Weather, fuel, system status

A diversion decision is really three questions asked at once — what's the weather doing, how much fuel do I have, and is the airplane fully working? Each one can force the decision, and each shapes where you're able to go.

Fuel — the hard floor

§91.151 sets the VFR minimum: enough fuel to reach the first point of intended landing plus 30 minutes (day) / 45 minutes (night) at normal cruise. That reserve is a floor, not a plan — and a diversion spends time and fuel you can't get back.

The math in the air is simple and unforgiving: fuel aboard − fuel to the alternate (with a wind allowance) must still leave the reserve intact. If it doesn't, you've turned a weather problem into a fuel emergency. When the numbers are close, the nearer field with fuel wins over a better one you might not reach.

Concretely: if you're holding 7 gallons and the airplane burns 8 gph, that's roughly 52 minutes of flying — and a 45-minute night reserve eats almost all of it, leaving about 7 minutes of truly discretionary fuel. That's the number that should drive the decision, not the comfortable-sounding "7 gallons."

Weather — usually the reason, always the test

Weather is what triggers most diversions — a destination dropping below minimums, a line of storms across the route — and it's the first test every alternate has to pass: it must have weather you can actually use. A closer field that's also socked in isn't an alternate, it's a trap.

Watch the trend, not just the snapshot. A destination losing 200 feet of ceiling per hour will close before your fuel runs out, so the decision clock is set by the weather, not the tank. (Reading the actual products — METARs, TAFs, radar — is Area I.C; here the skill is acting on what they're telling you.)

Aircraft status — what the airplane can still do

A partial failure changes the whole calculation. A rough-running engine, a charging-system failure, or a stuck gear or flap shrinks the set of usable alternates — you may now want a longer runway, a towered field with emergency services, or simply the nearest one — and it raises the urgency of deciding at all.

The mapping is intuitive once you say it out loud: a partial power loss wants the longest, closest runway it can reach into the wind; an electrical failure wants a non-towered field (no clearance to coordinate) or a tower you can reach before the battery dies; a rough engine over hostile terrain wants the nearest survivable surface, period. Diagnose what's wrong, then let the failure choose the field.

The principle is to match the alternate to the failure. A precautionary landing in a healthy airplane can afford to be picky; a sick airplane takes the best field it can reach right now, because the next few minutes may be all the choice you get.