The Overweight Departure and the Performance Chart
You are planning a departure from a 1,200 m asphalt runway at 1,500 ft elevation. OAT is 28°C, QNH 1005 hPa. Your aircraft's POH gives a take-off distance of 980 m at sea level, ISA, MTOM. The runway has a 1% upslope and the wind is calm. Your calculated take-off mass is 1,050 kg against an MTOM of 1,100 kg. The POH does not provide a factor for upslope. You have a 15-minute slot and a passenger who is anxious about the flight.
1. Is the departure within performance limits? Show your reasoning.
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Baseline: POH figure 980 m at sea level, ISA, MTOM (1,100 kg).
Corrections needed:
- Pressure altitude: QNH 1005 hPa is below 1013, so pressure altitude is higher than elevation. Roughly, 8 hPa ≈ 70 ft, so pressure altitude ≈ 1,500 + 70 ≈ 1,570 ft.
- Temperature: 28°C at 1,500 ft is well above ISA (ISA at 1,500 ft ≈ 15 - 2×1.5 ≈ 12°C). That is about +16°C ISA deviation.
- Mass: 1,050 kg vs 1,100 kg MTOM - a small reduction, which helps, but does not offset the density altitude penalty.
- Slope: 1% upslope increases take-off distance; the POH has no factor, so you cannot quantify it from the chart. A conservative rule of thumb is a significant percentage increase per 1% slope.
Conclusion: The 980 m figure is for sea level ISA at MTOM. At ~1,570 ft pressure altitude and +16°C ISA deviation, the distance will be substantially greater - likely well over 1,200 m. Even with the mass reduction, the departure is not within performance limits. The upslope makes it worse. You should not depart.
2. What would you do to make the departure safe, and how do you handle the passenger?
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Options:
- Reduce mass: Offload fuel or baggage to reduce take-off mass and recalculate. This may bring the distance within the runway, but the density altitude penalty is large.
- Wait for cooler conditions: Early morning or evening departure reduces OAT and hence density altitude.
- Use a different runway/aerodrome: A longer runway or one with a downslope or headwind component.
- Recalculate with the POH: Use the actual pressure altitude and temperature, apply the mass correction, and add a slope factor. If the resulting distance exceeds the runway available (including any clearway/stopway), do not depart.
Passenger: Explain that the aircraft cannot safely depart under current conditions and that you are delaying or changing the plan. Do not let the slot or the passenger's anxiety influence the decision. Safety and performance limits take precedence.