# Why is fuel usage discussed per time, not per distance?

I don't understand why sometimes the fuel used for a flight is discussed in terms of fuel used per time.
I think I have seen it a couple of times here - not sure whether it was in some common context.

I would think that it's much more relevant how much fuel is used per distance in almost every respect, except designing engines.

As an example, this answer to the recent question How do you compare private jet and commercial jet efficiency? uses "x/hr USD" as in "Hourly fuel costs for a private jet can run from 800-2000/hr USD." I'm sure I noticed similar cases a couple of times.
It's fuel cost per time, instead of fuel weight or volume, but that does not make a difference in regards to the question.

Maybe there is a reason based on some "tradition"?

• Do you mean for example 10 gal per min? Is that what you mean by fuel per time? Sep 30, 2014 at 21:25
• Yes, or more often tons per hour I think. Sep 30, 2014 at 21:28
• Generally, with the same weight, altitude, and indicated airspeed, fuel per unit time should be pretty consistent. With fuel per unit distance, you have to factor in the wind, which generally isn't relevant. Sep 30, 2014 at 21:29
• @VolkerSiegel Can you add some examples of where you saw that? Sep 30, 2014 at 21:45
• Since I wrote that answer I can tell you why I chose money/time. it was to make answering the question easier/more relevant since the OP asked question about costs. But, with that said I always do all my calculations in units of fuel/time (ex. pounds per hour-PPH), since that is what you need to calculate range (winds act for a time) and things like IFR reserves are based on time. Sep 30, 2014 at 23:38

Unlike with driving, in aviation the major variable when planning fuel consumption isn't how far you're traveling but how long you're traveling: Distance traveled in aircraft is a derived value (calculated from groundspeed and time - groundspeed also being a derived value affected by many factors including wind).

A pilot may not readily know their groundspeed (it can be quite different from airspeed), but at a given power setting they know an engine will burn a specific quantity of fuel per unit time, therefore fuel consumption is generally expressed that way (in gallons per hour or pounds per hour), and if they know how long they've been flying they can reliably estimate how much fuel they've burned, how much is left in the tanks, and how much longer they can keep flying before running out of fuel.

To illustrate why this is superior for flight planning consider the following (extremely simplified) flights fitting the following profile:

• A 200 nautical mile trip
• An airspeed of 100 knots
• A fuel burn of 10 gallons per hour
• A 25-gallon fuel tank.

Scenario 1: No Wind
With no wind the flight will take two hours to complete. The groundspeed is 100 knots.
The aircraft will burn 20 gallons of fuel.
The fuel efficiency is 10 miles per gallon.

In this scenario we land with 5 gallons of fuel left in the tank - that corresponds to 30 minutes, which is the minimum legal reserve for day VFR flight per FAA regulations.

With a 25 knot headwind the flight will take about 2:40 minutes to complete.
The groundspeed is 75 knots.
The aircraft will burn 27 gallons of fuel.
The fuel efficiency is 7.5 miles per gallon.

In this scenario we run out of fuel 2:30 into the flight. Running out of fuel is generally considered to be a Bad Thing: The passengers complain, and the FAA asks a lot of uncomfortable questions.

Scenario 3: 25 Knot Tailwind
With a 25 knot tailwind the flight will take about 1:40 minutes to complete.
The groundspeed is 125 knots.
The aircraft will burn 16 gallons of fuel.
The fuel efficiency is 12.5 miles per gallon.

In this scenario we land with almost an hour of fuel still in the tank - a very comfortable reserve.

On longer flights, and in slower aircraft, the effects of wind can be extremely pronounced, and a 5 or 10 knot difference from the forecast winds can mean the difference between arriving at your destination or having to add a fuel stop.

