How IFR Fuel Requirements Are Built Up

Federal Aviation Regulation 91.167 sets out the fuel a pilot must carry for flight under instrument flight rules. The requirement is cumulative and has three parts. First, enough fuel to fly from the departure airport to the airport of intended landing. Second, when an alternate airport is required, enough additional fuel to fly from that intended destination to the alternate. Third, forty five minutes of further flight at normal cruising speed and consumption.

That structure matters because the alternate leg is measured from the destination, not from the departure point. A diversion airport eighty five nautical miles beyond your destination adds the fuel for eighty five miles of flight after you have already burned everything needed to arrive. Pilots who measure the alternate distance from their departure field consistently underestimate the requirement.

This calculator lays out each element separately so you can see where the fuel goes. It adds two elements the regulation does not explicitly require but that any careful preflight should include: taxi and run-up fuel, which is burned before the wheels leave the ground and is not covered by any enroute calculation, and an allowance for a missed approach at the destination before the diversion begins.

The 1-2-3 Rule and When an Alternate Is Required

FAR 91.169 governs whether you must file an alternate at all. The test looks at the forecast for the destination during the window from one hour before to one hour after your estimated time of arrival. If the forecast throughout that window shows a ceiling of at least two thousand feet above the airport elevation and visibility of at least three statute miles, no alternate is required. If either value falls below the threshold at any point in the window, an alternate must be filed and its fuel carried.

Two details trip people up. The rule is driven entirely by the forecast, so a TAF that predicts deterioration triggers the requirement even when conditions on departure are perfect. And the airport you choose must itself meet alternate minimums at your expected arrival time there, which are typically six hundred feet and two miles for a precision approach and eight hundred feet and two miles for a non-precision approach unless the chart specifies otherwise. Some airports are marked as unusable for alternate filing entirely.

Enter the forecast ceiling and visibility in Advanced Options and the calculator evaluates the rule for you, showing whether the alternate leg is included in the total and which of the two conditions triggered it.

Wind, Weight and the Realities of Diversion

Fuel burn depends on time in the air, and time in the air depends on ground speed. A headwind on the diversion leg stretches the flight without reducing fuel flow, and diversions typically head into the same weather system that closed the destination. Professional Mode accepts a headwind component and recomputes the leg on ground speed rather than true airspeed, which for a typical light aircraft diversion can move the fuel requirement by fifteen to twenty five percent.

Weight is the other consideration. Fuel is heavy, and carrying a generous margin has to be checked against your maximum takeoff weight and centre of gravity envelope. The calculator reports total fuel weight in pounds regardless of whether you entered gallons or pounds, using six pounds per gallon for one hundred low lead avgas and six point seven for Jet A. On a warm day at a high field elevation with four seats occupied, the fuel you would like to carry and the fuel you are permitted to carry are not always the same number.

Treat the Legal Minimum as a Floor

Forty five minutes of reserve is the regulatory minimum for Part 91 instrument flight, not a recommendation. It assumes nothing goes wrong after you arrive at the alternate. It provides no room for a hold at a busy terminal area, a re-route around a line of convection, an approach flown twice, or a lengthy taxi at a congested field. Fuel exhaustion and fuel starvation remain persistent causes of general aviation accidents, and the accident reports overwhelmingly describe flights that were legal on paper at the moment of departure.

A widely used personal minimum is one hour of reserve rather than forty five minutes, with more when the forecast is marginal, the destination has a single runway, or the alternate is distant. Commercial operators impose their own fuel policies which are almost always stricter than the federal floor. The reserve field in Advanced Options lets you plan to whatever standard you have adopted.

Finally, remember what this tool is. It does not retrieve live weather, it has no winds aloft data, it does not know your airframe, and it cannot see terrain, airspace or notices to air missions. Use it to understand the shape of your fuel requirement and to test how sensitive it is to the variables. Then plan the actual flight from current official weather products and the performance section of your aircraft flight manual.