A rejected takeoff is the decision to stop an aircraft during the takeoff roll instead of continuing into flight. The most important reference speed for this decision is V1, often called the takeoff decision speed. Before V1, the crew is expected to stop for serious problems because there is enough planned runway to brake safely.
After V1, continuing the takeoff is usually safer because there may not be enough runway left to stop.
Understanding Aviation: The Rejected Takeoff
V1 is not a permanent number painted on the aircraft. It is calculated for each departure using the aircraft mass, runway length, runway slope, air temperature, airport elevation, wind, flap setting, and available engine thrust. A hot day or a high airport reduces engine and wing performance, so the speeds may change.
Water, slush, snow, or ice can greatly reduce braking grip. The crew receives the calculated speeds before departure, then includes them in the takeoff briefing. One pilot normally flies the aircraft while the other monitors instruments, calls important speeds, and watches for warnings or abnormal signs.
The stop itself is a carefully managed event, not simply hard braking. The pilot closes the thrust levers, applies maximum approved braking, deploys ground spoilers, and uses reverse thrust when appropriate. Ground spoilers reduce lift, placing more of the aircraft weight on the wheels.
This improves tire grip and makes wheel brakes more effective. Anti skid systems prevent the wheels from locking, much like anti lock brakes in a car. Brake energy is a major limit.
As speed rises, the energy stored in the moving aircraft rises with the square of speed. Brakes can become extremely hot after a high speed stop, so crews may need to wait before taxiing or approaching the wheels.
Takeoff performance planning compares two demanding cases. In one case, an engine problem occurs and the aircraft stops. In the other case, an engine problem occurs and the aircraft continues, lifts off, then climbs away safely on the remaining engine or engines.
The selected decision speed helps make both cases fit the runway and obstacle limits. This is why the number depends on conditions rather than being a simple rule such as a fixed speed for every flight. The calculations include a short recognition period before braking begins.
They use approved aircraft data and conservative safety margins. A runway may have extra paved area beyond its normal end, called a stopway, which can be included in some performance planning.
Students should notice that pilots do not stop for every small fault at high speed. A minor message, a nuisance warning, or an issue that can be handled after takeoff may not justify a stop near the decision speed. A fire warning, major engine failure, severe control problem, or unsafe takeoff configuration demands immediate attention.
The exact procedures come from the aircraft handbook and the airline or operator. Directional control is especially important because strong braking and uneven reverse thrust can pull the aircraft off its path. After stopping, the crew assesses fire risk, brake temperature, and the need for evacuation.
The key lesson is that the decision is prepared before the aircraft moves. Good planning gives the crew a clear action when time is short.
Key Facts
- V1 is the maximum speed at which the pilot can begin a rejected takeoff and still stop within the available runway under certified assumptions.
- Before V1, serious failures such as engine failure, fire warning, or unsafe configuration can justify stopping.
- After V1, the aircraft is normally committed to takeoff because stopping distance increases rapidly with speed.
- Kinetic energy to remove during braking is KE = 1/2 mv^2, so doubling speed makes the brakes absorb four times as much energy.
- Stopping distance under constant deceleration can be estimated by d = v^2/(2a).
- Accelerate-stop distance must be less than or equal to accelerate-stop distance available, often written ASDR ≤ ASDA.
Vocabulary
- Rejected takeoff
- A rejected takeoff is the planned emergency procedure of stopping an aircraft during the takeoff roll before liftoff.
- V1
- V1 is the takeoff decision speed used to decide whether to stop or continue the takeoff after a serious problem.
- Accelerate-stop distance
- Accelerate-stop distance is the runway distance needed to accelerate to a decision speed, recognize a failure, and stop the aircraft.
- ASDA
- Accelerate-stop distance available is the runway length plus any approved stopway that can be used for stopping after a rejected takeoff.
- Balanced field
- A balanced field is a runway planning condition where the distance to continue takeoff after an engine failure equals the distance to reject and stop.
Common Mistakes to Avoid
- Treating V1 as the speed where the pilot decides what to do is wrong because the reject action must be started by V1, not considered after passing it.
- Assuming stopping after V1 is always safer is wrong because the remaining runway may be too short and the brakes may overheat from the high kinetic energy.
- Ignoring reaction time is wrong because the aircraft continues accelerating and moving while the crew recognizes the failure and begins braking.
- Using runway length alone without considering weight, wind, temperature, slope, and runway condition is wrong because each factor changes acceleration and stopping performance.
Practice Questions
- 1 A 70,000 kg jet reaches 70 m/s just before a rejected takeoff. How much kinetic energy must the brakes, spoilers, reverse thrust, and drag remove to stop it? Use KE = 1/2 mv^2.
- 2 An aircraft begins braking at 68 m/s with an average deceleration of 3.4 m/s^2. Estimate the stopping distance using d = v^2/(2a).
- 3 A crew detects an engine fire warning just after passing V1 on a dry runway. Explain why continuing the takeoff may be safer than trying to stop, using runway distance and kinetic energy in your answer.