Fuel: Pounds vs. Gallons

[Halo]

The GlobalFlyer record today gets me to wondering why aircraft fuel lately seems to be stated more in terms of pounds rather than gallons.  

Seems much of that is temperature related and just how many pounds an aircraft can carry, fuel and overall.  But I remember things like external fuel tanks were usually stated in gallons instead of pounds, and fuel basically is still measured in gallons rather than pounds, right?

Are aircraft designed with fuel tanks measured in gallons or ability to carry x pounds of fuel?

When aircraft tanks are filled, do their indicators show gallons or pounds of fuel, or both?

[storch]

I believe it's all lbs now

[bunch]

Maybe because volume measure changes  with alt/barometer, but mass is nearly constant regardless of alt?  I measure my 100LL in gallons for flight time & pounds for weight & ballance

[gwshaw]

Short answer is that jets measure fuel flow in lbs, piston engines measure it in gallons. In reality both types of engines power is proportional to mass flow through the engine, so even piston engines should measure flow in lbs/min, but gallons are the traditional method.

One of these days I'll get around to writing up how to calculate mass flow and hp for a piston engine, but I'm too lazy to do it right now. "Not Much of an Engineer" but Sir Stanley Hooker has a fairly easy to follow method for doing it, which is how I've been doing it. There is also a more complicated NACA method for doing the same thing, which I'm still trying to digest and compare to Hooker's method.

Greg Shaw

[JB73]

i would have thought something to do with weight, meaning the plane was carrying XXX lbs of fuel, where they have to account for the fuel weight when planning the flight (takeoff weight) and all

[flakbait]

Fuel gage markings depend on the aircraft, and the owner. Most smaller a/c have the gage simply marked a lot like a car's gas gage, with hatch marks showing how much is left. Sometimes (Cessna piston-pumpers for one) there's a gallons reading beside a given hatch mark. On other aircraft (jets, turbo-props) it gets peasured in pounds, with roughly six pounds per gallon. The media usually likes using pounds when they report how much fuel a given aircraft type carries. It's more dramatic with bigger numbers and "sounds technical": 500 gallons is a lot, but 3,000 pounds sounds like more.

Aircraft owners can buy after-market gages if they want to see fuel in pounds, gallons, or just read hatch marks. Pounds is usually more useful for two reasons: if you have a fuel imbalance, you need to know it in pounds because the POH lists it in pounds. And you need to know the weight and balance of the aircraft. Too much weight and it won't fly, and if the balance is off by quite a bit you can have all sorts of problems.



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Flakbait [Delta6]

[gear]

My father was a lite airraft pilot and his ground school books refured to fuel load as pounds and burn rate as gallons per hour.
 The reason for that is for calculating total take off weight of the aircraft.

[Maverick]

The change to pounds is very simple.

100 gallons of fuel will change its VOLUME with temperature. If you pump 100 gallons of COLD fuel it takes up less volume than does 100 gal. of HOT fuel.

600 lbs. stays the same irregardless of volume taken up by the fuel.

[Rino]

Jet A is roughly 6.7 lbs per gallon, 100LL about 6 lbs per
gallon.  Water is about 8 lbs per gallon.

[Holden McGroin]

At Indy, Roger Penske was the first to start refrigerating his team's fuel.  He could fit more pounds of fuel in his car's 40 gallons than the opposition.  He could go a few more laps per tankful than the competition.

Now the rules specify the temperature of the fuel transferred to the car.

[Halo]

Good points.  I remember some of the fuel expansion/contraction from just filling my car's gas tank in cold weather.  

As in automobiles, I presume aircraft fuel tanks have overflow valves?  E.g., pump refrigerated fuel into an aircraft sitting in a hot desert and pretty soon lots of fuel is wasted unless the fuel is kept refrigerated or the aircraft gets airborne quickly?

[Rino]

Originally posted by Halo
Good points.  I remember some of the fuel expansion/contraction from just filling my car's gas tank in cold weather.  

As in automobiles, I presume aircraft fuel tanks have overflow valves?  E.g., pump refrigerated fuel into an aircraft sitting in a hot desert and pretty soon lots of fuel is wasted unless the fuel is kept refrigerated or the aircraft gets airborne quickly?

     Topping off aircraft in winter is very annoying, especially if
they request hangar.  If you don't leave expansion space, they'll
freaking vent fuel like mad.  It also works that way if you fuel at
night and later the sun heats the tanks.

[Cobra412]

As others have stated we calculate our fuel load by pounds.  You can get erroneous readings if you did it by gallons due to expanding and contracting fuel due to temperatures.  

Prior to calibrating our fuel quantity system we do fuel density checkout with a hydrometer.  The hydrometer tells us how much the fuel weighs at that point in time.  To calibrate the system to the empty side you don't need to know the fuels current weight.  You follow specific procedures of defueling the tanks and the adjust the pots to indicate 0lbs.  

Once you've done that you do a refuel and get a fuel density reading.  You then calibrate the system by referencing a table for each tank and how much it should read at a particular fuel density.  It's all about the accuracy difference when monitoring and indicating in pounds opposed to in gallons.  Going by pounds is the most accurate reading you can get.

Halo we use what is called a surge box in the F-15.  A perfect example is what happens in desert enviroments.  The temperatures will drop a fair bit at night and the rise drastically in the day.  If the aircraft was fueled at night and by the morning the temperatures rose enough for the fuel to expand beyond the tanks capacity it will open a vent valve and vent into the surge box.  If for some reason the surge box is full then it will vent via the wing tips vent ports.  The pressure will build up enough to open a butterfly valve and force the excess fuel out of the wings.  Once the pressure has falling the valve will typically close on it's own and stop venting fuel.  Then we have to get a fuel bowser and empty the surge boxes in the wings.

[Halo]

Thanks, Rino and Cobra.  Bet most people have no idea of the complexities of aircraft fueling.  Almost like aircraft are living things.  Long way from any idea of just hop in and go.

[Golfer]

It's not too complex.  I did find one of the guys that worked for me at the FBO (Fixed Base Operator...airport gas station) who claimed to be 'experienced' with fueling jets topped off our Citation and stuck it back in the heated hangar during winter.  I came to hate the stink of unburned jet fuel even more that day.  Burned jet fuel isn't so bad...especially if you're doing the burning

Like they said above, jet fuel (for regular jet engines as well as turboprops) is done in pounds because of the very wide ranges of temperatures that the fuel will likely see.  It's a lot colder in the flight levels (Above 18,000 feet) than the ground.  Because the fuel will expand and contract with temperature, I would assume (and have been told) thats where this came about.  Pilots order their fuel in pounds, line guys need to calculate using 6.7lbs/gallon how many gallons you pump into the airplane or each tank.  No real issues, just make sure you always as a pilot double check your fuel load prior to takeoff.  Every now and then someone could defy logic and put into your fancy schmansy jet 1000lbs instead of 1000 gallons and that is a less than good thing.