# Why is fuel quantity expressed in flight hours [duplicate]

Going through a few aircraft documentaries online, I've noticed the thing that they usually express the fuel quantity in flight hours.

Ex - A 747 burns 4 litres of fuel every second. So with a ton of fuel, it'll stay in air for roughly 3-4 minutes.

Why are flight hours a necessary term while consider fuel quantity? I mean shouldn't they be worried about how much distance they can cover in an emergency to reach an airport, instead of how long they can stay in the air? Seems a question related to endurance but couldn't find any proper explanation

It’s primarily due to wind.

Aircraft move through the air at a certain speed, which makes it tempting to give range in miles (or kilometers), but the air itself is moving as well, and often at a large enough fraction of the aircraft’s airspeed that it materially affects range across the ground.

For instance, say you have a Cessna 172 with four hours of fuel (plus reserves) and a cruise speed of 110kt. With a 30kt tailwind (140kt ground speed), your range is 560nm. But going the opposite direction with a 30kt headwind (80kt ground speed), your range is only 320nm—just over half as far!

To make it even more complicated, the winds will have different directions and speeds at different altitudes and locations, and all of that changes over time too, which is one of the reasons pilots (or dispatchers) have to carefully plan each flight’s route and fuel load, even for a “fixed” daily route between the same two airports.

In other vehicles, like automobiles, measuring fuel quantity in miles (or kilometers) makes more sense because there is usually a fairly direct correlation between fuel burn and mileage. This is not the case with aircraft. The distance that a plane can travel on a single tank of gas will vary widely, based on atmospheric conditions.

Headwinds may not affect cars very much, but a 40 knot headwind can just about cut a small plane's cruise speed by half. Prop planes don't work as efficiently at high altitudes, jets don't work very efficiently at low altitudes. It just makes more sense for a pilot to know how long the engines will keep running - then they can calculate the available distance based on the current conditions.

• Is it possible for you to add an example of how this is used in real life? Say an aircraft flying Eastward at 350 knots with 10 knots of headwinds, how/ what will I get out of it exactly? Pretty sure that's not all the required information but I hope you got my problem – Salmonbeing00 Aug 2 at 17:23
• I'd think that "measuring fuel quantity makes more sense" in any vehicle. Something missing there? – FreeMan Aug 2 at 18:59
• @Salmonbeing00 A 10 knot wind is fairly irrelevant, but the high altitude Jet Stream winds have been measured at up to 275 knots. On one occasion a flight from LA to London landed almost an hour ahead of schedule because of a 200 knot tail wind, and the schedule would already include the effect of an "average" 100mph wind adding about an hour to a 6 hour flight from east to west compared with still air, and reducing it by an hour flying from west to east. – alephzero Aug 3 at 1:28
• @Salmonbeing00, what specific help do you need ? The math is really quite simple, just subtract headwind or add tailwind. In the specific example you offered, at the end of an hour instead of traveling 350 nautical miles, with a 10 knot headwind you would have traveled 340. That’s what you will get out of it. Speed is just distance over time. Is this what you are asking? – Michael Hall Aug 3 at 15:32

The unit of measure used for fuel is entirely dependent on the context:

• When discussing endurance or talking to ATC you refer to fuel in flight time.
• When calculating weight and balance you refer to it by weight (kilograms or pounds).
• When computing cost or talking about tank capacity you refer to it by volume (liters or gallons).

As far as air traffic control is concerned, fuel endurance is the measure which air traffic control uses to initiate search and rescue if the aircraft becomes overdue. It is the most practical method for an aircraft with which communication has been lost.

Well, there's a simple way to think about it. A plane with no fuel will do only one thing, fall or just not move (the one leads to the other anyway)

So fuel keeps us up there and without it, we're pretty much doomed.

Given the cruise, climb and descent speeds and the known distance we calculate the time it will take to get to places. If we need to redirect to another destination for some reason we need to know how much longer we can stay in the air which means fuel in liters or gallons isn't useful.

So we rather calculate how long the engine can keep us airborne using fuel flow values in different flight sections and the total fuel to determine this.

• -1: planes do not "fall" when they run out of fuel. Besides being obvious knowledge for anyone who flies, noticeable examples are the Gimli Glider, US Airways 1549, and many others. – Martin Argerami Aug 3 at 5:39
• Gliding is falling, only very slowly and with a horizontal component ;) With the right thermals you can even fall upwards. – Michael Aug 3 at 7:32
• @Michael: no. When an aircraft is gliding, forces are equalized and there's no acceleration. Vertical speed is steady in calm air (with a decent pilot, at least). You wouldn't say you are "falling" when you walk down the stairs. Most thermals also require decent pilots ;) – Martin Argerami Aug 3 at 7:54