How Much Fuel Does IFR Really Require, Reading the Reg from Destination to Alternate to Reserve

Under Instrument Flight Rules, FAR 91.167 requires enough fuel to fly to your destination, then on to your alternate if one is required, and then for an additional 45 minutes at normal cruise speed. Whether you must carry alternate fuel at all is decided by the 1-2-3 rule, and if an alternate is required, that airport has to meet its own weather minimums. The legal minimum is not the same as a safe personal reserve.

What Does FAR 91.167 Actually Require for IFR Fuel?

The regulation breaks into three pieces, and every instrument pilot should be able to recite them: destination, alternate, reserve.

First, enough fuel to fly to your first airport of intended landing. Second, from that airport, enough to continue to your alternate - but only if an alternate is required. Third, on top of all of it, enough to fly for 45 minutes at normal cruise speed.

That third piece is the reserve, and it sits behind everything else. It is not fuel you plan to use; it is fuel that must still be in the tanks after you have theoretically flown the entire route including the leg to your alternate.

When Do You Need to File an Alternate? The 1-2-3 Rule

The trap in the regulation is the phrase “if an alternate is required.” Whether you carry alternate fuel depends entirely on the forecast weather at your destination, and that test is the 1-2-3 rule.

Here is how it reads: if, from one hour before your estimated time of arrival to one hour after, the destination is forecast to have a ceiling of at least 2,000 feet and visibility of at least 3 statute miles, you do not need to file an alternate.

One hour on either side. 2,000-foot ceiling. 3 miles visibility. One, two, three.

Miss any part of that forecast window and the alternate goes back on the flight plan - and the fuel to reach it goes back in the tanks.

What Weather Minimums Must Your Alternate Meet?

Deciding you need an alternate is only half the problem. Once you choose one, that airport must meet its own forecast weather minimums at your expected arrival time, and those numbers depend on the type of approach available.

If the alternate has a precision approach (an ILS or the like), you need a forecast ceiling of at least 600 feet and visibility of at least 2 statute miles.

If it has only a non-precision approach, the numbers rise to an 800-foot ceiling and 2 miles visibility.

600-2 for precision, 800-2 for non-precision. These are the standard alternate minimums, and the logic is simple: your alternate is your out, so the regulation demands it be in better shape than the destination that already went sour.

How Do You Work a Full IFR Fuel Problem?

Walk through a realistic example. Your destination forecast around arrival is a 1,000-foot ceiling and 2 miles visibility - below 2,000 and 3, so an alternate is required.

You pick a nearby field with an ILS. Its forecast for your arrival window shows a 1,000-foot ceiling and 3 miles visibility. That beats 600-2, so it is legal as an alternate.

Now the fuel adds up: enough to reach your destination, then from the destination to the alternate, then 45 minutes of cruise on top of all of it. Add taxi, takeoff, the climb, and a realistic hold if the system is busy, and the tanks fill up in a hurry.

This is why the math matters in the real world, where a forecast that looked fine at breakfast is a different animal by the time you are airborne.

Why the 45-Minute Reserve Is a Floor, Not a Plan

Here is a clearly labeled opinion: the 45-minute reserve is a legal minimum, not a personal one. It is the floor the FAA will not let you go below - it is not the number a careful pilot plans to land with.

Weather moves. Approaches get missed. You get held. A one-hour personal reserve on an instrument flight has saved far more pilots than it has ever inconvenienced. The regulation tells you where the cliff is; it does not tell you how close to stand.

One caveat: turbine and jet operations fall under different rules, and airlines run their own more conservative reserve policies. But if you are flying a piston single or twin under Part 91, it is 91.167 and the 1-2-3 rule that keep you legal - and the personal margin on top that keeps you comfortable.

Key Takeaways

• FAR 91.167 requires IFR fuel for three things: destination, alternate (if required), and a 45-minute reserve at normal cruise.
• The 1-2-3 rule decides whether you need an alternate: no alternate required if the destination forecast is at least a 2,000-foot ceiling and 3 miles visibility from 1 hour before to 1 hour after ETA.
• A required alternate must meet its own minimums: 600-2 with a precision approach, 800-2 with only a non-precision approach.
• Total fuel stacks up fast once you add taxi, takeoff, climb, and holding to the destination-plus-alternate-plus-reserve baseline.
• The 45-minute reserve is a legal floor, not a safe plan - a one-hour personal reserve is a wiser target in real IFR weather.