A Takeoff calculation is performed before takeoff in order to confirm that the actual weight is below the maximum permissible takeoff weight at particular aerodrome in the conditions prevailing. On modern aircraft this value and the values for the various speeds can be obtained from the FMS, after feeding in the relevant data for the airfield and conditions.
Following basic conditions are considered in the calculations:
- Airfield elevation
- Runway slope
- Air temperature
- Runway length and conditions
- Flap configuration
Using either charts or computer software the maximum permissible takeoff weight is determined and when it is confirmed that the actual weight is within the limits, it is necessary to find the takeoff speeds and thrust setting corresponding to the actual weight.
Following speeds are determined or, on modern aircraft, obtained from the FMS:
• V1 – decision speed, at which in case of engine failure the continued takeoff distance required, will not exceed the takeoff distance available;
• VR – rotation speed, at which aircraft nose is lifted from the ground (rotated) for takeoff;
• V2 – takeoff safety speed with critical engine inoperative, at which the aircraft can take of safely with critical engine inoperative.
Typical takeoff calculation errors
Calculating and entering takeoff performance parameters into aircraft systems involves a number of steps that create potential opportunities for errors. The following list provides examples of the types of errors that have been identified from investigations into related accidents and incidents:
- the ZFW (Zero Fuel Weight) is inadvertently used instead of the TOW (Takeoff Weight) - an aircraft weight is incorrectly transcribed or transposed into an aircraft system or when referencing performance manuals; for example, a weight of 234.000 kg or 224.000 kg is used instead of 324.000 kg
- V speeds are incorrectly transcribed or transposed when manually entered into an aircraft system
- aircraft data from a previous flight is used to calculate the V speeds
- takeoff performance parameters are not updated as a result of a change in flight conditions; for example, a change in the active runway, intersection departure or ambient temperature
- selecting the incorrect value from the loadsheet or take-off data card
- using the wrong performance charts for the aircraft type
- inadvertently selecting the wrong table or column/row in the performance charts
- using the incorrect value when referencing the performance charts - failing to convert values into the required unit of measurement (for example pounds to kilograms)
Typical takeoff calculation errors consequences
In the event the above errors are not detected and corrected prior to takeoff, the following adverse consequences may occur:
- tailstrike: when aircraft rotation is initiated at a speed below that required for the aircraft’s weight, lift-off may not be achieved. In response, the pilot may increase the nose-up attitude of the aircraft, which may result in the tail contacting the runway
- reduced takeoff performance: during the takeoff,the crew may observe that the aircraft’s performance is not as expected; the aircraft may appear ‘sluggish’ or ‘heavy’
- degraded handling qualities: after takeoff, there may be a reduced margin between the aircraft’s actual speed and the stall speed until the aircraft accelerates up to the normal climb speed. If the V2 speed is also erroneous, this may not occur until after the aircraft passes through the acceleration height - rejected takeoff: if the aircraft fails to accelerate or lift-off as expected, the crew may reject the takeoff
- runway overrun: if the aircraft fails to stop after a rejected takeoff or the aircraft fails to liftoff, the aircraft rollout may extend beyond the end of the runway resulting in an overrun
- TO/GA (Takeoff/Go around) engine thrust: if the aircraft fails to accelerate or lift-off as expected, the crew may select take-off/go-around (TO/GA) engine thrust (the maximum thrust that the engines will supply)
- increased runway length required: early rotation increases drag and significantly increases the distance from rotation to liftoff.
- overweight takeoff: this may occur if an erroneous TOW (Take Off Weight) is used to determine whether a runway is acceptable for the takeoff
- reduced obstacle clearance: if the takeoff is commenced at low speed, the aircraft will not achieve the climb gradient required, and the clearance between any obstacles along the take-off path will be reduced.