Xjyuk

There is another scenario other than "line up and wait" that the other two answers cover...

A "flight" of more than one aircraft may be cleared as a group to take-off. This can consist of many aircraft sharing the runway during take-off (and even landing).

So you can have multiple aircraft on the runway when:

• A "line up and wait" order is issued to line up on the runway after another aircraft takes-off or lands. This is usually done so that the departing aircraft can get prepared while waiting for wake-turbulence to clear.


• ATC clears multiple aircraft as a "flight". The aircraft must be part of a coordinated group and a single aircraft is the "lead" aircraft for the flight that receives/coordinates with ATC for clearances. The entire flight group will enter the runway and take off in a pre-coordinated fashion.

There are specialized scenarios as well, for example during Airventure Oshkosh. Aircraft will be required to "land on the dot" and multiple aircraft may be on the runway for take-off at the same time, although ATC coordinates them so that they don't depart at the same time, they are asked to line up "on a dot" (a colored circle on the runway) and will then be cleared to depart from there.

As an extreme example:

116 aircraft lined up on the runway and cleared for take-off at the same time.

Yes. It's a Minimum Runway Occupancy Time procedure, and is applied at specific airports after safety and layout assessments.

One such airport is London Heathrow. Below is from the UK AIP:

(...) should remain behind the subject aircraft but may cross the runway holding point (subject to there being no illuminated red stop bar) and enter the runway upon receipt of the clearance. There is no requirement for the subject aircraft to have commenced its take-off roll before entering the runway. Pilots must be aware that there may be a blast hazard as the aircraft on the runway applies power.

(Emphasis mine.)

Yes, it is possible, under some circumstances. However, your general understanding that only one aircraft is allowed on a runway at one time is correct most of the time.

There are a few obvious cases, also covered in the other answers. For instance, an aircraft can be allowed to cross or taxi on a runway behind another aircraft which is taking off or landing. An aircraft waiting to depart can also be cleared to enter/line up on a runway behind another aircraft taking off or landing. These examples are pretty simple, because only one aircraft is actually using the runway for takeoff or landing, the other is just taxiing behind. But actually, two aircraft can takeoff or land on the same runway at the same time in accordance with the below, and as stated in ICAO Doc 4444 section 7.10 on Reduced Runway Separation.

1. An aircraft can land on a runway while another aircraft in front of it is also landing, provided that the first aircraft has touched down, is in motion and will vacate the runway without backtracking (doing a 180 on the runway).




2. An aircraft can land on a runway while another aircraft in front of it is taking off from that runway, provided that the first aircraft is airborne.




3. An aircraft can take off from a runway while another aircraft in front of it is also taking off, provided that the first aircraft is airborne (but still over the runway, and thus not clear of the runway yet).

ICAO suggests some pretty strict requirements in order to use these rules, such as the exact distance needed between the aircraft involved, as well as the type of aircraft, runway length etc. However, some states have pretty lenient rules. In Denmark, for example, it is largely up to the air traffic controller on duty to decide if conditions are safe to use reduced runway separation. The phraseology used is a little different as well:

1. Instead of the usual "cleared to land", the phraseology used is "Land behind preceding landing (aircraft type)". The second aircraft will obviously have been informed of the first one, and weather conditions must permit the pilot to see the first aircraft at all times.




2. Similar to 1, the phraseology is "Land behind preceding departing (aircraft type)".




3. A normal "cleared for takeoff" is used, but the pilot of the second aircraft must have been informed of the preceding aircraft and have it visually in sight.

I've seen this often in Atlanta. An aircraft will start its takeoff roll, with the next aircraft behind the line. Once rolling aircraft is about halfway down the runway, the next aircraft will slowly start to move onto the runway.

If you're on board on the side you'll be taking off on, you can typically watch the preceding aircraft as it reaches V1 and rotates, just before your aircraft finishes its turn to line up with the runway.

If a runway has multiple crossing taxiways then ground control can clear several aircraft to cross the runway at the same time.

If a runway is not being used for landings or takeoffs (in favor of another runway) then it can be used like any other taxiway (other than the clearance and read-back requirements). On busy airports during peak hours this could mean the runway can be use as a holding point for aircraft waiting for their gate to become available.

I have seen that once on an international airport which is also a military base. 4 large Helicopters rolled onto the runway and took off in a short sequence.

Historically, scrambling military aircraft would involve several planes taking off simultaneously in formation, so this guaranteed more than one aircraft on the runway at once. In addition, the next formation to take off was generally lined up and ready to go behind them. For interception duties, this enabled as many planes as possible to take off in as short a time as possible, giving the pilots an invaluable few extra minutes to gain height in preparation. This can be clearly seen in this Pathe footage, where Spitfires and Hurricanes launch in formations of three aircraft.

Strategic bombers adopted (and still use) a similar technique called minimum takeoff interval, designed to get nuclear bombers airborne as rapidly as possible for a retaliatory counterstrike in the event of attack by nuclear missiles. With the high turbulence from preceding aircraft, takeoff does become very much more risky, but it remains standard practise.

Having more than one aircraft on the runway at once is therefore entirely familiar for military ATC.