Hot and High Departure from Lichinga
You are flying a Piper PA-31 Navajo (MTOW 2 948 kg, twin 310 hp piston engines) from Lichinga (FQLG), elevation 4 500 ft, runway 08/26, 1 800 m long, asphalt, slope 0.8% uphill on 08. Temperature 32 °C, QNH 1 012 hPa, wind 080/08 kt. Take-off mass 2 800 kg. The AFM gives, for 2 800 kg at sea level ISA: ground roll 480 m, 50 ft screen 780 m. The AFM correction: +12% per 1 000 ft pressure altitude, +8% per 10 °C above ISA, +15% for a 1% uphill slope. Runway 08 is the only usable direction because of a 200 ft obstacle on the 26 approach.
1. Compute the pressure altitude, ISA deviation, and corrected take-off distance to 50 ft, and decide whether the departure is legal.
Read the worked answer
Pressure altitude = elevation + 30 × (1013 − QNH) = 4 500 + 30 × (1013 − 1012) = 4 530 ft.
ISA temperature at 4 530 ft = 15 − 1.98 × 4.53 ≈ 6.0 °C. Actual 32 °C → ISA deviation ≈ +26 °C.
Corrections (multiplicative on the 780 m sea-level ISA figure):
- Altitude: +12% × 4.53 = +54.4% → ×1.544
- Temperature: +8% × 2.6 = +20.8% → ×1.208
- Slope (0.8% uphill on 08): +15% × 0.8 = +12% → ×1.12
780 × 1.544 × 1.208 × 1.12 ≈ 1 632 m to 50 ft.
Decision. Runway 08 is 1 800 m, so the corrected distance (1 632 m) leaves only ~168 m margin - under the usual 15% safety factor (1 632 × 1.15 ≈ 1 877 m) this exceeds the available runway. The departure is not legal/safe as planned. Options: reduce mass (offload fuel/cargo), wait for cooler conditions, or use runway 26 if the obstacle can be cleared with a climb gradient check. Do not depart on 08 at this mass and temperature.
2. If you must depart, what mass reduction or operational change is required, and how would you verify it?
Read the worked answer
Work backwards: allowable 50 ft distance with 15% factor = 1 800 / 1.15 ≈ 1 565 m. Required correction factor = 1 565 / 780 ≈ 2.006. Current factor is 1.544 × 1.208 × 1.12 ≈ 2.089 - slightly over. A small mass reduction (or a lower temperature) will close the gap.
Practical steps:
- Recompute using the AFM mass-correction chart; offload enough fuel/payload to bring the corrected distance below 1 565 m.
- Confirm the climb gradient clears the 200 ft obstacle on 26 if that direction becomes viable.
- Verify the AFM figures are for the actual configuration (flaps, gear, mixture) and that no contaminated runway exists.
- Record the calculation and decision in the flight documentation.