Fuel Reserve vs. Forecast Deterioration on a Cross-Country
You plan a 95 NM cross-country from a rural strip to a coastal aerodrome in a recreational aircraft cruising at 85 kt TAS. Still-air flight time is 67 minutes; planned taxi and climb add 12 minutes. Forecast winds give a 10 kt headwind outbound, so planned endurance required is about 92 minutes. Fuel onboard at start-up is 62 litres; burn is 15 litres per hour. At the halfway point the AWIS at destination reports visibility 6 km in haze and a 15 kt crosswind gusting to 22 kt, and your calculated remaining fuel is 44 litres. Your aircraft's crosswind limit is 12 kt. You hold a Passenger Carrying Endorsement and one passenger is aboard.
1. Determine whether you can legally and safely continue to the planned destination, and state the fuel figure you must protect.
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Work the numbers before deciding. Planned endurance required is 92 minutes; at 15 L/h that is 23 L. The 62 L onboard gives 248 minutes of endurance, so at the halfway point (46 minutes elapsed) you have used about 11.5 L and hold roughly 50 L, not 44 L - reconcile the discrepancy, because an unexplained 6 L gap suggests either a richer mixture setting, a leak, or a gauge error, and any of those is a reason to land short. The protected figure is the fixed reserve plus the variable reserve your planning rules require; with a deteriorating forecast, treat the destination as unavailable and divert to an alternate within 20 minutes. Independently, the gusting 22 kt crosswind exceeds your 12 kt limit, which alone forbids the approach. Divert, land, and re-plan on the ground.
2. If you divert to an alternate 18 NM away with a 15 kt tailwind, show the fuel and time margin you would land with and explain the decision rule you applied.
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At 85 kt TAS plus 15 kt tailwind, groundspeed is 100 kt, so 18 NM takes about 11 minutes. Fuel used is 15 L/h × 11/60 = 2.75 L. If you held 44 L at the diversion decision, you land with about 41 L, or roughly 164 minutes of endurance - comfortable, but only because the alternate is close. The decision rule is: identify a point before which you will divert, compute the fuel needed to reach the alternate plus the fixed reserve, and never let the plan depend on the destination becoming usable. Because the crosswind already exceeds your limit, the diversion is not discretionary - the rule is applied to a known-unusable destination, so the alternate must be one where the crosswind is within 12 kt.