Aircraft Speeds during Takeoff

For the aircraft, characteristic speeds are denominated with a capital V entailed by a subscript, they are referred to as V-Speeds. V-Speeds are provided to the operator in Indicated Air Speed (IAS) or Calibrated Air Speed (CAS), according to the speed displayed on his speed indicator. During the takeoff roll in normal, AEO conditions, there are four major V-Speeds that need to be considered for the performance calculation. These are the Takeoff Decision Speed V1, the Rotation Speed VR, the Liftoff Speed VLOF and the safe climb speed V2. In OEI conditions, the speed at which the engine fails, VEF, is another very important parameter. V1 is the highest speed at which a takeoff can still safely be aborted and represents one of the most important speeds in the takeoff performance determination.

There are a number of other relevant V-Speeds which are acting as limitations due to aerody-namic or mechanic properties of the aircraft. In fig: 3.1, they are indicated with a red band for the mechanical and with a blue band for the aerodynamically limiting speed regions.

4 This refers to the fact that older aircraft may still be certified under older regulatory requirements if the original aircraft has been certified according to older standards applicable at the time of initial certifica-tion.

Fig. 3.1 Takeoff Speeds and ground distances in AEO condition (Airbus 2002)

From a mechanical point of view, one limiting speed is the maximum tire speed VTIRE, justi-fied from the maximum centrifugal forces on the aircraft tires. Another limitation is the Max-imum Brake Energy Speed VMBE. This is the takeoff mass dependent maximum speed up to which the aircraft brakes are able to dissipate the kinetic energy of the airplane in a stopping case. If the aircraft is faster and has kinetic energy higher than the maximum brake energy limit, the aircraft braking system cannot sustain sufficient braking action to bring the aircraft to a complete stop. As a consequence, the highest speed at which a takeoff could be aborted is VMBE, an upper boundary for V1. CS-25.109 specifies that a flight test demonstration of the maximum brake kinetic energy accelerate-stop distance must be conducted with no more than 10% of the allowable brake wear range remaining on each of the wheel brakes.

Aerodynamically justified characteristic V-Speeds have to be flight-tested, they will be found in the AFM of a certified aircraft. They can be approximated as a function of the stall speed of the aircraft at 1g vertical acceleration, VS,1g or VS. The estimation for VLOF is presented ac-cording to Roskam VII, the VR and V2 estimations according to GJE EXTGFD-003.

(3.1)

(3.2)

(3.3)

Also, the minimum control speeds have to be determined in flight test. CS-25.149 specifies the VMC as follows:

VMC is the calibrated airspeed, at which, when the critical engine is suddenly made

inoperative, it is possible to maintain control of the aeroplane with that engine still inoperative, and maintain straight flight with an angle of bank of not more than 5º.

The stability properties of the aircraft and the means to stabilize the aircraft on the ground are different to those in the air; therefore there is a distinction between VMCG (ground) and VMCA

(air). Due to this stability criterion, the rotation speed VR can never be lower than VMCA. Likewise, the decision speed V1 can never be lower than VMCG. The aircraft in an OEI condi-tion would not be able to sustain stable condicondi-tions. This is of importance for the takeoff per-formance calculation when the V1 and VR speeds coincide with the VMC requirements, usually for low takeoff masses.

The Minimum Unstick Speed VMU shown in Fig. 3.1 is the lowest speed at which the aircraft can get airborne. However, due to the large Angle of Attack (AOA) necessary, the lift-to-drag ratio is unfavorable and not optimal to clear the obstacle height at minimum distance.

The following procedure describes a takeoff in AEO conditions as shown in Fig. 3.1. The aircraft will accelerate from the brake release point passing the Takeoff Decision Speed V1

towards the rotation speed VR. The nose of the aircraft is rotated (about 3°/s⁵) and the aircraft lifts off at liftoff attitude at the speed VLOF and climbs out accelerated. The objective is to pass an obstacle at a specific height at the end of the runway at the safe climb speed V2. In case of a wet runway, when the required obstacle clearance height is reduced to give a performance benefit, the V2 can still be reached at the original obstacle clearance height defined for dry runways.

In OEI conditions, an engine is assumed to fail at a specific speed VEF as defined in CS-25.107. Should this critical malfunction occur before the decision speed V1, the pilot will take action to abort the takeoff run. Should the engine fail after passing V1, the pilot will not abort the takeoff run because the remaining stopping distance would not be sufficient, and continue the takeoff under OEI conditions.

The following definition from CS-25.107 serves as a baseline for the further discussions and calculations of the OEI condition in this report. It marks the difference between VEF and V1.

(a)(2) V1, in terms of calibrated airspeed, is selected by the applicant; however, V1 may not be less than VEF plus the speed gained with the critical engine inoperative during the time interval between the instant at which the critical engine is failed, and the instant at which the pilot recognises and reacts to the engine failure, as indicated by the pilot's initiation of the first action (e.g. applying brakes, reducing thrust, deploying speed brakes) to stop the aeroplane during accelerate-stop tests.

5 this pitch rate is a common orientation but may vary for specific aircraft and/or configurations

The most critical VEF is so close before V1 that the engine failure recognition occurs exactly at V1. This is the most critical and highest speed, because at V1 both GO or STOP decisions co-incide and the aircraft will either have to accelerate with OEI from this lowest speed to pro-ceed with a takeoff, or brake from this maximum reject speed down to a full stop. V1therefore defines the boundary between the GO and the STOP case and its knowledge therefore is of major importance in the takeoff preparations and will be determined in this report. In contrast to the other speeds discussed, it is also dependent of the runway length available.