Fuel and Alternates on a Marginal VFR Cross-Country
You are planning a VFR flight of 2 h 40 min still-air time in a four-seat single. Usable fuel is 180 L; the engine burns 32 L/h in cruise and 38 L/h in the climb, and you plan a 12-minute climb. The destination forecast is TEMPO 4,000 m visibility and BKN 1,800 ft for your arrival window; the alternate 25 minutes away has a PROB30 for the same conditions. Your company standard (and your own discipline) is a 45-minute final reserve plus 30 minutes holding. Weather at the destination is expected to be at or just above your personal minima.
1. Calculate your fuel state at destination and decide whether the flight can be dispatched as planned.
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Climb fuel. 12 minutes at 38 L/h = 0.2 h x 38 = 7.6 L.
Cruise fuel. Total still-air time 2 h 40 min = 2.667 h. Subtract the 0.2 h climb, leaving 2.467 h in cruise at 32 L/h = 78.9 L. Total trip fuel = 7.6 + 78.9 = 86.5 L.
Reserves. Final reserve 45 min at cruise burn = 0.75 x 32 = 24 L. Holding 30 min = 16 L. Alternate 25 min = 0.417 x 32 = 13.3 L. Total reserves = 53.3 L.
Total required. 86.5 + 53.3 = 139.8 L against 180 L usable. That leaves about 40 L, or roughly 75 minutes of cruise fuel, as contingency. On the face of it the flight is feasible.
But the decision is not just arithmetic. The destination TEMPO puts visibility and cloud at the edge of your personal minima, and the alternate carries a PROB30 for the same deterioration. A 30-minute hold plus a 45-minute final reserve assumes you can actually reach and use the alternate. If the alternate is also affected, the reserves are not a real safety net. The correct call is to delay departure, or to nominate a second alternate clear of the TEMPO area, before accepting the plan.
2. During cruise, ATC puts you in a 20-minute hold. Recalculate and state your action.
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Recalculate. A 20-minute hold at cruise burn costs 20/60 x 32 = 10.7 L. Your contingency of about 40 L now falls to roughly 29 L, about 55 minutes of cruise fuel. The reserves themselves (final reserve, holding, alternate) are intact, so you have not yet infringed the planning minima.
However, the hold also delays your arrival. You will now arrive later, potentially into a worse part of the TEMPO window, and your alternate may be deteriorating at the same time. The fuel is adequate; the weather risk is not improved.
Action. First, confirm the hold duration and ask ATC for the reason and expected onward clearance. Second, reassess the destination and alternate weather with the latest information. Third, if the destination is trending below your minima, divert early rather than holding and then diverting with less fuel. The key point is that a hold consumes contingency, not reserves, and the decision to divert should be made while the contingency is still intact, not after it has gone.