are 1 second shorter than intended, but that doesn't mean you should aim for it. Pilots shouldn't intentially fly imprecisely, they should try to fly the procedure exactly as it is. It would make no sense to publish values for acceptable deviations, since pilots have no way to accurately determine if they are within those margins anyway. Safety margins were considered by procedure designers when they created the holding pattern, but should not be considered by pilots, who should fly it exactly as published. $\endgroup$ – expeditedescent Feb 27 at 18:06

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