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Finals and landings

Old 26th Apr 2020, 20:50
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Finals and landings

Curious to know how long the "landing approach" has to be... for example if you are flying west to east, but have to land in a westward direction, how far past the airport is it normal to fly before turning round and heading in to land? (This is for passenger planes, 777, A380 etc). Are there guidelines for descent altitude versus distance from airport?
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Old 26th Apr 2020, 22:20
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Originally Posted by Paul Lupp View Post
Curious to know how long the "landing approach" has to be... for example if you are flying west to east, but have to land in a westward direction, how far past the airport is it normal to fly before turning round and heading in to land? (This is for passenger planes, 777, A380 etc). Are there guidelines for descent altitude versus distance from airport?
Altitude vs distance for an instrument runway is a function of the ILS glideslope angle. That's typically 3 degrees, equivalent to just over 300 feet per nautical mile.

Airlines typically require approaches to be stabilised by 1000', so that's a bit over 3 nm from the runway, although aircraft will usually join the ILS farther out than that.

https://www.skybrary.aero/index.php/Stabilised_Approach
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Old 27th Apr 2020, 09:39
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Paul,

It all depends on the type of approach you make. If you are making a visual approach then there is no need to go much above 2 miles past before making a descending turn to line up with the runway a couple of miles out. If you are flying an ILS then a little further, but not much, probably 4-5 miles minimum. If you are flying a non-precision approach then you will probably go further still. This all depends on where you are and the surrounding terrain, for instance at Amsterdam for an ILS approach they will feed you in quite happily at 5 miles or just less, whereas if you are at Innsbruck you will go over 20 miles out before joining the approach.

Guidelines for descent vs altitude, we generally use 3 time table, so from 30000' we multiply the number of thousands of feet by 3, in this case 90, thats how many miles we roughly need. Having said that we do have ways of reducing that if necessary.
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Old 27th Apr 2020, 15:10
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If you look at a typical set of approach plates like these https://vau.aero/navdb/chart/EGPH.pdf you can see distances marked plus there are tables of time and distance as well
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Old 27th Apr 2020, 17:52
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Another factor is the design of the airspace. There are numerous things that can affect it, such as adjoining airports/ airfields, obstacles such as telecoms towers and tall buildings, and even people who live under the flightpaths (some are very vocal and sometimes it's just easier to avoid them...). So for example, on a multi runway airport, you could fly further on one runway compared to the other.

These days, environmental considerations are taken into account and reducing the distances flown (albeit they may be small) is a big factor in the airspace design.
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Old 27th Apr 2020, 20:26
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Can anyone tell me when planes arriving at Heathrow over London join the ILS - is it earlier? And does confirmation that the landing is stabilised have to take place earlier? When I land at Gatwick I am always surprised at how late the final approach appears to be established until I remind myself that it's normal and that Heathrow is the exception.
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Old 27th Apr 2020, 22:04
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Although the traffic is scarce these days, a picture here: https://www.flightradar24.com/51.48,-0.4/12 is worth a thousand words.

The stabilised criteria need to be achieved by 1000' above field elevation, i.e. 3 NM before the runway threshold (on a typical 3 landing slope that gives 5.2% steepness).

Our understanding to join the ILS "fully" is to acquire its glide-path (vertical) plane as well. That is 10 NM or so for LHR. The capture of the lateral (localizer) signal for the extended centerline better be at least 2 NM prior - that's what pilots need.

However, given the constraints of the airspace (keeping room for corridors of the aircraft departing in various directions) as well as to have enough playground for the ATC who need to make all the various birds walk a nice line in a steady and synchronised pace (speed over ground to achieve and maintain proper in-trail spacing), the aircraft would be actually aligned on the final track at about 15 - 18 NM before touchdown.

At Gatwick with its single runway, there will be departing aircraft in between the landing ones. The cadence of arriving traffic is thus about half of that in Heathrow, and an orderly queue can be arranged within a shorter distance from the field. Which is very desirable for all the other traffic in vicinity.

Keep in mind that planes arriving into and departing from the airport zone will cover one mile every 13 seconds, you cannot ever have too much airspace. For a simple turn into the opposite direction a corridor almost 8 miles wide is the minimum.

Last edited by FlightDetent; 28th Apr 2020 at 09:54.
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Old 28th Apr 2020, 08:15
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At places like LHR you can see the 4 aircraft at east in a nice line for landing - that's 12-15 miles. But as FD says that's more to corall the traffic into the system than any ILS driven constraint
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Old 28th Apr 2020, 13:11
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Originally Posted by Asturias56 View Post
At places like LHR you can see the 4 aircraft at east in a nice line for landing - that's 12-15 miles. But as FD says that's more to corall the traffic into the system than any ILS driven constraint
It's also for noise considerations, making arrival flightpaths more predictable. The Heathrow AIP requires that aircraft be stabilised on the ILS by 2500' (roughly 8 nm from the runway) in daytime and 3000' (10 nm) at night.
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