Fuel reserve and alternate decision on a cross-country leg
You are flying a light single-engine aeroplane on a 210 NM cross-country leg at 95 KTAS. Planned fuel at engine start is 110 litres; the engine burns 30 litres per hour at your chosen power setting. Taxi and run-up use 3 litres. The destination forecast is marginal: broken cloud at 1800 ft with a 40 percent probability of temporary ceiling at 1200 ft, and the only alternate is 35 NM away. You have already been airborne 1 hour 50 minutes and have 55 litres remaining according to the gauges. Your operator's standard is a 30-minute final reserve plus a 45-minute alternate allowance, and you must land with at least 20 minutes of fuel remaining after the alternate.
1. Compute your usable endurance at this point and determine whether the planned destination plus alternate plus reserves is still achievable.
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Fuel burn is 30 L/h, so 55 litres gives 55/30 = 1.83 h, or about 1 h 50 min of total remaining endurance.
Required allowance: 30-minute final reserve = 15 L; 45-minute alternate allowance = 22.5 L; the 20-minute post-alternate buffer = 10 L. That totals 47.5 L just in allowances, leaving only 7.5 L, or 15 minutes, of cruise fuel.
The alternate is 35 NM away, about 22 minutes at 95 KTAS. You cannot reach the alternate, hold the reserves, and still have the post-alternate buffer. The destination remains reachable only if the weather holds, but the 40 percent probability of a 1200 ft temporary ceiling makes that a gamble. The correct decision is to divert now toward the alternate or the nearest suitable aerodrome while the margin still exists, and to declare a fuel situation if the diversion will erode the final reserve.
2. If you decide to continue to the destination, what specific conditions would make that decision defensible, and what should you prepare before entering the terminal area?
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Continuing is defensible only if you can verify, from a current observation or ATIS, that the ceiling and visibility at the destination are comfortably above your minima and are not trending downward, and if the destination has a runway and approach you can use without a circling manoeuvre in marginal cloud. You should also confirm that the alternate weather is above its minima, so the diversion option remains open.
Before entering the terminal area, prepare a diversion plan with headings, distances and times to the alternate, brief the approach and missed-approach procedure, set up the radios and navigation aids, and identify a personal decision point (for example, 10 NM out) at which you will divert if the weather is not clearly acceptable. If ATC can offer a straight-in approach or vectors that shorten the track, request them. Advise ATC of your fuel state so they can prioritise you if holding becomes necessary.
3. Explain how the 30-minute final reserve should be interpreted in flight, and why it is not simply fuel to be burned.
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The final reserve is a protected quantity intended to cover unforeseen delays, a missed approach, and a diversion to a suitable landing area. It is not cruise fuel. In flight, you treat it as a floor: once the fuel state reaches the reserve, you should be landing or committed to landing, and you should declare an emergency or urgency condition if you cannot.
In this scenario the reserve plus alternate allowance plus buffer already consumed 47.5 of the 55 litres, so the reserve was effectively being eroded by any further cruise. The practical interpretation is that the reserve defines the point at which the flight must be concluded, not the point at which you begin to worry. Good airmanship is to identify the erosion early, when a diversion is still cheap, rather than at the moment the reserve is breached.