Tankering fuel into a weight-limited mountain departure
A twin turboprop cargo flight is planned from mountain airport A (elevation 6,200 ft, runway 1,500 m, 1.5% upslope, OAT 24°C, QNH 1008) to B, then B to C. The operator's figures are: BOW 4,500 kg, MTOW 7,600 kg, MLW 7,300 kg, MZFW 6,400 kg. Performance software gives a maximum take-off weight at A of 7,250 kg for the actual conditions. Planned payload is 1,700 kg. Trip fuel A-B is 520 kg, B-C is 890 kg, and the final reserve plus contingency for each sector is 300 kg. Taxi fuel is 30 kg at each airport. A NOTAM states that the fuel bowser at B is unserviceable for 48 hours, so all fuel for the B-C sector must be carried from A. The next airport with fuel, D, is 35 minutes from B, but D's runway is only 1,200 m and has no instrument approach. The dispatcher asks whether the flight can carry the planned payload by tankering.
1. Calculate the maximum payload if all B-C fuel is tankered from A, and identify the limiting weight.
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Fuel that must be on board at B for the B-C sector is trip 890 kg + reserve/contingency 300 kg + taxi 30 kg = 1,220 kg. That fuel has to be carried from A, so take-off fuel at A is A-B taxi 30 kg + A-B trip 520 kg + 1,220 kg = 1,770 kg.
At B, the take-off weight is BOW + payload + 1,220 kg. The MZFW limit gives payload <= 6,400 - 4,500 - 1,220 = 680 kg. Check the other limits: take-off weight at A with 680 kg payload is 4,500 + 680 + 1,770 = 6,950 kg, below the 7,250 kg performance limit and below MTOW 7,600 kg. Landing at B is 4,500 + 680 + (1,770 - 520) = 6,430 kg, below MLW 7,300 kg.
So the maximum payload with full tankering is 680 kg, not 1,700 kg. The limiting constraint is MZFW at B, because the tankered fuel is payload that is carried but not burned before B. MTOW is not the answer; it is not the active limit on this sector.
2. Why is a simple MTOW check misleading in this case?
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MTOW only limits the weight at the start of the flight. Here the aircraft lands at B with more than 1,200 kg of fuel still on board because that fuel is for the next sector. The relevant limit at B is the zero-fuel weight, which excludes fuel but includes payload. Adding tankered fuel directly reduces the payload that can be carried under MZFW, one kilogram for one kilogram, until MZFW is reached.
The departure performance limit at A is also lower than MTOW: 7,250 kg, not 7,600 kg, because of elevation, slope and temperature. A planner who checks only 'MTOW 7,600 kg' may think there is 1,400 kg of margin, when the real margin is much smaller. Both the performance-limited take-off weight and the MZFW at the tankering airport must be checked, together with MLW at the landing airport.
3. Compare tankering with a tech stop at D and state what you would verify before choosing.
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Full tankering allows only 680 kg payload, an offload of 1,020 kg. A tech stop at D avoids carrying the B-C fuel from A, so more payload can be carried on A-B, but it adds a landing and take-off at a 1,200 m runway with no instrument approach. The aircraft would still need fuel to D plus reserve, and D's fuel must actually be available and suitable.
Before choosing, verify D's take-off and landing performance at the forecast temperature and wind, the runway surface and slope, obstacle clearance, weather and alternate availability, and whether the fuel at D is confirmed by NOTAM or direct contact. Also check that the extra sector does not breach crew duty or cargo commitments.
If D is marginal or the weather is uncertain, reducing payload may be the safer commercial choice. If the temperature rises by 10°C, the A departure limit will fall further and even 680 kg may not be achievable. The decision should be based on the limiting weight at each airport, not on the posted MTOW.