Let's say an commercial passenger jet is accelerating down the runway, taking off. The ATC receives some signal that he/she should cancel all takeoff clearances.
What happens in the control room?
Aborting a take-off at high speed is dangerous, even if the aircraft is travelling below V1. If the aircraft is already rolling, the controller will make a quick decision whether the situation justifies an aborted take-off. For example, if there is traffic infringing the runway and a crash is imminent the controller would attempt to stop the aircraft. If an aircraft is attempting to take-off from a taxiway in snowy conditions it might be safer to just let him take-off unless there is other traffic on the taxiway.
If the aircraft hasn't started rolling the decision is of course easily made.
What does the ATC do and how does he/she make sure pilots understand the cancellation?
Using standard phraseology is the best way of making sure the message comes across, especially in international aviation.
If the aircraft is not yet rolling, the proper ICAO phraseology is:
Tower: Big Jet 345 hold position, Cancel take - off, I say again cancel take - off (due to vehicle on the runway)
Big Jet: Holding, Big Jet 345
If the aircraft already started rolling the controller would say:
Tower: Big Jet 345 stop immediately, (Big Jet 345 stop immediately )!
Big Jet: Stopping, Big Jet 345
No immediate verbal reply is to be expected as the flight crew have other things to care about at that moment. The effect of their actions should be clearly visible from the tower.
How do the pilots in the aircraft taking off respond? Are they trained for this scenario?
The crew will assess whether they can stop safely. Below V1, they will probably abort the take-off, above probably not. The final judgement is made by the captain. He might reason that it is safer to get airborne and pass the infringing traffic overhead than running into it at full brakes. They are trained for aborted take-offs, but since there are so many non-standard situations the final decision is made on the basis of a mix of training and expert judgement.
Is a takeoff cancelled even after V1?
Since ATC does not know when the aircraft passes V1, a controller might attempt to cancel the takeoff after V1. The flight crew will only attempt to cancel the take-off after V1 if it is clear that there is no way the aircraft can become - and stay - airborne safely. In such a case it becomes damage control rather than damage prevention.
Tower will tell the pilot that take off clearance is revoked and the pilot will hit the brakes hard in the standard abort takeoff procedure unless the plane already passed V1 then he will continue the takeoff while hoping the reason won't hit the plane.
After V1 the plane is committed to taking off because it won't be able to stop in time before the end of the runway.
Your quote suggests that AM-18 aborted take-off because of ATC instruction, but if you read the article carefully it's not the case.
The AM-18 was accelerating for take-off when the crew noticed something is wrong with the tires (probably blown tire already) and aborted just below V1 due to their own decision.
- Exactly what happens in the control room?
I don't know exactly, but there are various sensors designed to warn of possible runway incursions. Apparently the sensors detect aborted take-off and alerted the controller.
In some places, including USA, the controllers will clear aircraft to take-off or land when they expect the runway will be clear by the time the aircraft reaches it. So they have to cancel that clearance when there is an incident and the previous aircraft does not vacate the runway as happened here. In other places, e.g. Europe, they don't do this, so they don't have to cancel any clearances, but the pilots are often receiving their clearances at the last moment which has it's own disadvantages.
- What does the ATC do and how does he/she make sure pilots understand the cancellation?
They cancelled clearance of the next flight that was just about to line up behind AM-18. I believe "cancel" is standard wording, so something like "tower, XYZ, take-off clearance cancelled; line up and hold." They know the pilot understood since the pilot reads back the instructions in reply.
If the aircraft has already commenced take off roll (which didn't happen here), time becomes critical, so then the instruction is just "XYZ, stop immediately", often repeated twice, or until the aircraft starts to brake or the pilots answer.
It does happen now and then. See for example this incident.
- How does the pilots respond? Are they trained for this scenario?
If the clearance is changed, the pilots simply follow the new instructions.
If "stop immediately" is requested, pilots abort the take off if possible.
If the pilot does see the problem (nobody will request aircraft in take-off roll to stop unless there is a serious risk of collision), which they often do, they may decide to continue the take-off if it's more likely they'll lift off before the obstacle than that they stop before it. Captain is the final authority responsible for safety of the aircraft, so it's their call.
- Is a takeoff cancelled even after V1?
No, because the aircraft is no longer able to stop on the remaining runway. Instead the pilot may rotate earlier. The Vr is higher than Vmu, so they can rotate at slightly lower speed and they can increase thrust if they are doing reduced thrust ("flex") take-off, which they now usually do.
For example in this incident ATC instructed the crew to abort (but failed to follow correct phraseology). At the same time the crew noticed the problem themselves, but they were already above V1 and decided they can fly over the obstacle and not stop before it and therefore continued the take-off.
Note that at V1 the aircraft is already long way down the runway, so the obstacle is likely near the end of the runway and likely can be already seen by the pilot, so the pilot decides how they try to avoid the collision.
A takeoff clearance cannot be revoked. Once you issue a clearance the aircraft is CLEARED for takeoff.
If the tower realizes that something that will obstruct or threaten an aircraft taking off is present, all they can do is inform the PIC of the situation. At that point it is up to the PIC to figure out what to do. They might try to abort the takeoff or perform an evasive maneuver.
To clarify you this for you: a tower cannot tell an aircraft to abort a takeoff because they are not qualified to make that determination. The guy in the tower neither is qualified nor can see the instruments of the aircraft so they have absolutely no idea whether it is safe to abort a takeoff or not. Therefore, they will never tell a pilot to abort a takeoff once it is underway. All they can do is cancel the clearance (See Section 9. Departure Procedures and Separation in FAA ATC guidelines). The phraseology is:
[Call sign] Cancel Takeoff Clearance [state reason]
Of course, if the aircraft has reached V1 it will not abort.
In situations I have been in where something bad has happened is that the tower will just tell the pilot what to do. Like if some plane tries to land the wrong way on a runway when you are taking off, the tower will say "[call sign] Bear right immediately, traffic dead ahead!" So, you basically try to veer to the right to avoid the head on collision.
To answer your specific questions:
Exactly what happens in the control room?
In a crisis scenario, the tower guy will say whatever he/she thinks is necessary to avoid a collision. Once I had a near collision because another guy was not where he supposed to be, but I was the one with comm, so she moved me by screaming "TURN LEFT RIGHT NOW !!!" Phraseology goes out the window when you are 0.5 seconds away from a huge fireball with multiple fatalities.
What does the ATC do and how does he/she make sure pilots understand the cancellation?
(I gave the phraseology above.)
How do the pilots in the aircraft taking off respond? Are they trained for this scenario?
If you can safely abort, you abort and slam on the brakes. If not, you keep going and try to maneuver around whatever it is, moose, truck, whatever. Pilots are trained to never abort when they reach V1. Other than that it is experience and nerves that count, no more training.
Is a takeoff cancelled even after V1?
Cancelling is what the tower does. Aborting is what the pilot does. The tower cannot "cancel after V1" because they have no way of knowing if the plane is at V1 or not. A pilot will almost never abort after V1 because the consequences of going 100 miles an hour off the runway are almost always worse than the consequences of going 100 mph in the air. If an unavoidable obstruction occurs, like a plane rolls onto the runway in front of you, and you are at V1 you still takeoff, you just try to do it sooner.
