Rotation on a Short Hot Runway
A twin-engine light aircraft is cleared for take-off from a 1,600 m asphalt runway on a day when the outside air temperature is 34 °C, the aerodrome pressure altitude is 1,500 ft, and the surface wind is reported as 210°/12 kt. The runway in use is 03, giving a 30° crosswind component of 6 kt and a headwind component of about 10 kt. The pilot plans to rotate at the published Vr of 95 kt IAS. During the take-off run the pilot notices that acceleration is slower than expected, and the aircraft reaches 95 kt only about 1,150 m along the runway. The pilot's instinct is to hold the aircraft on the ground a little longer to build extra speed before rotating, because the density altitude is high and the aircraft feels sluggish.
1. Evaluate the pilot's instinct to delay rotation. Explain, using lift and drag behaviour, why holding the aircraft on the ground longer does or does not improve the situation on this runway.
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Delaying rotation is generally the wrong response here, and the reasoning rests on the lift equation and on how drag changes with speed on the ground.
Lift is L = ½ρV²SC_L. In the ground run the aircraft is at a fixed low angle of attack, so C_L is small and roughly constant. The only variable the pilot controls is V, and lift rises with V². Holding the aircraft down means the wing continues to produce only the low-C_L ground-run lift while the aircraft accelerates. The moment the aircraft is rotated, C_L jumps to the take-off value and lift increases sharply at essentially the same speed. Waiting for extra speed therefore buys lift very inefficiently compared with simply rotating.
Meanwhile, every extra knot on the ground adds parasite drag (rising with V²) and rolling friction time, so the ground roll lengthens and the aircraft consumes more of the remaining 450 m. On a hot, high-density-altitude day, ρ is already reduced, so the same IAS corresponds to a higher true airspeed and a longer ground roll; the aircraft is genuinely sluggish, and the correct response is to recognise the degraded performance, not to extend the exposure.
The safer judgement is to rotate at Vr as planned, or to reject the take-off if the achieved acceleration suggests the aircraft will not reach a safe screen height before the runway end. Holding down to build speed converts a marginal situation into a worse one.
2. The aircraft becomes airborne at 95 kt and the pilot selects flap up immediately. Describe what happens to the stall margin and to the climb gradient, and state the better technique.
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Retracting flap immediately after lift-off at 95 kt removes the extra lift coefficient the flap was providing. The aircraft is now flying at a lower C_L for the same weight, which means it must fly at a higher angle of attack to hold level or climb, so the margin between the current angle of attack and the stall angle shrinks. In gusty or shear conditions near the ground this is a real stall risk, and the published Vr already assumes a particular flap setting.
Climb gradient also suffers. Rate of climb depends on excess power, and excess power depends on the difference between power available and power required. Removing flap reduces induced drag but also reduces lift at a given speed, forcing a higher angle of attack and higher induced drag until the aircraft accelerates. The transient is a period of reduced climb performance at the worst possible height.
The correct technique is to rotate at Vr, establish a positive rate of climb, then retract flap in stages at or above the published flap-retraction speed, accelerating to the scheduled climb speed before selecting flap up. On a hot, high-density-altitude departure the pilot should also fly the published obstacle-clearance speed and accept a shallower gradient rather than trading stall margin for a momentarily better-looking climb.