Climb performance
Climb speeds and gradients
Climb performance separates two ideas: climb rate (feet per minute, affected by excess power) and climb gradient (height gained per unit horizontal distance, affected by excess thrust). Certification requires a take-off climb segment structure: the first segment to 35 ft at V2, the second segment to at least 400 ft with take-off flap and gear up, the transition segment, and the final segment to 1,500 ft. The net gradient is the gross gradient minus the required reduction (0.9% twin, 0.8% three, 0.5% four engines). Obstacle clearance uses the net flight path.
What changes the climb
Higher mass, higher altitude, higher temperature, and engine bleed air all reduce climb gradient. Climbing at too low a speed wastes performance, but too high a speed also reduces excess thrust, so VY (best rate) and VX (best angle) matter. For a jet, the scheduled climb speed is usually V2 plus a margin to V2+10 or V2+15 kt. A common question trap is confusing rate with gradient: into a headwind, gradient improves while rate is unchanged; with a tailwind the reverse. Another trap is using gross instead of net gradient for obstacle clearance.
Worked example and rule of thumb
A twin-engine aircraft has a gross second-segment gradient of 3.4%. Net gradient is 3.4 − 0.9 = 2.5%. Over a 4,000 m horizontal distance to an obstacle, the net height gained is 0.025 × 4,000 = 100 m. If the obstacle is 90 m high with a 15 m required clearance, the aircraft clears it by 100 − 105 = −5 m, so the take-off mass must be reduced. Rule of thumb: 1% gradient equals 60 ft per nautical mile.