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DocLightning
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Why Are Holding Patterns Oval?

Sun Nov 28, 2010 6:09 am

Why not just have the aircraft fly in a circle?

Sorry, this sort of thing occurs to my bored brain on cold, wet Saturday evenings.
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akiss20
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RE: Why Are Holding Patterns Oval?

Sun Nov 28, 2010 6:16 am

Quoting DocLightning (Thread starter):
Why not just have the aircraft fly in a circle?

Sorry, this sort of thing occurs to my bored brain on cold, wet Saturday evenings.

How would you know if you are keeping the fix centered/your radius is good? (Also constant banking is annoying as hell to maintain accurately)
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DocLightning
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RE: Why Are Holding Patterns Oval?

Sun Nov 28, 2010 6:52 am

Quoting AKiss20 (Reply 1):

How would you know if you are keeping the fix centered/your radius is good?

I dunno. Why would flying in a circle preclude that?

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swiftski
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RE: Why Are Holding Patterns Oval?

Sun Nov 28, 2010 7:11 am

Quoting DocLightning (Reply 2):
I dunno. Why would flying in a circle preclude that?

The pattern as it is works well. Flying overhead the fix, then making rate one turns, timing the outbound leg and tracking a bearing/radial inbound allows you to actually be quite accurate. Also, as pointed out, there is less time spent in the turn than if the pattern was a circle.
 
Alias1024
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RE: Why Are Holding Patterns Oval?

Sun Nov 28, 2010 7:32 am

Quoting DocLightning (Thread starter):
Why not just have the aircraft fly in a circle?

Sorry, this sort of thing occurs to my bored brain on cold, wet Saturday evenings.

Wind

If you maintained a constant bank angle for the turn your circles would be pushed downwind. The whole idea of a holding pattern is to keep you in one area.
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SAAFNAV
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RE: Why Are Holding Patterns Oval?

Sun Nov 28, 2010 8:23 am

Also, Holds are based on Rate 1 turns, meaning 1 180 turn takes 1 minute. Below Fl145, you go straight for 1 minute, and turn inbound again for another minute, and home on the radial for another minute. That gives you 4 minutes to complete one hold.

The above is for Still Air Conditions. We use a Rule-of-Thumb: for every 10Kts Tailwind Component on the Inbound leg, you add 10sec on the outbound leg, so as to keep the whole pattern at 4 minutes.

So when ATC gives you an Estimated Approach Time, you can play around with the timings so that you can be overhead the fix and continue with the Approach to within a few seconds.

Hopes this helps.

Erich
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SAAFNAV
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RE: Why Are Holding Patterns Oval?

Sun Nov 28, 2010 8:54 am

Quoting DocLightning (Reply 2):

I dunno. Why would flying in a circle preclude that?

If you have very strong Crosswinds, the wind would squash your circle.

Longer legs gives you opportunity for straighten it out.  
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Vmcavmcg
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RE: Why Are Holding Patterns Oval?

Sun Nov 28, 2010 9:25 am

Quoting SAAFNAV (Reply 5):
and home on the radial for another minute

Not to nit pick, but you don't home you track in on the radial if you are holding over a VOR and track the bearing if you are holding over an NDB.

As everyone said, the purpose of holding is to position your self over a specific point. If you had winds and held in a circle, then at some point in time you would not be able to maintain the point as you would not be able to bank steep enough to compensate for the winds. In reality you would have an oval holding pattern.
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SAAFNAV
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RE: Why Are Holding Patterns Oval?

Sun Nov 28, 2010 10:37 am

Quoting Vmcavmcg (Reply 7):

Not to nit pick, but you don't home you track in on the radial if you are holding over a VOR and track the bearing if you are holding over an NDB.

Yep, technically that is correct, but if you use the 'tail' of the RMI for both, it is pretty much the same thing.
Different terminology, but in essence you use it the same.

Regards,

Erich
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sfotom
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RE: Why Are Holding Patterns Oval?

Sun Nov 28, 2010 11:52 am

Aircraft attitude gyros knowledge of "up" and "down" gradually drifts with time. There is a very gentle correction system within the gyro that resets it to vertical based on the pull of gravity. The correction system of a attitude gyro is gentle (slow) enough that it is normally not affected by the aircraft maneuvering, however if an aircraft was to enter a constant bank for an extended time the gyro would begin loose it's correct reference to "up" and "down". Instrument holding patterns are "racetrack" in shape so that this does not happen.
 
Quokka
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RE: Why Are Holding Patterns Oval?

Sun Nov 28, 2010 1:19 pm

A related question: would the "shape" of the oval vary according to the airport that is being approached( sometimes longer, sometimes shorter). I have in mind approaches to airports that are in close proximity to another one, for example LHR and LGW, or are the holding areas sufficiently far enough apart to avoid overlap?
 
Pihero
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RE: Why Are Holding Patterns Oval?

