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In Is there a 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)

Looking at FlightRadar24, the time between aircraft appears to be of the order of about 85 seconds according to my stopwatch.

enter image description here

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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    $\begingroup$ Nice picture (if you took it). :) $\endgroup$
    – Farhan ♦
    Jul 10, 2014 at 13:29
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    $\begingroup$ @Farhan: It's a nice image even though I didn't take it. Attibution added below image. $\endgroup$
    – RedGrittyBrick
    Jul 10, 2014 at 15:04

5 Answers 5

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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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    $\begingroup$ In the case of the "being made aware to expect a late landing clearance", would following aircraft's PIC be in a "ready to abort" mental mode until clearance is granted (even more then perhaps normal)? $\endgroup$
    – CGCampbell
    Aug 21, 2014 at 15:03
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    $\begingroup$ If you use: expect late landing clearance , you let the pilot know that a landing clearance is to be expected, so he would not be in a mental state to expect a go-around. $\endgroup$
    – SentryRaven
    Aug 21, 2014 at 15:21
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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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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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    $\begingroup$ I still have a hard time buying that the second one is minutes away from the photograph... $\endgroup$
    – user541686
    Jul 12, 2014 at 8:17
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    $\begingroup$ @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. $\endgroup$
    – jwenting
    Jul 14, 2014 at 9:18
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    $\begingroup$ 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... $\endgroup$
    – user541686
    Jul 14, 2014 at 10:16
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    $\begingroup$ @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. $\endgroup$
    – SentryRaven
    Jul 15, 2014 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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  • $\begingroup$ Also known as: Reduced Runway Separation in ICAO-land. :) $\endgroup$
    – SentryRaven
    Jul 10, 2014 at 13:56
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    $\begingroup$ This is incorrect. Read closer on Same Runway Separation. There is one for successive departures and one for separating arriving aircraft from other aircraft (Arrival or departure). For arriving aircraft, you need to ensure that preceding arrival has cleared of the runway and preceding departure has cross the departure end of the runway. Except during sunset and sunrise, (a) and (b) you quoted apply. But not (c). You just can't separate Cat III arriving by this $\endgroup$
    – fjch1997
    Dec 5, 2018 at 19:30
  • $\begingroup$ Concur with @fjch. This answer (and image) is correct for an arrival behind a departure, not for an arrival behind an arrival. $\endgroup$
    – randomhead
    Dec 4, 2021 at 23:26
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In the United States, FAA has some quite complicated rules on landing clearance.

Anticipated Seperation

Contrary to the rest of the world, FAA ATC clears aircraft to land unless there is a reason not to; for example, traffic holding in position.

a. Landing clearance to succeeding aircraft in a landing sequence need not be withheld if you observe the positions of the aircraft and determine that prescribed runway separation will exist when the aircraft crosses the landing threshold. Issue traffic information to the succeeding aircraft if a preceding arrival has not been previously reported and when traffic will be departing prior to their arrival.

b. Anticipating separation must not be applied when conducting LUAW operations, except as authorized in paragraph 3−10−5b2. Issue applicable traffic information when using this provision.

For departures,

Takeoff clearance needs not be withheld until prescribed separation exists if there is a reasonable assurance it will exist when the aircraft starts takeoff roll.

This means that ATC needs not to wait for the extra few seconds to issue takeoff clearance when we all know that the aircraft taking off won't be able to line up and start takeoff roll until some 10 seconds later.

Same Runway Seperation

In case of issuing landing clearance, preceding arrival has to be cleared of the runway and preceding departure has to have crossed the departure end of the runway.

a. Separate an arriving aircraft from another aircraft using the same runway by ensuring that the arriving aircraft does not cross the landing threshold until one of the following conditions exists [...]

  1. The other aircraft has landed and is clear of the runway. [...]

  2. The other aircraft has departed and crossed the runway end. [...]

According to AIM 4−3−20 b., The definition of being clear of the runway is not actually crossing the hold short line.

An aircraft is considered clear of the runway when all parts of the aircraft are past the runway edge and there are no restrictions to its continued movement beyond the runway holding position markings.

Except

Landing clearance can sometimes be given even if both aircraft remains on the runway.

At the time of landing traffic crossing the threshold, the following must be ensured if the preceding traffic is departing

If you can determine distances by reference to suitable landmarks and the other aircraft is airborne, it need not have crossed the runway end if the following minimum distance from the landing threshold exists:

(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. (See FIG 3-10-5.)

[...]

if the preceding traffic has landed.

Between sunrise and sunset, if you can determine distances by reference to suitable landmarks and the other aircraft has landed, it need not be clear of the runway if the following minimum distance from the landing threshold exists:

(a) When a Category I aircraft is landing behind a Category I or II− 3,000 feet.

(b) When a Category II aircraft is landing behind a Category I or II− 4,500 feet.

Note the difference between when preceding aircraft is a departure and when it is an arrival. The reason being that Category III aircraft (airliners) is not guaranteed to be able to stop within 6,000 ft and not hit the traffic ahead which is only at taxi speed.

Note that all of the above apply only to the issuance of landing clearance, takeoff clearance follows another set of rules that is quite similar but not all the same.

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