Hot and High Charter Departure Using the Wrong Performance Chart
A Part 135 operator plans a charter from Goroka, elevation about 5,200 ft, to Madang using a twin-engine aircraft with MTOW 5,670 kg and basic empty weight 3,300 kg. The load sheet shows eight passengers at 80 kg each, 450 kg cargo and 600 kg fuel. The runway is 1,100 m long, wet, with a 1% uphill slope; OAT is 28°C; QNH is 1008 hPa; forecast wind is 10 kt tailwind. The AFM performance charts are in the aircraft, but the operator's flight planner has used the dry-runway, sea-level chart and says the aircraft is below MTOW, so the takeoff is legal. The flight is 45 minutes; the pilot plans 30 minutes reserve fuel. The destination forecast is fine, but there is a 40% chance of thunderstorms near Madang. The chief pilot asks you to review the load and fuel plan before signing the flight authorisation.
1. Check the load sheet. Is the aircraft within MTOW, and what else must be checked before the takeoff performance can be accepted?
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The arithmetic is straightforward: basic empty weight 3,300 kg plus eight passengers at 80 kg, which is 640 kg, plus 450 kg cargo plus 600 kg fuel equals 4,990 kg. That is 680 kg below the 5,670 kg MTOW, so the aircraft is not overweight on those figures.
Weight alone does not authorise the takeoff. The centre of gravity must be inside the AFM envelope for the actual loading, including cargo distribution and fuel burn. More importantly, the performance calculation is invalid. The planner used a sea-level, dry-runway chart. Goroka is about 5,200 ft pressure altitude, and with QNH 1008 the pressure altitude is roughly 5,350 ft. The calculation must use the AFM chart for that pressure altitude, 28°C, a wet runway, 1% uphill slope and a 10 kt tailwind, and must account for the 50 ft obstacle and any contaminated-runway factor. A tailwind and wet runway both increase takeoff distance. On a 1,100 m strip, the aircraft may be performance-limited even though it is under MTOW. If the correct chart cannot show a legal margin, the takeoff cannot be authorised.
2. Evaluate the fuel plan and the thunderstorm risk. What reserve and alternate planning would you require?
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The planned 600 kg fuel for a 45-minute flight leaves 30 minutes reserve. That is probably below the PNG CAR Part 135 reserve for this operation, and it is below the operator's own standard if the manual requires 45 minutes. The legal minimum must be checked, but the practical answer is that 30 minutes is not enough with a 40% chance of thunderstorms near Madang. Convective weather can force a diversion, holding or a missed approach.
I would require fuel to destination plus a suitable alternate plus the required reserve, and I would identify an alternate with weather and runway suitable for the aircraft. If no alternate is available, the flight should be delayed or cancelled. The pilot should also consider that the destination forecast is not a guarantee; the 40% risk is a planning trigger, not a reason to rely on luck. The authorisation should not be signed until the correct performance chart, weight and balance, fuel reserve and alternate are documented.