DALLAS – Even before you have set foot on board the aircraft that is flying you to your destination, a plethora of calculations will have been performed by the pilots that are flying you. The computations relating to takeoff performance, which are conducted prior to each flight, are at the heart of these.
Previously, such calculations were performed using cumbersome paper charts that required interpolation to extract the required data. Nowadays, such calculations are performed quickly and very accurately using tablets or laptops on board.
No two flights are alike when it comes to takeoff performance. Factors that influence performance include aircraft weight, environmental conditions, and runway characteristics. The essential information that a pilot needs to know before takeoff is how much engine thrust should be used, how much flap should be deployed, and finally, what the takeoff speeds should be.
The use of the term speeds in its plural form may surprise you. Before beginning the takeoff run, pilots must be familiar with three different speeds. These are known as V1, VR (Rotate), and V2. V1 is the decision speed at which the pilot is committed to taking off regardless of what happens. VR is the speed at which the aircraft can be rotated and allowed to lift off from the runway. Finally, V2 is the minimum safe airspeed during the climb out if an engine fails after V1.
A heavier aircraft will result in all three speeds increasing. There is another interesting factor that can influence V1 when the runway is wet. Because stopping distance is likely to be greater on a wet runway than on a dry runway, a more noticeable split between V1 and VR is common in wet conditions. This provides more stopping distance at the most crucial point, which is at V1, after which a take-off can no longer be aborted.
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Less Is More
Since aircraft engines are expensive and complex components, it makes engineering sense to use them as efficiently as possible. Consequently, most takeoffs are performed at less than full power. Due to the calculations that are performed prior to takeoff, the engines are frequently configured to apply just enough thrust to meet the minimum performance requirements on takeoff. The two methods for determining how much thrust should be used are referred to as the assumed temperature or fixed derate methods.
Because the thrust generated by an engine decreases as temperature rises, there will always be a maximum temperature at which the engines can generate just enough thrust for takeoff at a given weight. This maximum temperature is known as the ‘assumed temperature’, and it is calculated prior to takeoff and programmed into the flight management computer. In simple terms, you are tricking the engines into thinking it is hotter than it actually is, causing the engines to produce less thrust.
The fixed derate method employs a pre-programmed setting, resulting in a lower thrust setting being generated. The crew may be shown this setting in the flight management computer as a percentage reduction from full power. Assumed temperature takeoffs are more common than fixed derate takeoffs since the former allows for greater flexibility in setting the optimum takeoff thrust.
Flaps and slats on an aircraft deploy to allow slower airspeeds to be flown. While flaps increase lift, they also increase drag, which is the opposing force to the engines’ thrust. When the flaps are first deployed, the amount of lift produced outweighs the amount of increased drag; however, as the flaps near the end of their deployment, the amount of drag increases significantly in relation to the additional lift that follows.
For that reason, on takeoff, only enough flap is used to allow the aircraft to safely depart from the runway while also ensuring the aircraft climbs as efficiently as possible. Modern computer programs that are used to calculate takeoff performance data, will determine the best flap setting based on the environmental and airport characteristics.
Wind direction and speed, outside air temperature, air pressure at sea level, and the surface condition of the runway, are the four environmental data items that are assessed for takeoff performance. Depending on the aircraft type, the position of the center of gravity during takeoff can also be entered to obtain more accurate data.
Take-off performance is influenced by the location of the center of gravity, with a more forward center of gravity being the most limiting. Therefore, shifting the center of gravity aft can permit slightly less thrust to be used while potentially allowing for more payload.
When it comes to wind, a headwind requires less runway, whereas a tailwind increases takeoff distance. Temperature has a similar effect to changes in air pressure. If the temperature rises or air pressure falls, then the air then becomes less dense. Accordingly, engines must work harder to provide the necessary thrust. Higher temperatures or lower air pressure will be more limiting.
The runway condition also has a significant impact on takeoff performance, and the most common categories are wet or dry. A runway can also be classified as contaminated during more severe weather conditions. A contaminated runway is one in which more than 25% of the runway is covered by a substance that is more than 3mm deep.
Contaminants include compact snow, dry snow, slush, and standing water. Crosswind limitations are frequently more restrictive when a runway is contaminated, and the maximum depth of contamination varies depending on what the runway is covered with.
You are entitled to believe that take-off performance is a unique subject in its own right! And so, watch this space for an overview of how landing performance is calculated on the flight deck.
Featured Image: 8Q-IAI Maldivian Airbus 321-211. Image: Alberto Cucini/Airways