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Often flights will leave the cruising altitude long before the actual landing. They will go down to a couple of thousand feet and stay at that altitude and circle for quite a while in waiting loops before it's actually their turn to land. It can be seen basically all the time at Heathrow Airport (LHR). But also for example in Munich Airport (MUC), where flights often come far down over the city and then make wide turns towards the airport.

So I'm wondering: if it's already clear ahead of time (and I'm sure mostly it is) that it's not their time to land yet, why not circle at higher altitudes? Often those waiting loops occur over dense populations and thus create quite some noise on the ground.

Peter Mortensen
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silent
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    A couple of thousand feet? The lowest level for Heathrow's stacks is 7000ft, and aircraft will enter higher than that. See Heathrow arrival paths – CatchAsCatchCan Aug 16 '19 at 20:29
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    It’s still an altitude at which you can clearly hear the planes on the ground – silent Aug 16 '19 at 20:31
  • Interesting link though! – silent Aug 16 '19 at 20:37
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    "creating quite some noise" Username checks out :) – Bianfable Aug 16 '19 at 20:41
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    @silent Noise intensity falls off with the square of distance. An aircraft at 7000ft is already about one tenth as noisy as one at 'a couple of thousand feet'. Having lived near Biggin Hill for many years I can tell you that the traffic noise from the M25 was far more intrusive than the passing jets. – CatchAsCatchCan Aug 16 '19 at 20:46
  • @ReddHerring: Biggin Hill is closed to commercial traffic, which could have something to do with the low aircraft noise levels there. – Vikki Aug 16 '19 at 21:10
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    @sean We're not talking about traffic visiting the airfield, but traffic using the Biggin hold while en route to Heathrow. – CatchAsCatchCan Aug 16 '19 at 21:47
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    @ReddHerring While it's true the intensity falls of rapidly, that's exactly a good reason to hold higher than they currently do. Unfortunately there's a whole lot of reasons why it's also a good idea to hold lower and noise pollution often isn't a primary concern. The M25 being noisier in a certain area doesn't make air traffic less of a problem all of a sudden. Even though LHR has it sorted better than some other major airports in that regard. – Mast Aug 17 '19 at 08:48
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    Since you specifically talk about LHR and flying low over the city, I think you might be confusing holding stacks with landing approaches. Planes typically want to land into the wind and the prevailing winds in the UK are from the west or southwest. That means that planes at Heathrow usually want to land from the east, which means that a plane coming from the west will fly across London, then do a 180-degree turn over central or east London at fairly low altitude before flying west to land. – David Richerby Aug 17 '19 at 11:02
  • Fair enough. The other example of MUC might be actually more what my question is about. There holding stacks - at least like in LHR - are much less common. But still planes will run long approaches on a mostly steady, low altitude – silent Aug 17 '19 at 11:09
  • As far as noise is concerned, they are also slowly washing off speed during this descent, which is somewhat of a powered glide. ATC can also interleave traffic on approach with less complexity. It can also depend upon the origin of the traffic. – mckenzm Aug 18 '19 at 23:57
  • The cruising altitude is a way to reduce drag while flying at high speeds. There's no point in flying fast while circling - it's not going to make you reach your destination faster, and you're just wasting fuel. – Luaan Aug 20 '19 at 07:49

3 Answers3

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It's better to be low(-ish) and ready for a spot to open, than high and far from that spot. As to why, for busy international airports the answer is really simple:

► There isn't a way to manage it near perfectly (yet).

To understand that statement, requires some prerequisites, so I'll try to simplify and summarize the basics:

  1. There is the concertina effect: it's when fast vehicles slow down, the spacing between them go down. So the nearer the airport for landing, the more squeezing happens.

  2. Add to that that the arriving planes come from all or most directions, and the same low-ish airspace is being used by departures as well.

  3. The trajectory of each plane depends on each plane's load and performance, and the wind it experiences. While wind is forecast, the forecasts aren't perfect.

A solution to that is for all planes to send their estimated trajectories to the air traffic management (ATM). That is one of the projects being worked on in Europe, but it's at least a decade from full deployment. One part of it is the extended arrival management (E-AMAN).

Why wasn't it worked on decades ago has to do with the history of data communication in aviation. And it boils down to money and return of investment. Initially some of the users, like the airlines, didn't see a benefit of the high cost of the high tech solutions they'd have to add to their planes.

  1. Seasonal adverse weather en route or in the terminal area complicates matters even more.

  2. Busy international airports with multiple runways land planes every 30 seconds or so, so you can imagine the impact of the slightest delay one plane headed to that airport experiences. So arrival management is first come, first served.

Related: What are the negative associations with Continuous Descent Approach?

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    "Perfectly managed" would be to slow down their cruise before reaching the destination if there was a landing backlog, and have planes (nearly) never circle in holding patterns? But still land 1 plane / 30 sec. – Peter Cordes Aug 17 '19 at 13:57
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    @PeterCordes: Sure, and to do that you need to know each plane's estimated trajectory, hence one of the paragraphs and the link. –  Aug 17 '19 at 14:12
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Airports can accept landing aircraft at a (mostly) fixed, constant rate. However, inbound aircraft arrive at different times and rates based on weather and other factors, regardless of the schedules. This means, at times, aircraft will be coming in faster than the airport can accept them, from many different directions. And airplanes can't just stop mid-air to wait their turn.

The answer is to put aircraft in a "holding pattern", which is a racetrack (not circle) shape that repeatedly passes over the same fixed point, to delay them. When you do this at multiple altitudes, it becomes a "holding stack".

Arriving aircraft are directed into the top of the stack, 1000ft above the previous one. Aircraft at the bottom of the stack are released at a fixed rate to continue toward the airport, and all the aircraft above them are shifted down 1000ft.

The depth of the stack will vary depending on numerous factors, but at congested airports like Heathrow, every arriving plane is pretty much guaranteed to do at least a few turns in holding. And yes, this can mean a lot of noise on the ground for those who live under the stacks. That is the price of airports not being able to expand to increase their arrival rate.

StephenS
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    Thank you for your answer. However my question was more about why the holding pattern occurs on rather low altitudes. Why need often need to wait in line (or in stacks...) is clear to me – silent Aug 17 '19 at 09:24
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    @silent It depends how close the stack is to the airport. In general, the bottom of the stack will be how high airplanes should be if the stack isn't in use. The standard descent angle is 3° (about 1000ft per 1.5nmi), so if the bottom of the Heathrow stacks is at 7000ft, I'd expect them to be about 10nmi from the airport. – StephenS Aug 17 '19 at 17:19
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As an aside to the hold explanations furnished above: to save on fuel, an airliner will reduce power to commence descent at a distance from the airport which will place it at either 1) its anticipated hold altitude, or 2) the landing pattern entry altitude, subject to ATC's instructions, upon arrival in the vicinity of the airport.

While at reduced power, the pilot will trim the aircraft for the optimum power-on glide angle that lets him or her make best use of the plane's gravitational potential energy during the descent and approach, and thereby minimize fuel burn during that phase of the flight. The pilot will adjust this to conform to ATC instructions by adding power or deploying spoilers, flaps, or gear as required.

niels nielsen
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