Sun Nov 28, 2010 1:26 pm

First of all, the shape of a holding pattern is not oval, it is as sfotom in the shape of a race track : one 180° turn in one direction, followed by an "outbound straight leg and then back to the departure point through another 180° turn and an inbound leg to the origin.
That's in theory and with no wind.
The wind is going to mess everything up and the pattern will be misshaped, but just understand that if one doesn't correct for wind, at the end of the inboud leg, one would found oneself having to "clim back up-wind " to reach the origin of the hold.
That remark was at the beginning of a few holdong correction techniques, one in particular is to steer outbound with triple the drift angle for that leg....etc...

Quoting SAAFNAV (Reply 5):
Holds are based on Rate 1 turns,

Correct for slow airplanes... but that rate one, worth around 15° of bank per 100Kt TAS is not very comfortable for jets that hold at 210kt +. We know expect the bank at 25° for comfort. The speed for which the pattern has been designed for is on the AIPs.

Quoting SAAFNAV (Reply 6):
If you have very strong Crosswinds, the wind would squash your circle.

Coprrect again, but it does everything, doesn't it ?
The reason for not flying circles are about 1) Fuel consumption as the load factor in the turn forces a thrust increase and 2) passenger comfort.

Of course all the above is totally moot with an advanced EFCS/AP system, which flies a pattern quite nicely. Fun to watch, but with some strong winds, it also have to adapt with potatoid racetrack patterns.
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ThrottleHold
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RE: Why Are Holding Patterns Oval?

Sun Nov 28, 2010 7:54 pm

Fkying in a turn at a constant altitude burns more fuel. That's why you will hear crews requesting longer outbounds than published. I personally requested, and was approved, 50nm legs holding for JFK one wintery night.
 
xero9
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RE: Why Are Holding Patterns Oval?

Mon Nov 29, 2010 3:42 pm

While this likely has nothing to do with it, imagine anyone who is nervous about flying being in a constant turn for say 20 minutes?

But about the winds.. I have a question. If the winds are say from 270, would the long stretch be from east west? And if the winds are say from 180, the long stretch would be from north south? Just curious.
 
Pihero
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RE: Why Are Holding Patterns Oval?

Mon Nov 29, 2010 8:56 pm

Quoting xero9 (Reply 13):
If the winds are say from 270, would the long stretch be from east west? And if the winds are say from 180, the long stretch would be from north south? Just curious.

The holds are well defined, both as to the inbound radial/bearing and the direction of the turns. the aircrews have to cope with the actual wind conditions.
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tb727
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RE: Why Are Holding Patterns Oval?

Mon Nov 29, 2010 9:49 pm

Quoting Alias1024 (Reply 4):

Wind

If you maintained a constant bank angle for the turn your circles would be pushed downwind. The whole idea of a holding pattern is to keep you in one area.

Yeah, it's crazy how far you get pushed if you just do circles even down low. I was out with a Captain upgrade trainee west of Detroit working on his steep turns before his checkride and just kept doing 360 after 360 and we ended up drifting about 15 miles, then we would fly a straight line back to the SW into the wind and start over again. The track didn't show up on flightaware though, it would have been a great visualization.
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oly720man
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RE: Why Are Holding Patterns Oval?

Mon Nov 29, 2010 10:07 pm

Quoting Quokka (Reply 10):
A related question: would the "shape" of the oval vary according to the airport that is being approached( sometimes longer, sometimes shorter). I have in mind approaches to airports that are in close proximity to another one, for example LHR and LGW, or are the holding areas sufficiently far enough apart to avoid overlap?

To see it in action, go here

http://casper.frontier.nl/egkk/

and set the date/time to Friday 26th November 06:00 and set the speed to 25x. Click the show tracks box to see the flightpaths. Screenshot here




There are 3 holds for LHR as can be seen by the racetracks, NW, NE and SE of London. The nearest one to LGW (runway indicated by the white line, centre bottom) has the planes at around 11000ft, rather higher than the aircraft approaching LGW. As far as I know the hold(s) for LGW are south of LGW.


Planes generally fly the oval but there's an occasional 360, presumably for shorter spacing.
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Pihero
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RE: Why Are Holding Patterns Oval?

Tue Nov 30, 2010 12:24 am

Why not look at the official STAR charts for both Heathrow and Gatwick ?
Here is the LHR South-East arrival procedure ;
STAR Biggin
and the East/South-East STAR chart for LGW :
STAR Timba
You have a few examples of defined arrival holding stacks.

Quoting oly720man (Reply 16):
there's an occasional 360, presumably for shorter spacing.

It's generally an interrupted hold for spacing, and it could be a continuous turn toward the approach path.

Btw, maps like the above have basically no meaning. One would need the official trajectories in order to understand the situation as close to reality as possible.

Spelling spelling spelling

[Edited 2010-11-29 16:32:59]
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pilotpip
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RE: Why Are Holding Patterns Oval?

Wed Dec 01, 2010 5:43 am

Quoting Pihero (Reply 11):
The reason for not flying circles are about 1) Fuel consumption as the load factor in the turn forces a thrust increase

This perhaps more than anything. When we get holding instructions one of the first things we do is consult the performance charts, determine our max endurance airspeed and ask for it. If holding instructions are given we want to do everything we can to avoid diverting. Conserving as much fuel as possible leading up to and in the hold allows us to stretch the amount of time we are in the hold.
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