The legal number is 30 minutes of cruise fuel still in the tanks when you land during the day, 45 at night. That number has never stopped anyone from running a tank dry. 14 CFR 91.151 tells you what to plan for and says nothing about how you know what’s actually on board, and that gap is where fuel exhaustion lives. A reserve checklist closes it with three items the regulation never asks for: measure the fuel instead of reading the gauge, convert gallons into a clock time before you start the engine, and write down a bingo time that ends the flight whether or not you feel like ending it.
Read the fuel threads on Pilots of America and the same conclusion comes back every time: fuel exhaustion is entirely preventable. Then read what the posters actually describe. Somebody who trusted a gauge. Somebody who skipped the visual check because the fuel truck had just been there. Somebody 20 minutes from home with 15 minutes of fuel who decided that was close enough. Not one of them misunderstood the regulation.
VFR fuel reserve requirements are a floor
Know the actual text, because it’s more specific than pilots remember. 91.151(a) prohibits beginning a flight under VFR unless, considering wind and forecast weather conditions, there’s enough fuel to reach the first point of intended landing and then fly for at least 30 minutes at normal cruising speed during the day, 45 at night. Rotorcraft get 20 minutes. Under IFR, 91.167 wants destination, then the alternate when one is required, then 45 minutes at normal cruising speed.
Two details matter. First, “considering wind and forecast weather conditions” is regulatory text, not advice, which means a plan built on no-wind numbers isn’t compliant. Second, 91.103(a) names fuel requirements explicitly in the preflight action rule for IFR flights and any flight not in the vicinity of an airport. You are required to have worked this out.
What none of that gives you is a margin worth having. Thirty minutes at cruise burn in a trainer is four or five gallons. That is one hold, one runway change, one 15-knot headwind that showed up 3,000 feet lower than forecast. My personal reserve is an hour still in the tanks at the destination. The only thing it has ever cost me is a fuel stop.
The gauge is certified to be right exactly once
This is the part that surprises pilots, and it explains most of the forum stories. The rule the legacy general aviation fleet was certified under, 14 CFR 23.1337(b)(1), required that each fuel quantity indicator be calibrated to read zero during level flight when the fuel remaining in the tank equals the unusable fuel supply. Read it again. The needle has to be honest at empty. Nothing in that rule ever required it to be honest at half. The current performance-based replacement, 23.2430(a)(4), asks for a means to determine total usable fuel available and leaves the engineering open.
So a float-type sender in a 40-year-old wing, reading a bit over a quarter in bumpy air, tells you almost nothing. Treat the gauges as trend instruments: they confirm fuel is leaving the tanks in roughly the order and rate you expected. They are not a quantity.
A totalizer is better and has its own failure mode: it’s only as good as the number somebody typed in after fueling. Enter 40 when the truck put in 30 and it will confidently report an hour of fuel you don’t have, right up until the engine disagrees.
The preflight fuel checklist
Six items, and they go in the section you actually run, not on a card in your flight bag.
- Visual check, both tanks, every flight. Look in. Not at the gauge, not at the fuel receipt, not at the line guy’s word. This single item is the one the forum threads keep identifying as the missing step.
- Dipstick or tabs, and write the number down. A calibrated stick gives you gallons. Filling to the tabs gives you a repeatable known quantity when you need useful load more than range, and your POH lists what that quantity is. Either way you leave the wing with a number, not an impression.
- Sump both tanks and the strainer. Water and the wrong grade of fuel are separate problems from quantity, and this is where you catch them.
- Caps secure and seated. A poorly seated cap siphons fuel in flight, and you’ll never see it happen from the cockpit.
- Totalizer set to the measured quantity. Whatever you measured, that’s what goes in the box.
- Usable fuel, not total. Your POH’s usable figure is the only one that flies, and it varies by model and by whether long-range tanks are installed.
Fuel reserve calculation for a cross country: work it in time
Gallons hide the answer. Minutes reveal it. Here’s the arithmetic, using round example numbers so you can drop your own POH figures in.
Say 50 usable gallons and a planned burn of 9.5 gph, which is deliberately above the book cruise figure because I’d rather find fuel in the tanks than a surprise. Allow 1.5 gallons for start, taxi, and takeoff. Set the personal reserve at one hour, so 9.5 gallons. That leaves 39 gallons of trip fuel, and 39 divided by 9.5 is 4.1 hours: 4 hours 6 minutes of usable flying.
