Diversion Fuel Over the Rocks
You are flying a light single-engine piston aircraft from a coastal airport to an inland destination, planned 2 h 10 min en route at 115 KTAS. Fuel on board at start was 180 L usable; the flight manual gives 32 L/h at your planned power setting. You planned 45 min fixed reserve plus 10 min taxi. One hour into the flight, ATC advises the destination is now forecast IFR with a 2 000 ft broken layer and you must hold for 20 min before an approach, or divert to an alternate 35 min away at your current ground speed. Your fuel gauges read just under half on both tanks, and the totaliser shows 68 L used. Winds aloft have been stronger than forecast, and your actual ground speed has averaged 98 kt.
1. Using the totaliser and the fuel plan, work out your actual fuel consumption and remaining usable fuel. What does this tell you about your original planning assumptions?
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Totaliser shows 68 L used in 1.0 h → actual consumption ≈ 68 L/h, roughly double the 32 L/h book figure. That gap is too large to be a gauge error; it suggests either a mixture/leaning problem, a fuel leak, or that the '32 L/h' figure was for a lower power setting than you are actually using. Remaining usable fuel = 180 − 68 = 112 L. At 68 L/h that is about 1 h 39 min of endurance, which is far less than the 1 h 10 min still to run plus 45 min reserve plus a 20 min hold. The original plan (2 h 10 min × 32 L/h + reserves) was built on a consumption figure you have not achieved, so the plan is invalid regardless of what the gauges say. Treat the totaliser figure as the truth and re-plan from it.
2. Compare holding 20 min then flying the approach versus diverting now. Which do you choose and why?
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Hold option: 20 min hold at 68 L/h ≈ 22.7 L, plus the remaining 1 h 10 min to destination at 68 L/h ≈ 79 L, plus approach and missed-approach allowance. Total ≈ 102 L+ against 112 L remaining - no legal reserve left and no margin for a missed approach. Divert option: 35 min at 68 L/h ≈ 39.7 L, leaving about 72 L (≈ 1 h 04 min) - ample reserve, and you land with fuel in hand. The hold also depends on the weather actually lifting, which is not guaranteed. Choose the diversion now: it preserves the fixed reserve, removes dependence on an uncertain forecast, and is the lower-risk option. Note that the decision must be made early - delaying the diversion eats the same fuel you are trying to protect.
3. What would you do differently on the next flight of this type to avoid this situation?
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Build the plan on a verified consumption figure, not a book number: measure actual burn over at least one full tank and use that. Lean correctly for the cruise power setting and altitude, and confirm with EGT/CHT and the totaliser. Plan with the forecast winds but carry a wind allowance, since the actual ground speed (98 kt vs 115 KTAS) shortened the still-air range. Set a personal fuel-reserve trigger (e.g. divert when remaining fuel equals diversion burn + fixed reserve + 15 min) and write it on the plog. Cross-check totaliser against gauges at each waypoint so a consumption anomaly is caught in the first 30 min, not at the point of no return.