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Why are holding patterns oval rather than circular?

It’s been suggested that the wings-level portions are intended to enable a traditional mechanical AI to erect.

I don’t really see that, because the famous pendulous vanes in a vacuum AI, or the equivalent mechanism in an electric AI, will sense the same straight-down gravity vector in straight-and-level flight as they will sense in a coordinated turn (of any turn rate). The only difference detectable inside an aircraft is that the G is slightly greater during the turns, but does any mechanical horizon sense that? This is sort of relevant.

It may thus be more to do with entry and exit procedures and getting aligned with the holding fix, ready to continue the procedure.

Administrator
Shoreham EGKA, United Kingdom

Interesting question.
Aligning plays a part most probably (eg course reversal/racetrack vs holding).
It’s also a bit difficult to time segments of a circle.

Who was the first pilot to ever fly a navaid defined holding pattern? Elroy Jeppesen maybe?

Freelance IRI / CB-IR Instructor
LOWG | Worldwide

Seen as holding pattern rather than stage of an approach it makes perfect sense, it can easily be extended procedurally.

It also enable Z-axis of the brain to settle as well

Holding with continuous Rate1 turns is disorientating and will drift with the wind easily, having a wing level segments seems helpful for disorientation and the lines bring the circles back where they should, I think it started as “holding circles” where the turns evolved into race track to stay attached to the fix and to stay comfortable ?

I used to hold with Rate1 turns, not part of any procedure just me sorting stuff out, the big worry was busting CAS on the 4th turn, ahem spiral

I think it also allow adjusting time to get back to the fix exactly after 4min, although honestly with today’s tools that is no longer a big concern, the same reason why not many do them these days…

Last Edited by Ibra at 18 Nov 21:48
ESSEX, United Kingdom

Peter wrote:

I don’t really see that, because the famous pendulous vanes in a vacuum AI, or the equivalent mechanism in an electric AI, will sense the same straight-down gravity vector in straight-and-level flight as they will sense in a coordinated turn (of any turn rate).

In straight and level the force the AI senses is directed downwards. In a constant turn, the force will be at an angle to the vertical. So the AI would eventually show straight and level although the aircraft is turning.

ESKC (Uppsala/Sundbro), Sweden

Three other reasons I can think of.

If holding to sort out a problem, the straight legs give calm time for troubleshooting or preparation

The inbound straight leg gives the opportunity to get properly oriented in case of drift etc

In the pre autopilot days, the straight legs gave the pilot a chance to steady altitude etc

Upper Harford, United Kingdom

Maybe fuel flow / balance of the aircraft?

We're glad you're here
Oxford EGTK

I can think of three reasons:
- easier to adjust to x wind
- easy to shorten or extend by ATC
- fuel imbalance and/ or unporting

I would think about 2 reasons:

- The only way to fly a prescribed track circle with a traditional nav aid (i.e. no GPS type area navigation) is a DME-Arc. That is quite demanding to fly accurately. Therefore the racetrack shape where at least the outbound track is accurately defined is much easier to fly.
- Thinking completely the other way round: The best holding pattern for operational reasons would be a straight line (easy to fly and accurately to track with traditional nav aids). A straight line, however, has the disadvantage that it needs lots of space and takes you quickly away from where you want to go. Therefore you need to compromise and build turns into your pattern. A square would be the next best options – however, to fly it accurately with traditional navaids you need 4 of them at each corner of the square. That is expensive. A racetrack like shape, again, is the best compromise between flying straight lines and restricting the procedure space with only navaid.

Germany

VFR holdings are usually circles

Airborne_Again wrote:

In straight and level the force the AI senses is directed downwards. In a constant turn, the force will be at an angle to the vertical. So the AI would eventually show straight and level although the aircraft is turning.

That’s right. In that sense, the holding pattern should be an “8” and not an oval.

ENVA ENOP ENMO, Norway
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