Mountain Airport Landing Distance and the 80% Rule
An aircraft is planned to land at a mountain airport with a 1,050 m runway. Landing mass is 1,150 kg. The AFM factored landing distance at sea level, ISA, dry runway, no wind, is 520 m. The AFM provides these additive percentage corrections to the factored distance: pressure altitude 2,000 ft +12%, 4,000 ft +26%; temperature ISA+10°C +8%, ISA+20°C +18%; 10 kt tailwind +20%; wet runway +15%; 1% downslope +5%. Airport elevation is 3,800 ft, QNH 1013 hPa, temperature 24°C, wind 10 kt tailwind, runway 1% downslope, and the runway is wet. The crew must decide whether to land at the planned mass or reduce mass. Company procedure requires the corrected factored landing distance to be no more than 80% of the runway length. The aircraft can land 80 kg lighter by burning fuel, but that would leave the final reserve intact. Determine whether a landing at the planned mass is acceptable and what margin exists.
1. Calculate the pressure altitude and ISA temperature deviation at the destination.
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With QNH 1013 hPa, pressure altitude equals elevation = 3,800 ft.
ISA temperature at 3,800 ft ≈ 15 − 2 × 3.8 = 7.4°C. The actual temperature is 24°C, so the deviation is 24 − 7.4 = +16.6°C above ISA.
2. Determine the corrected factored landing distance at the planned mass and compare it with the runway and the company 80% rule.
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Interpolate the altitude correction between 2,000 ft (+12%) and 4,000 ft (+26%): at 3,800 ft, fraction = 1,800 ÷ 2,000 = 0.9, so correction = 12 + 0.9 × 14 = 24.6% → 1.246.
Interpolate temperature between ISA+10°C (+8%) and ISA+20°C (+18%): at ISA+16.6°C, fraction = 6.6 ÷ 10 = 0.66, so correction = 8 + 0.66 × 10 = 14.6% → 1.146.
Apply the other factors: tailwind 1.20, wet runway 1.15, downslope 1.05.
Corrected distance = 520 × 1.246 × 1.146 × 1.20 × 1.15 × 1.05 = 1,076 m.
The runway is 1,050 m, so the corrected distance already exceeds the runway by about 26 m. The 80% rule requires no more than 840 m, so the exceedance against company procedure is about 236 m. A landing at the planned mass is not acceptable.
3. State the operational decision and the data needed before accepting a reduced landing mass.
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Do not land at the planned mass. The options are to reduce mass, use a different runway, or divert. Burning 80 kg of fuel is not proven sufficient: it is about a 7% mass reduction, while the distance must fall by about 22% to meet the 80% rule. Without the AFM landing-distance-versus-mass table, the crew cannot assume that 80 kg will close the gap.
Before accepting a reduced mass, obtain the AFM factored landing distance for the actual mass and configuration, apply the same altitude, temperature, wind, runway condition, and slope corrections, and confirm the result is at or below 840 m. If that cannot be shown, divert. The final reserve must remain intact throughout.