All conventional aircraft flights start at the point of departure with a take-off. In this phase, the aircraft is transferred from its stationary, ground-borne, state into a safe airborne state. Since the maneuver takes place in close proximity to the ground, and at low airspeed, there is relatively high risk to the safety of the aircraft. The maneuver must be carried out in a manner that will reduce the risk of an incident occurring to an acceptably low level of probability.
In the conventional take-off maneuver, the aircraft is accelerated along the runway until it reaches a speed at which it can generate sufficient aerodynamic lift to overcome its weight. It can then lift off the runway and start its climb. During the take-off, consideration is given to the need to ensure that the aircraft can be controlled safely and the distances required for the maneuvers do not exceed those available.
In this section, we will discuss take-off performance terminology, which mainly includes the definitions of some distances and airspeeds, and the methods to calculate “V” speeds and take-off power.
TAKE-OFF PERFORMANCE TERMINOLOGY
The space available for take-off is limited by the dimensions of the runway and the area beyond the runway in the take-off direction. The runway is defined as a rectangular area of ground suitably prepared for an aircraft to take off or land. At the end of the runway, there may be a stopway or clearway.
Clearway — a plane beyond the end of a runway which does not contain obstructions and can be considered when calculating take-off performance of turbine-powered transport category airplanes. The first segment of the take-off of a turbine-powered airplane is considered complete when it reaches a height of 35 feet above the runway and has achieved V2 speed (take-off safety speed). Clearway may be used for the climb to 35 feet.
For turbine-powered airplanes, a clearway is an area beyond the end of the runway, centrally located about the extended centerline and under the control of the airport authorities. Clearway distance may be used in the calculation of take-off distance.
Stopway — an area designated for use in decelerating an aborted take-off. It cannot be used as a part of the take-off distance but can be considered as part of the accelerate-stop distance.
A stopway is an area beyond the take-off runway, not any less wide than the runway, centered upon the extended centerline of the runway, and able to support the airplane during an aborted take-off.
Regulation requires that a transport category airplane’s take-off weight be such that, if at any time during the take-off run the critical engine fails, the airplane can either be stopped on the runway and stopway remaining, or that it can safely continue the take-off. This means that a maximum take-off weight must be computed for each take-off. Factors that determine the maximum take-off weight for an airplane include runway length, wind, flap position, runway braking action, pressure altitude and temperature.
In addition to the runway-limited take-off weight, each take-off requires a computation of a climb-limited take-off weight that will guarantee acceptable climb performance after take-off with An engine inoperative. The climb-limited take-off weight is determined by flap position, pressure altitude and temperature.
When the runway-limited and climb-limited take-off weights are determined, they are compared to the maximum structural take-off weight. The lowest of the three weights is the limit that must be observed for the take-off. If the airplane’s actual weight is at or below the lowest of the three limits, adequate take-off performance is ensured. If the actual weight is above any of the limits a take-off cannot be made until the weight is reduced or one or more limiting factors (runway, flap setting, etc.) is changed to raise the limiting weight.
After the maximum take-off weight is computed and it is determined that the airplane’s actual weight is within the limits, then V1 (take-off decision speed), VR (rotation speed) and V2 are computed. These take-off speed limits are contained in performance charts and tables of the airplane flight manual, and are observed on the captain’s airspeed indicator. By definition they are indicated airspeeds.
When the aircraft starts the take-off at rest on the runway, take-off thrust is set and the brakes released. The excess thrust accelerates the aircraft along the runway and, initially, the directional control needed to maintain heading along the runway would be provided by the nose-wheel steering. This is because the rudder cannot provide sufficient aerodynamic yawing moment to give directional control at very low airspeeds. As the airspeed increases the rudder will gain effectiveness and will take over directional control from the nose-wheel steering. However, should an engine fail during the take-off run the yawing moment produced by the asymmetric loss of thrust will have to be opposed by a yawing moment produced by the rudder. There will be an airspeed below which the rudder will not be capable of producing a yawing moment large enough to provide directional control without assistance from either brakes or nose-wheel steering or a reducing in thrust on another engine. This airspeed is known as the Minimum Control Speed, Ground, VMCG. If an engine failure occurs before this airspeed is reached, the take-off run must be abandoned.
During the ground run the nose wheel of the aircraft is held on the runway to keep the pitch attitude, and hence the angle of attack in the ground run, A g, is low. This will keep the lift produced by the wing to a small value so that the lift-dependent drag is minimized and the excess thrust available for acceleration is maximized. As the aircraft continues to accelerate, it will approach the lift-off speed, VLOF, at which it can generate enough lift to become airborne. Just before the lift-off speed is reached, the aircraft is rotated into a nose-up attitude equal to the lift-off angle of attack. The rotation speed, VR, must allow time for the aircraft to rotate into the lift-off attitude before the lift-off airspeed and becomes airborne; this is the end of the ground run distance, SG. The lift-off speed must allow a sufficient margin over the stalling speed to avoid an inadvertent stall, and a consequent loss of height. This may be caused by turbulence in the atmosphere or any loss of airspeed during the maneuvering of the aircraft after the lift-off. The lift-off speed will usually be taken to be not less than 1.2 VS1, where VS1 is the stalling speed of
the aircraft in the take-off configuration. This will give a lift coefficient at lift-off of about 0.7 CLmax and provide an adequate margin of safety over the stall, if the aircraft is over-rotated to a greater angle of attack at the rotation speed then lift-off can occur too soon and the aircraft start the climb at too low an airspeed. This can occur if, for example, the elevator trim control is set incorrectly or turbulence produces an unexpected nose-up pitching moment. The minimum speed at which the aircraft can become airborne is known as the minimum unstuck speed, VMU. It occurs when extreme overrotation pitches the aircraft up to the geometry limited angle of attack with the tail of the aircraft in contact with the runway. Tests are usually required to measure the take-off performance in this condition.
