Fuel decision on a Maputo–Inhambane VFR leg
You are flying a Cessna 172S (Lycoming IO-360, 180 hp) from Maputo (FQMA) to Inhambane (FQIN), a planned 250 NM leg. Fuel at engine start is 44 US gal usable. Taxi and run-up burn 2 gal. Planned cruise is 65% power at 9,500 ft, giving a true airspeed of 118 kt and a fuel flow of 9.6 gal/h. Forecast winds along track average a 15 kt headwind component. ATC has advised that a large formation of parachute operations near Xai-Xai may force a 15-minute hold en route, and you must land at Inhambane with a 45-minute VFR reserve. The aircraft's total usable capacity is 53 gal.
1. Compute the fuel required for the flight as planned, including the possible hold and the reserve. Does the 44 gal on board suffice?
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Leg time. Groundspeed = 118 − 15 = 103 kt. Time = 250 / 103 ≈ 2.43 h (2 h 26 min).
Fuel.
- Taxi/run-up: 2.0 gal
- Trip: 2.43 h × 9.6 = 23.3 gal
- Hold: 0.25 h × 9.6 = 2.4 gal
- Reserve: 0.75 h × 9.6 = 7.2 gal
- Total: ≈ 34.9 gal
On-board usable after taxi = 42 gal, so the plan closes with roughly 7 gal (about 44 min) of margin - acceptable on paper, but the margin is thin enough that any further headwind or hold eats it.
2. Halfway along, the headwind increases to 30 kt and the hold is confirmed at 25 minutes. Recompute the fuel situation and state the judgement you would make.
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Revised numbers. Remaining distance ≈ 125 NM. New groundspeed = 118 − 30 = 88 kt. Remaining time = 125 / 88 ≈ 1.42 h (1 h 25 min).
Remaining fuel required.
- Trip: 1.42 × 9.6 ≈ 13.6 gal
- Hold: 0.42 h × 9.6 ≈ 4.0 gal
- Reserve: 7.2 gal
- Total ≈ 24.8 gal
Fuel actually remaining at the halfway point (assuming everything so far matched plan) ≈ 42 − 2 − (1.25 × 9.6) ≈ 28 gal. Margin is only about 3 gal (≈ 19 min). That is below a comfortable VFR decision margin.
Judgement. Do not press on hoping the wind eases. Options ranked: (1) divert to a nearer suitable aerodrome with fuel available (e.g. Inhassoro/Chidenguele area strips if confirmed serviceable); (2) reduce power and lean to a lower fuel flow, accepting a slower but still safe groundspeed; (3) if no diversion is practical, declare a fuel situation to ATC early rather than late. The correct answer is to act while the margin still exists - a 19-minute margin with a known hold ahead is not a margin, it is a plan to become a statistic.