Aircraft Performance: Distances, Gradients and Decision Points
Aircraft performance converts mass, atmosphere, runway and procedure into distances, gradients, speeds and range. NZ ATPL questions are often multi-step and hinge on identifying the limiting condition.
Take-off, landing and climb
- Take-off distance grows with mass, density altitude, tailwind, upslope and contaminated runway. Use AFM data for actual mass and conditions; do not extrapolate beyond chart limits. V1, VR and V2 are constrained by field length, obstacle clearance, tyre speed, brake energy and minimum control speeds.
- Landing distance grows with mass, density altitude, tailwind, downslope and poor braking. Regulatory landing distances include factors; reverse thrust may be excluded unless the AFM and operator approvals allow it.
- Climb gradient (%) = 100 × ROC / (groundspeed in ft/min). ROC required = gradient × groundspeed (kt) × 101.27.
Range, endurance and decision points
Specific range (air nautical miles per unit fuel) peaks near the speed for minimum drag; long-range cruise trades speed for fuel. PNR is the point from which the aircraft can return to the departure aerodrome with required reserves. The critical point is equidistant in time between two alternates, allowing for wind.
Worked example: A net climb gradient of 3.3% is required. Groundspeed is 140 kt. Required ROC = 0.033 × 140 × 101.27 = 468 ft/min. If the aircraft achieves 420 ft/min, the gradient is 420 / (140 × 101.27) = 2.96%, so the departure is not compliant.
Traps
Using TAS instead of groundspeed in gradient calculations is a common error. Wind affects groundspeed and therefore gradient and PNR; it does not change TAS. Do not use unfactored landing distance as the regulatory figure. Distinguish accelerate-stop distance from take-off distance available, and check whether slope is uphill or downhill before applying a correction.