Now the leg. Three hours 20 minutes at no-wind planning speed, plus 25 minutes for the forecast headwind, gives 3:45. Compare it to 4:06 and your real margin is 21 minutes. Not the hour you thought you’d budgeted, because the reserve is already spoken for. Twenty-one minutes is one vector around a TFR, or one full-stop at the wrong airport.
That number, the margin, is the whole output of the worksheet. If it’s under 30 minutes I plan a fuel stop, and the stop always costs less than it looks like it will.
Bingo time catches “a few more miles”
Everything above happens on the ground, where you’re reasonable. The in-flight half exists because the pilot at 4,500 feet with the destination 25 miles ahead is a different person, and that person is very good at arithmetic that comes out favorably.
So take the decision away from him. Before takeoff, add your trip-fuel endurance to your planned takeoff time and write down the latest wheels-down time. Back up 30 minutes from that and write down a bingo time next to it. Both go on the kneeboard in ink.
The rule attached to those two numbers is one sentence: at bingo time, if the destination is not both in sight and inside 30 minutes, I am going to the nearest suitable airport. No recomputing. The recomputing is the failure mode. Add three en route items to hold it up: a fuel log entry every 30 minutes with tank, gauge indication, and time; a groundspeed check against the plan at the first waypoint, because a slow groundspeed is a fuel problem before it’s a schedule problem; and a switch schedule for the selector, because starvation with fuel still aboard is its own category of engine failure. In a Piper, switching on the clock beats switching on a hunch.
Where these items go
A fuel planning checklist doesn’t work as a separate worksheet. It works when the items sit inside the checklist you already run, which is the whole argument in how to build a checklist and why the preflight flow is the right place for the visual check. Add a two-line “Fuel Plan” block to your Before Start section for measured quantity, endurance, and bingo time, then a fuel item to your cruise checkpoint. On a 172 checklist or a PA-28 in the editor, that’s a five-minute edit.
Bingo time has to be legible in turbulence, so think about which of these lives on paper versus the EFB. Mine is on the kneeboard even when everything else is on glass. And since your personal reserve is a number you set when nothing is riding on it, it belongs with the rest of your personal minimums.
Confirm your airplane’s usable fuel and cruise burn against your current POH rather than this post, and if you want a second opinion on the personal reserve figure, ask the CFI who has watched you plan a cross-country.
Common questions
What are the VFR fuel reserve requirements?
14 CFR 91.151 says you may not begin a flight under VFR unless, considering wind and forecast weather, you have enough fuel to reach the first point of intended landing and then fly for at least 30 minutes at normal cruising speed during the day, or 45 minutes at night. Rotorcraft get 20 minutes. Under IFR, 91.167 requires destination, then alternate when one is required, then 45 minutes at normal cruising speed.
Why can't I trust the fuel gauges in a small airplane?
Because they were only ever certified to be accurate at one point on the scale. The rule the legacy GA fleet was certified under, 14 CFR 23.1337(b)(1), required each fuel quantity indicator to read zero during level flight when the fuel remaining equals the unusable supply. Nothing required the needle to be right at half. The current performance-based rule, 23.2430(a)(4), asks only for a means to determine total usable fuel available.
How do I calculate fuel reserve for a cross country?
Work in minutes, not gallons. Take usable fuel from your POH, subtract a start-and-taxi allowance, then subtract your personal reserve in gallons at your planned burn rate. Divide what's left by that same burn rate and you have trip fuel expressed as hours and minutes. Compare that to your planned time en route including the forecast headwind. The difference is your actual margin, and it is usually smaller than it feels.
What is bingo fuel and how do you set it?
Bingo is a clock time you write down before takeoff, past which you stop going to the destination and start going to the nearest suitable airport. Add your trip-fuel endurance to your takeoff time to get the latest legitimate wheels-down time, then back up 30 minutes. When the clock hits bingo and the destination isn't a sure thing, you divert. The point is that the decision was already made on the ground.
What is the difference between fuel exhaustion and fuel starvation?
Exhaustion means there is no usable fuel left on board. Starvation means the fuel is there but it isn't reaching the engine, most often because the selector is on a tank that's already dry. Both stop the engine and each has a different fix: exhaustion is a planning failure, starvation is a switching-discipline failure. A good checklist has items for both.