Take-off Performance: Field Length, Obstacle and Climb-Limited Cases
The three take-off cases
Take-off performance is the most restrictive of three cases. Field length: TODR must not exceed TODA and ASDR must not exceed ASDA; for a dry runway the aeroplane must reach 35 ft by the end of TODA with one engine failed at V1. Obstacle: the net take-off flight path must clear all obstacles by the required margin (35 ft vertical, or 90 m + 0.125 × D laterally for close-in obstacles). Climb: the aeroplane must meet the scheduled gross gradients - 3.3% second segment for twins, 2.7% for three, 2.4% for four, reduced by 0.8% for net - with gear up, one engine inoperative, V2, take-off flaps.
Segments
First segment (to 35 ft, all engines), second (to 400 ft, one engine out, V2, flaps take-off), third (to acceleration altitude, V2), fourth (to 1,500 ft, flaps up, V2 or VY). Questions often ask which segment limits a hot/high departure: usually the second segment.
Speeds
V1 must satisfy V1 ≥ VMCG and V1 ≤ VR ≤ V2, with V2 ≥ 1.13 VS1 and V2 ≥ 1.10 VMCA. Increasing V1 lengthens the accelerate-stop distance but shortens the one-engine-out distance; the balanced field is the optimum.
Worked example
TODA 2,400 m, obstacle 1,200 m from brake release, 60 ft high, net second-segment gradient 2.4%. At 1,200 m the net height is roughly 1,200 × 0.024 ≈ 29 m (95 ft) plus 35 ft screen - just adequate if the required vertical margin is 35 ft; if the obstacle were 120 ft the take-off mass must be reduced until the net path clears it.
Traps
Using gross rather than net gradient; forgetting the 35 ft screen; applying obstacle accountability to close-in obstacles without the lateral rule; mixing TODR with TOR.