Xjyuk

Often flights will leave cruising altitude longe before 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. Can be seen basically all the time at LHR. But also for example in 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.

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.

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




5. 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?

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.

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.