• In your edits to the question you noted that operating costs are often expressed as dollars-per-hour as well - this is done for similar reasons: Fuel is a dominating factor in operating costs (and for the reasons above we track that per unit time). In addition many maintenance items are dictated on a time(hours)-in-service basis (oil changes, hot section replacement in turbine engines, inspection cycles, etc.), so it's convenient to break down operating costs that way. Sep 30, 2014 at 22:48
• You might want to clarify that the legal reserve under (day) VFR is 30 minutes, not 5 gallons :-) Sep 30, 2014 at 23:39
• I would think ground distance traveled would often be a known rather than derived quantity (after all, in many cases the purpose of a flight is to travel between distances a known distance apart), but air distance traveled would be a derived quantity. In your example, the plane travels 10 nautical miles per gallon through the air, but depending upon tailwind/headwind would have to travel between 160 and 266 nautical miles through the air to move 200 miles on the ground. I would expect that the airspeed which would yield the optimal number of ground miles per gallon would... Oct 1, 2014 at 0:01
• ...vary depending upon windspeed, though I'm not sure how much. Would that expectation be correct in practice? Oct 1, 2014 at 0:06
• @supercat The as-flown distance is 200 miles in all of the examples (the aircraft is traveling along a straight (actually great-circle) 200 mile line from point A to point B) - I've included an image with an example flight fitting the hypothetical profile. The way I've always seen flight planning done wind doesn't change the flight plan distance (which is always measured relative to the Earth's surface), it changes your velocity. You could work it as a distance modification, but the math is much harder so we don't do that :) Oct 1, 2014 at 2:20

It’s an interesting question. Most calculations that pilots do are based on fuel flow per hour. Several regulations include these types of calculations as well: you must have a fuel reserve for (say) 30 or 60 minutes beyond your final destination.

For some questions, eg comparing a small plane to a car in fuel economy, fuel per distance is interesting.

But, some other reasons why fuel per distance may not be as useful as fuel per hour:

• Wind. A tailwind versus a headwind can dramatically change the fuel per mile, but doesn’t change the fuel per hour (if your airspeed is the same)
• Comparison to other planes. If everyone compares figures per hour, then you have a solid basis for comparison. Of course, comparing a jet’s fuel per hour compared to a prop plane’s fuel per hour would be silly, but planes in the same class may have somewhat comparable cruising speeds
• Taxi, climb and descent. The cruising speed of a plane might not be that relevant for short flights, when most of the time is not spent cruising
• Pilot Operating Handbooks (POH)s tell you many figures in gallons per hour, including fuel usage while climbing, at different speeds, different atmosphere conditions etc

These are just a few. I guess neither fuel per hour nor fuel per distance is useful by itself—it depends entirely on what you want to compare.

As explained in other answers, fuel/time is better for planning, but now let's move to the cockpit!

Fuel/time is easily and accurately measured by the vehicle instrumentation and, as a plane throttle is mostly kept fixed, the value won't change much allowing you to use it to do estimations. A very convenient one is the residual flight-time: this tells you how much you can wait for landing and how much this wait will cost. From the fuel/time is not hard to get the range of autonomy once you know the ground speed.

Fuel/distance, instead, will make your life much harder if you need to know how much you can wait above an airport!

On the contrary, driving a car you want to know how far you can get to reach a gas station, not for how long you can drive around it, that's why fuel/distance is much better in this case.

For a given amount of fuel, the engines will only run for a certain amount of time. Distance is irrelevant. If you circle going nowhere for one hour, you've still consumed one hour's worth of fuel. If your airspeed is 100 knots and flying into a 100 knot headwind, you're still consuming fuel even though the distance traveled is 0.

• If there's a 100-knot headwind, you probably shouldn't be flying. Jan 7, 2019 at 3:58

Technically, the best metric would be fuel flow, per weight. For a simple answer, you burn far more fuel when you're heavier, than when you're lighter.

• This is true, there could be a base fuel flow figure for a basically empty plane, and then additional fuel flow for each unit of weight carried. In reality, there are generally different fuel flow figures given for different weights, and other variables as well. Apr 27, 2016 at 16:22