Turboprop Step Climb: Fuel Versus Time
A twin turboprop is planned for a 520 NM sector at FL190. Forecast wind gives a 40 kt headwind component; cruise TAS is 310 kt and cruise fuel flow 620 kg/h. Climb to FL190 consumes 120 kg and 16 min; descent consumes 70 kg and 14 min. The destination alternate requires 25 min at cruise fuel flow, and the operator's final reserve is 45 min. Taxi fuel is 30 kg and contingency is 5% of trip fuel. Usable fuel at engine start is 2,300 kg. In cruise, ATC offers FL210: TAS 320 kt, headwind component 20 kt, cruise fuel flow 590 kg/h. The climb from FL190 to FL210 would add 40 kg and 7 min; descent fuel and time are unchanged. The crew must decide whether the higher level gives a worthwhile fuel margin or merely burns time. Assume no step-climb penalty and that the aircraft is already at FL190 when offered.
1. Calculate the total fuel required and the start-up fuel margin for the FL190 plan.
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Ground speed at FL190 = 310 − 40 = 270 kt. Cruise time = 520 ÷ 270 = 1.926 h.
Cruise fuel = 620 × 1.926 = 1,194 kg. Trip fuel = 1,194 + 120 climb + 70 descent = 1,384 kg. Contingency = 5% × 1,384 = 69 kg. Final reserve = 620 × 45/60 = 465 kg. Alternate = 620 × 25/60 = 258 kg. Taxi = 30 kg.
Total required = 1,384 + 69 + 465 + 258 + 30 = 2,207 kg. With 2,300 kg onboard, the start-up margin is 93 kg.
2. Recalculate the requirement if the crew accepts FL210 after reaching FL190. Use the extra climb fuel and time only once.
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At FL210, ground speed = 320 − 20 = 300 kt. Cruise time = 520 ÷ 300 = 1.733 h.
Cruise fuel = 590 × 1.733 = 1,023 kg. Total climb fuel = 120 + 40 = 160 kg. Trip fuel = 1,023 + 160 + 70 = 1,253 kg. Contingency = 5% × 1,253 = 63 kg. Final reserve = 590 × 45/60 = 443 kg. Alternate = 590 × 25/60 = 246 kg. Taxi = 30 kg.
Total required = 1,253 + 63 + 443 + 246 + 30 = 2,035 kg. The saving relative to FL190 is about 172 kg. Total time also falls by about 5 min because the extra 7 min climb is more than offset by the higher ground speed.
3. Recommend whether to accept FL210 and state the assumptions that could reverse the decision.
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Accept FL210. It reduces the required fuel by about 172 kg and slightly reduces total time, so it improves both fuel margin and schedule. The decision assumes the forecast wind and fuel flow are accurate, the aircraft can reach FL210 without a significant step-climb penalty, and no additional holding is required.
The advantage would shrink if the FL210 headwind were much stronger than forecast. At roughly 60 kt headwind or more, the higher level would no longer show a clear fuel benefit. If turbulence or temperature requires a different power setting, the crew should recalculate before committing.