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In Minimum separation between a landing aircraft and another crossing its runway? it was explained that once an aircraft has clearance to land, that runway is reserved for the exclusive use of that aircraft.

At busy airports you can usually see a long line of four or more airliners descending a glideslope towards a runway.

http://cdn.londonreconnections.com/assets/planes.jpg Image by nustyR AirTeamImages (found via LondonReconnections)

It seems as if, by the time the previous aircraft has turned off the runway, the following aircraft has a very short time to obtain clearance or decide to go around.

At what point is each aircraft given clearance to land by ATC?

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Nice picture (if you took it). :) –  Farhan Jul 10 at 13:29
    
@Farhan: It's a nice image even though I didn't take it. Attibution added below image. –  RedGrittyBrick Jul 10 at 15:04
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4 Answers 4

up vote 18 down vote accepted

Usually, the clearance is given when the runway is clear of traffic or, at least in Europe, when two conditions are met:

  • Reduced Runway Separations is in use and the criteria for issuing the landing clearance are met.

  • The previous traffic will have left the runway when the following aircraft crosses the threshold. (Non-withheld landing clearance)

321.5 Landing clearance need not be withheld until prescribed separation exists if there is reasonable assurance that the appropriate separation will exist when the aircraft crosses the runway threshold. However, do not clear an aircraft to land before a preceding landing aircraft has crossed the runway threshold.

Aircraft can also be made aware to expect a late landing clearance.

Clearing multiple aircraft to land in a sequence on the localizer is not used in countries operating under ICAO rules. This is more of an FAA procedure, as described in the other answers.

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In addition to @SentryRaven's answer, remember that telescopic compression makes that separation seem much smaller than it actually is.
Those aircraft are minutes apart, not seconds, the correct amount of time for one to clear the runway and still give the next a chance to abort the landing if needed.
In other words, the proximity you perceive from that image is an optical illusion, it's not real.

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I still have a hard time buying that the second one is minutes away from the photograph... –  Mehrdad Jul 12 at 8:17
    
@Mehrdad having done such photography extensively (until the airport here almost made it impossible) I know these pictures very well. It's what you see during a normal busy approach cycle, which has the aircraft spaced about 90 seconds apart. –  jwenting Jul 14 at 9:18
    
Ah, ok, I guess I can believe 90 seconds, but when you said "minutes" (plural) I assumed at least 3 or 4 minutes, which seemed quite a stretch... –  Mehrdad Jul 14 at 10:16
    
@Mehrdad In Europe, minimum separation on the ILS is 3nm, sometimes reduced to 2.5nm under certain conditions (not taking into account wake separation). The average final approach speed of a commercial airliner is around 140-150 GS. 3/140 = 1.285 * 60 = 77 seconds, so the 90 seconds estimate above is quite correct. Depending on available runways and departure spacing, the separation will be inceased to 4-5nm, to make room for departures. –  SentryRaven Jul 15 at 6:13
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FAA has the following rulings:

(a) Category I aircraft landing behind Category I or II- 3,000 feet.

(b) Category II aircraft landing behind Category I or II- 4,500 feet.

(c) When either is a category III aircraft- 6,000 feet.

This picture shows the separation of Category I, II and III.

Separation

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Also known as: Reduced Runway Separation in ICAO-land. :) –  SentryRaven Jul 10 at 13:56
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Also, in the USA, landing clearance can be given to multiple aircraft at once for the same runway- eg. "JetBlue 1203, number two following an Airbus A320 on a two mile final, wind 260 at 8, runway 22 left, cleared to land." This is called "anticipated separation", which allows aircraft to get their landing clearance with much more space. However, in Europe @SentryRaven is correct.

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