Fuel, Mass and the Point of No Return
You are planning a VFR cross-country in a single-engine piston airplane. Basic empty mass is 760 kg, maximum take-off mass 1150 kg. Pilot 80 kg, passenger 75 kg, baggage 25 kg, usable fuel 150 L at 0.72 kg/L (108 kg). Forecast: 25 min to top of climb, 2 h 10 min cruise at 145 KTAS, 20 min descent and approach. Cruise fuel flow 32 L/h, climb 38 L/h, descent 25 L/h. Reserve policy is 45 min at cruise flow plus 10 min taxi. Departure and destination airfields are both at 600 ft elevation; no alternate is required. During your pre-flight you notice the fuel gauge reads full but the dipstick suggests about 10 L less than the calculated usable capacity.
1. Compute the take-off mass and the total fuel required, then decide whether the flight can be flown as loaded.
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Mass: 760 + 80 + 75 + 25 = 940 kg empty-plus-load. Fuel 150 L × 0.72 = 108 kg, so take-off mass = 1048 kg, which is 102 kg below the 1150 kg limit. Fuel: taxi 10 min at roughly 10 L/h ≈ 1.7 L; climb 25 min at 38 L/h ≈ 15.8 L; cruise 130 min at 32 L/h ≈ 69.3 L; descent 20 min at 25 L/h ≈ 8.3 L; reserve 45 min at 32 L/h = 24 L. Total ≈ 119 L. Usable capacity is 150 L, so the trip is comfortably within both mass and fuel limits - but the dipstick indicates only about 140 L on board, which still covers 119 L with 21 L margin. The correct decision is to fly, but to verify with a second dipstick reading and to note the reduced margin in the flight log rather than trusting the gauge.
2. Halfway through cruise you are 45 min from destination with 55 L remaining. A 15 kt headwind develops. Does your reserve hold, and what is your decision rule?
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Recompute honestly rather than hoping. At 145 KTAS into a 15 kt headwind, groundspeed is 130 kt. Remaining distance ≈ 45 min at 145 kt ≈ 109 NM, which at 130 kt takes about 50 min. Fuel burned ≈ 50/60 × 32 ≈ 27 L, leaving 28 L on arrival - above the 24 L reserve. Margin is thin, so the decision rule is: if remaining fuel at the next checkpoint falls below 50 L, divert to the nearer of the two suitable airfields identified before departure. Never let a reserve become a plan; the reserve exists to absorb error, not to be spent.