Why can't aircraft be cleared to land before the preceding aircraft turns off the runway?

Question

For example, let's say we have an Airbus A330 in front, which has just landed, well past the threshold and looks like it's going to be alright. Next in line to land is a small general aviation aircraft, such as a Cessna 172.

Now it looks like there's little to no risk of the two aircraft colliding and there's already more than enough space to stop the aircraft, so why can't a controller use their judgement to be able to clear the aircraft behind to land? It seems like that it's a waste of time when you know that there's going to be more than ample time for the previous aircraft to turn off, so what's the problem with that?

Answer 1

Actually, the FAA doesn't always require the preceding airplane to clear the runway if they have the required separation unless it is dark (when it is very hard to visually estimate distances).

According to the FAA Air Traffic Control Order Section 10. Arrival Procedures and Separation:

3-10-6. ANTICIPATING SEPARATION

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.

EXAMPLE- “American Two Forty-Five, Runway One-Eight, cleared to land, number two following a United Seven-Thirty-Seven two mile final. Traffic will depart prior to your arrival.”

“American Two Forty-Five, Runway One-Eight, cleared to land. Traffic will depart prior to your arrival.”

NOTE- Landing sequence number is optional at tower facilities where the arrival sequence to the runway is established by the approach control.

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.

EXAMPLE- “American Two Forty-Five, Runway One-Eight, cleared to land. Traffic will be a Boeing Seven-Fifty-Seven holding in position.”

And the separation is specified in the same document:

3-10-3. SAME RUNWAY SEPARATION

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 or unless authorized in para 3-10-10, Altitude Restricted Low Approach.

1. The other aircraft has landed and is clear of the runway. (See FIG 3-10-1.) 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.

Answer 2

People have assumed that you are referring to US operations. But in the UK and Australia (and possibly many more countries), ATC can not give 'conditional' landing clearances. Clearance to land in these countries is not given until the runway is clear. The US ATC can give "cleared to land number 2 behind the 737 on final" but you won't hear that where I live (down under).

However, unlike in the USA, the UK and Australia (and again, possibly others), ATC may give a clearance of "Behind the inbound 737 line up and wait, behind". You can make safety arguments against either of these procedures, it's just a quirk of the international system that there are different philosophies in different places.

Answer 3

Although this is obviously not real-world, VATSIM does try to mimic it as closely as possible.

London Luton's VMATs has this to say:

3.2.6 Land after procedures

Normally, only one aircraft is permitted to land or take-off on the runway in use at any one time. However, when the traffic sequence is two successive landing aircraft, the second one may be allowed to land before the first one has cleared the runway in use, providing that:

• It is during daylight hours

• The preceding aircraft does not intend to backtrack in order to vacate

• There is no evidence that braking action may be adversely affected

• The second aircraft will be able to see the first aircraft clearly and continuously until it is clear of the runway

• The second aircraft has been warned

In place of the standard "cleared to land", the instruction "Land after (first aircraft type)". The responsibility for ensuring separation then lies with the pilot of the second aircraft

Answer 4

The Doc4444 (google it) is the bible of air traffic control. It is the base for all international standards of separation, and in section 7.9.1 "Separation of landing aircraft and preceding landing and departing aircraft using the same runway" we read:

Except as provided in 7.10 and Chapter 5, Section 5.8, a landing aircraft will not normally be permitted to cross the runway threshold on its final approach until the preceding departing aircraft has crossed the end of the runway-in-use, or has started a turn, or until all preceding landing aircraft are clear of the runway-in-use.

Now there are some national deviations from this in for example the US, but the basic principle is for a controller to NOT put him/her self in a monitoring situation.

If the controller were to clear nb2 for landing while nb1 is still on the runway, and something happens to nb1 that unables nb1 from vacating the runway, the controller is in a hurry to restore separation by cancelling the landing clearance to nb2.

If the controller instead follow the doc4444 and don't clear nb2 for landing, and something happens to nb1, the controller can do whatever he needs to do to help nb1 (send out fire trucks or whatever the problem demands), and IF the controller forgets to instruct nb2 to go around, that should be the default behavior of nb2 anyways since he never got a clearance to land (if no landing clearance is received when reaching the "missed approach point" the pilot will initiate a go-around). That way, separation was maintained the whole time and needed not to be restored.

Controllers are trained to always take risk into consideration, and eliminating it whenever possible. In the case of your cessna, depending on the experience of the pilot (many small planes are flying for schooling purposes) he may not touch down in the touchdown zone, especially if there is a crosswind. Students sometimes make a few bounces before settling on the runway etc.

Your reasoning saying that "there would be more than enough space" could also be used to argue for allowing a departure with a smaller aircraft when a larger one is way down the other end of the runway, "the little one would be airborne long before the big one anyways", right? But a controller will think "IF the little one has an engine failure (yeah, the small ones usually only have one engine...) on initial climb, he will glide back down on the runway. So we don't allow any type of obstacle on the runway in front of a departure either, no matter the size =)