During the take-off run, should an engine fail between the minimum control speed (ground) and the rotation speed, the decision either to abandon or continue the take-off will have to be made. This decision is based on the distances required either to stop the aircraft or to continue to accelerate to the lift-off speed with one engine inoperative. There will be a point during the acceleration along the runway at which the distances required by the two options are equal. This point is recognized by the indicated speed of the aircraft and is known as the take-off decision speed, V1. The decision speed also determines the minimum safe length of runway from which the aircraft can take off. If an engine fails before the decision speed is reached, then the take-off is abandoned, otherwise the take-off must be continued.
Once the lift-off has been achieved the aircraft must be accelerated to the take-off safety speed (V2). This is the airspeed at which both a safe climb gradient and directional control can be achieved in the case of an engine failure in the airborne state; this phase of the take-off path is known as the transition. The ability to maintain directional control in the climb is determined by the Minimum Control Speed, Airborne, VMCA. The minimum control speed, airborne, will be greater than the minimum control speed, ground, VMCG, since the aircraft is not restrained in roll by the contact between the landing gear and the runway. In the event of an engine failure in the
climb, the aircraft will depart in yaw, which will cause the aircraft to roll and enter a spiral dive if the yaw cannot be controlled. The take-off is complete when the lowest part of the aircraft clears a screen height of 35ft above the extended take-off surface. The distance between the lift-off point and the point at which the screen height is cleared is known as the airborne distance, SA.
The total take-off distance required will be the sum of the ground nm distance, SG, and the airborne distance, SA. To ensure that the take-off is performed safely, the take-off distances will be suitably factored to allow for statistical variation in the take-off performance of the individual aircraft and in the ambient conditions.
V1 (take-off decision speed) is the speed during the take-off at which the airplane can experience a failure of the critical engine and the pilot can abort the take-off and come to a full safe stop on the runway and stopway remaining, or the pilot can continue the take-off, safely. If an engine fails at a speed less than V1, the pilot must abort; if the failure occurs at a speed above V1, the pilot must continue the take-off.
The take-off decision speed, V1, is the calibrated airspeed on the ground at which, as a result of engine failure or other reasons, the pilot is assumed to have made a decision to continue or discontinue the take-off V1 is also the speed at which the airplane can be rotated for take-off and shown to be adequate to safely continue the take-off, using normal piloting skill, when the critical engine is suddenly made inoperative. Vef is the calibrated airspeed at which the critical engine is assumed to fail. VEF must be selected by the applicant but must not be less than 1.05 VMC or, at the option of the applicant, not less than VMCG.
It is important to know that the critical engine failure speed is an obsolete term for V1 which is now called take-off decision speed.
VR (rotation speed) is the IAS at which the aircraft is rotated to its take-off attitude with or without an engine failure. VR is at or just above V1.
V2 (take-off safety speed) ensures that the airplane can maintain an acceptable climb gradient with the critical engine inoperative.
VMU (minimum unstick speed) is the minimum speed at which the airplane may be flown off the runway without a tail strike. This speed is determined by manufacturer’s tests and establishes minimum V1 and VR speeds. The flight crew does not normally compute the VMU speed separately.
V1 is computed using the actual airplane gross weight, flap setting, pressure, altitude and temperature. Raising the pressure altitude, temperature or gross weight will all increase the computed V1 speed. Lowering any of those variables will lower the V1 speed.
A wind will change the take-off distance. A headwind will decrease it and a tailwind will increase it. While a headwind or tailwind component does affect the runway limited take-off weight, it usually has no direct effect on the computed V1 speed. The performance tables for a few airplanes include a small correction to V1 for very strong winds. For those airplanes, a headwind will increase V1 and a tailwind will decrease it.
A headwind, in effect, gives an airplane part of its airspeed prior to starting the take-off roll. This allows the airplane to reach its take-off speed after a shorter take-off roll than in no wind conditions. High rotation speeds and lower air density (high density altitude) both have the effect of increasing the take-off distance.
A runway slope has the same effect on take-off performance as a wind. A runway that slopes uphill will increase the take-off distance for an airplane and a downslope will decrease it. A significant slope may require an adjustment in the V1 speed. An upslope will require an increase in V1 and a downslope will require a decrease. An uphill runway will have the effect of decreasing an airplane’s rate of acceleration during the take-off roll thus causing it to reach its take-off speeds (V1 and VR) further down the runway than would otherwise be the case. An uphill runway will also necessitate an increased V1 speed in some airplanes.
If there is slush on the runway or if the antiskid system is inoperative, the stopping performance of the airplane is degraded. This requires that any aborted take-off be started at a lower speed and with more runway and stopway remaining. This means that both the runway-limited weight and the V1 used for take-off be lower than normal.