Interdependency of Take-off speeds

It is important to know the take-off speeds and the purpose behind them. Most of the speeds are defined in the CAP 698 in section four and I would advise all students to have a working knowledge of these speeds. We will look at a typical Class A departure from brake release to the end of the Take-off, which is achieved when the aeroplane attains the screen height.

The first speed the aeroplane must exceed during departure is Vmcg, the reason being, is that this speed guarantees the crew directional control, using the rudder alone, in the event of an engine failure and full power/thrust on the remaining engines. Obviously if we reject the take-off at this point then Vmcg is not required. But if we continue the departure, which we will be forced to do shortly, then we do have directional control of the aeroplane on the ground.

The next speed that is reached is Vef, this is the assumed speed of engine failure. This is a test pilot speed, that is set by the manufacturer, but must occur at least one second before V1. The purpose of this speed is to prove to the authorities and the pilot community that when we decide to continue the take-off after engine failure, which we must, once V1 has been called, the aeroplanes performance is sufficient to achieve the take-off in the take-off distance available.

This is followed by V1, which is our decision speed, although in reality it is the final point at which we can reject the take-off in the event of a major failure. The speed must be equal to or greater than Vmcg, equal to or less than Vmbe, and Vr.

Following V1 is Vmbe, which is the maximum brake energy speed. When brakes are applied, they convert the kinetic energy of the aeroplane into heat energy within the braking system. The brakes will only absorb so much heat before the fade and fail. So the maximum speed attainable before this occurs is Vmbe, and this speed must be equal to or greater than V1.

Vmu, is Velocity Minimum Unstick, this speed is obtained during flight testing by the test pilots. The aeroplane has its aft end strengthened and the test pilots accelerate along the runway until they have sufficient control to rotate the aeroplane to its maximum. This attitude is maintained until the main wheels leave the ground and that is minimum unstick speed. The purpose of this is to ensure that when a line pilot rotates the aeroplane at Vr, there is sufficient lift being generated using normal pitch attitudes for the aeroplane to leave the ground.

As we continue down the runway, we are approaching the point the aeroplane will become airborne and because of this the aeroplane must exceed Vmca. This guarantees that when we become airborne, we have directional control in the event of an engine failure.

Vr is the speed where the operating pilot will pull back on the controls to achieve a pitch attitude of 10-15 degrees, increasing the angle of attack and the value of lift so that lift equals weight and the aeroplane becomes airborne. It is important that we call the correct speed, and the rotation technique is correct, so that the aeroplane becomes airborne within the take-off distance available.

Shortly after rotating the aeroplane, we will become airborne and this occurs at Vlof, or lift-off speed. Vr should a speed be such that Vlof, is achieved at a minimum of 1.1 Vmu and 1.05 Vmca.

The next speed is Tyre Speed, and we must not exceed this speed on the ground or damage may occur to the tyres.

V2min/V2 follows which we should achieve no later than the screen height, which is 35 feet for a Class A performance aeroplane. This speed must be equal to or greater than 1.13 Vsr for 2 and 3 engine turboprops and all turbojets without provision for obtaining a significant reduction in the one engine inoperative power-on stalling speed. Or 1.08 Vsr for turboprops with more than 3 engines and turbojets with provision for obtaining a significant reduction in the one engine inoperative power-on stalling speed. It must also be equal to or greater than 1.1 Vmca.