Hot, High and Uphill: Take-Off Performance Judgement
Runway 08 at an airfield 1,200 ft AMSL: TORA, TODA and ASDA are all 1,700 m, dry, with a 1.0% uphill slope in the take-off direction. QNH 1008 hPa, OAT 32 °C, wind 080/12 kt. Aircraft mass 2,300 kg, flaps 5, engine bleeds on, anti-ice off. Your AFM extract, already corrected for pressure altitude and temperature at 2,300 kg, gives a take-off distance to 35 ft of 1,480 m and an accelerate-stop distance of 1,520 m, both at zero wind and zero slope. Company corrections: reduce distance by 1% for every 2 kt of headwind component (maximum 15%), applied to both take-off and accelerate-stop distance; add 10% per 1% of uphill slope; add 15% to the accelerate-stop distance only for a wet runway; each 5 °C above the AFM reference temperature of 32 °C increases take-off distance by 9%. Company minima require a 10% margin over TODA for the take-off distance required; no margin is required over ASDA.
1. Compute the pressure altitude, the wind and slope factors, the take-off and accelerate-stop distances required, and decide whether the take-off may be made.
Read the worked answer
Pressure altitude = 1,200 + 30 × (1,013 − 1,008) = 1,350 ft; use the 1,400 ft/32 °C AFM data, which is conservative.
- Wind: runway 08, wind 080/12 kt → 12 kt headwind component → 12 ÷ 2 = 6% reduction → factor 0.94.
- Slope: 1.0% uphill → factor 1.10.
TODR = 1,480 × 0.94 × 1.10 = 1,530 m. With the company margin: 1,530 × 1.10 = 1,683 m against a TODA of 1,700 m - legal, with only 17 m to spare.
ASDR = 1,520 × 0.94 × 1.10 = 1,571 m against an ASDA of 1,700 m - acceptable. If your AFM or operator prohibits headwind credit on the accelerate-stop case, use 1,520 × 1.10 = 1,672 m, which is still within ASDA.
The take-off is legal, but the margin on the take-off distance is thin: any temperature rise, a wind shift, or a heavier passenger load removes it. Record the figures used and brief the V1 rejection case, since the uphill slope lengthens the acceleration run.
2. The OAT rises to 38 °C before departure. Recalculate and state your options.
Read the worked answer
6 °C above the 32 °C reference = (6 ÷ 5) × 9% = +10.8% on take-off distance.
TODR = 1,530 × 1.108 = 1,696 m; with the 10% company margin, 1,865 m, which exceeds the 1,700 m TODA by 165 m. The take-off is not permitted at 2,300 kg.
Equivalently, the uncorrected distance after wind and slope must not exceed 1,700 ÷ 1.10 = 1,545 m, and you are at 1,696 m, so you need about 9% less distance.
Options, in order of practicality:
- Reduce mass. Take the AFM mass-correction for 38 °C and find the mass whose distance satisfies the 1,545 m requirement; this extract does not contain that correction, so the actual reduction must come from the AFM, but a roughly 3% per 100 kg sensitivity suggests several hundred kilograms.
- Delay the departure until the temperature falls or the wind strengthens.
- Use another flap setting or runway if the AFM and the aerodrome data support it.
The accelerate-stop case must be re-derived from the AFM at 38 °C as well, because this extract gives no temperature correction for it - do not assume it is unchanged.
3. A shower leaves the runway wet while the temperature stays at 32 °C and the mass at 2,300 kg. What changes, and is the take-off still possible?
Read the worked answer
Take-off distance is unaffected by the runway condition in this data set, so TODR remains 1,530 m and the 1,683 m margin figure still applies.
The accelerate-stop distance now carries the wet factor: ASDR = 1,520 × 0.94 × 1.10 × 1.15 = 1,807 m, which exceeds the 1,700 m ASDA by 107 m. The take-off is not permitted at 2,300 kg, because the rejected take-off case - not the all-engines case - has become limiting.
Actions: reduce mass until the wet ASDR fits within 1,700 m; check whether the AFM offers a different flap setting with a shorter wet accelerate-stop distance; or wait until the runway is assessed as damp or dry. Do not accept the sector on the basis that "the take-off distance still works" - the limiting case in a wet runway rejection is the one that puts you off the end. If headwind credit on the accelerate-stop distance is prohibited by the AFM, the wet figure is 1,923 m and the problem is significantly worse.