Reserve fuel, and the alternate nobody plans
The regulation says 45 minutes at cruise for IFR. Most turboprop operators fly to a number that is bigger and personal. JetDesk lets you set that number once, then checks every leg against it before you file.
01What a leg has to land with
Every leg starts with departure fuel: full usable by default, or a figure you enter if you are tankering less. JetDesk subtracts the leg burn (block time at your cruise gallons per hour plus taxi and takeoff) and gets landing fuel. That landing fuel has to cover two things: the burn to an alternate, if you name one, and your reserve.
02Minutes or gallons
Reserve in minutes scales with your cruise burn, which is the honest way to state it when you fly more than one airplane. Reserve in gallons is what most pilots actually carry in their head for one airplane: "I do not land with less than forty." Settings takes either. If you enter gallons, that wins; leave it at zero and the minutes figure applies.
03The alternate is the whole story
A Meridian burning 40 an hour at 260 knots carries about 170 usable. Nantucket to White Plains is 165 nm, about 50 minutes of block, which is 38 gallons with taxi. Nobody worries about reserve on that leg until the destination goes below minimums and the alternate is Bradley, 70 miles back the other way. That is another 20 gallons, and 45 minutes of reserve is another 30, so the leg really needs 88. If you left with 60 because the ramp fee at the stop was not worth a top-off, you are 28 short with the weather deciding for you.
That is why the fuel line on a leg card names the alternate and its burn separately. Set it while you are still planning, and the app tells you what departure fuel it takes to make it work.
04Departure fuel by weight
Full usable is not always available. Five people in a Meridian take the max fuel by weight to about 108 gallons, well under the tanks, and 200 lb of bags takes it to 78. When your profile has weights, JetDesk caps departure fuel at what max takeoff weight allows and the leg card says so, so the reserve check uses fuel you can actually load.
05Habits that keep this honest
Log fuel used after each flight. Three legs in, JetDesk tells you whether your book burn is optimistic and offers to fix it. Reserve planning on a burn figure that is ten percent low is reserve planning on a lie.
More field notes
Why block time swings twenty minutes
How the FAA winds aloft forecast becomes a groundspeed for each leg, and why the return leg costs more than the outbound one.
Weight before fuel: payload runs out first
Zero fuel weight, max fuel by weight and landing weight for a Piper Meridian with five aboard, and how JetDesk shows it per leg.
What the owner report should say
The monthly numbers an aircraft owner actually wants, and why estimate versus actual matters more than the total.