Aborted takeoff

What happens when? Aborted Takeoff

Tim Takeoff
02.11.2020
5 minutes

For many, the takeoff is the most exciting part of the flight. The feeling of being pressed into the seat, the roar of the engines and the tingling in the stomach. But what happens if the takeoff must be aborted?

When the thrust levers are moved forward, the crew is faced with many enormously important tasks in a very short time. We have already gone into this in more detail in our article on aircraft takeoff. Many parameters have to be monitored, because the launch can be aborted at any time under certain circumstances and up to a certain speed. This abort speed is called V1.

The power of decision lies with the captain

In most cases, the decision to abort is borne exclusively by the captain. The first officer can shout out problems aloud, the commander then decides whether to continue (“Continue!”) or to stop (“STOP!”), which he must clearly shout out. He has a difficult task, because he weighs up all criteria for an abort before taking a decision. In case of major problems, such as an engine failure, fire or a burst tire, the decision is made quickly. Things become more complex with errors that are not immediately obvious.

Criteria for aborted takeoff

Since takeoff is known to be very loud and very shaky and everything happens very fast, it is often not easy to detect minor errors at first sight. However, the aircraft systems help you very much in this respect. Almost all components are monitored and can trigger a warning if they are not functioning. For less critical things there is a “master caution” (yellow): here you are called to be careful. However, it can be disregarded by the crew if it is a known problem that does not jeopardize the takeoff. However, if a “master warning” (red) occurs, the takeoff must be aborted. It only triggers in case of critical problems and should always be considered more closely.

No chaos on the runways

There are also situations in which not the crew but the air traffic controller on the tower prescribes a takeoff abort. Mistakes are human, so even in a very critical area, such as a runway, mistakes and misunderstandings can occur. At many airports, several runways and taxiways intersect. If a pilot misunderstands something and, despite a stop instruction, suddenly rolls over the stop mark to cross the runway, the controller or pilot must react quickly. In bad weather and fog, the pilot has to rely on the controller. He has displayed all taxiing traffic on a ground radar screen. It is nevertheless a good habit among pilots to switch off the taxiing lights while standing at the taxiing stop, e.g. at night, to signal to everyone: Yes, I have understood the instruction and I will stop.

STOP! – aborted takeoff

If an incident does occur and the captain decides to abort, he must clearly declare this. Simultaneously, he suddenly pulls the thrust lever to idle and activates the reverse thrust. This is the system trigger for numerous things that now happen very quickly, and must happen.

Autobrake System

Pulling the thrust lever out during aborted takeoff activates the automatic brake system (“Autobrake System”) at maximum level. The hydraulic brakes, which are located inside the rims, give full pressure on the brake discs at that moment. The “antiskid system”, similar to the ABS of a car, controls the brake so that the wheels do not lock, because this would cause the aircraft to lurch. In addition, the air brakes on the upper side of the wing are activated. The negative acceleration is similar to that of an anchor throw. After the aircraft comes to a standstill, the tower is informed of the abort, if it has not already detected it of its own accord. The pilots then decide on a new takeoff attempt, or in case of smoke and fire, on an immediate evacuation on the runway.

Energy conversion during aborted takeoff

During braking, a large part of the energy is initially stored by the brakes. Only after the brakes have come to a standstill does this energy convert back into heat within the brake materials. The maximum temperature inside the brakes is reached approximately 10 minutes after standstill. This delay is a certification requirement so that the jet could still roll off the runway safely after a takeoff abort. Because a technical “predetermined breaking point” is imminent.

When the tires “blow off”

This is what pilots call it when the brakes have absorbed so much energy and converted it into heat that the pressure in the surrounding tires increases. For safety reasons, the tires of airplanes are not filled with air as is usual in cars. Instead, nitrogen is used because it is not flammable and, unlike the air we breathe, does not contain water that could freeze at high altitude.

The tires are equipped with melting valves that release all the nitrogen at a certain temperature. This has the effect of reducing the pressure inside the tire, but also has the advantage of cooling the red-hot brakes. Nevertheless, this is no reason for the fire department on the ground not to be particularly careful.

One pit stop, please!

If the tires are too hot and have been blown off, they must be replaced. At this moment, the aircraft initially comes to a halt. However, if a takeoff abort is necessary, then a set of new brakes and tires is probably the least of the problems. Aborted takeoffs are also practiced again and again in the flight simulator.

A very impressive video about an aborted takeoff at maximum speed and maximum weight during the certification of Boeing’s B787 Dreamliner. You can see very well the glowing brakes and the blowing off of the tires:

by Tim Takeoff

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