[Josh]

Why is that aviation fuel seems to always be measured in pounds vs. a volume? Because the temp changes could change the volume of fuel? To make gross weight estimates easier with the fuel figure?

[Sardaukar]

jua, on Wed 28 May 2008 1557, said:

Why is that aviation fuel seems to always be measured in pounds vs. a volume? Because the temp changes could change the volume of fuel? To make gross weight estimates easier with the fuel figure?

Good question...I'd think it is latter. Also, considering payload and maximum take off weight, it is useful to measure fuel also as weight.

[rmgill]

Simple value for trim and max take off weight issues. Saves converting back and forth.

[shep854]

Primarily due to volume changes as temp changes. Weight is mainly used in jets that operate in sub-freezing (water, that is) at cruise. It does have the added advantage of making weight & balance easier--as long as the correct measurement system is used. An Air Canada jet got things mixed up a while back (7/23/1983), fuel was loaded in pounds rather than kg, which meant the plane had rather less than half of the intended fuel load. Fortunately, the pilot was a glider pilot, and safely landed the plane on an abandoned airfield being used as a drag strip. Look up "Gimli Glider" on Wikipedia.

[Brasidas]

Definitely performance.

Working on an FMS at present, and it has over four default weight values used in computing performance.

You do your weight and balance based on weight and distribution, not volume.

You do your aircraft approach configs based on weight not volume.

You do your departure performance configs based on weight not volume.

The volume you carry does define how much weight you carry, however I think it is notable that Jet A is widely known to be approx 6.8 lbs per gallon instead of the number of gallons being used to determine your weight.

[Paul in Qatar]

shep854, on Thu 29 May 2008 0311, said:

Primarily due to volume changes as temp changes. edia.

Seconded.

[Ivanhoe]

Fuel is used in an exothermic chemical reaction. The amount of usable energy in the fuel tanks is dependent on mass, not volume. Recall the "golden number" of recip engines is the SFC, in lb/HP/hr.

[Brasidas]

Paul in Saudi, on Mon 2 Jun 2008 1303, said:

Seconded.

You burn fuel way faster than any volume expansion due to dipole moment through cooling could cause problems. Even if it did, just vent the tanks, excess gone.

[aevans]

Brasidas, on Mon 2 Jun 2008 2320, said:

You burn fuel way faster than any volume expansion due to dipole moment through cooling could cause problems. Even if it did, just vent the tanks, excess gone.

Aviation fuel tanks have pressure relief valves, don't they?

[shep854]

aevans, on Mon 2 Jun 2008 2327, said:

Aviation fuel tanks have pressure relief valves, don't they?

General aviation aircraft have vented tanks, to prevent a vacuum as fuel as used. Otherwise, the tank would (has) collapsed. The vent also allows fuel to escape if it expands.

When I mentioned the use of fuel weight due to expansion/contraction due to temperature variations, this also has a large effect on measurement. At subzero temperatures, a given weight of fuel obviously takes up less volume. If a fuel system is measuring volume, it will be in error at other than the temperature it was calibrated. Weight of a given volume obviously does not change with temperature. The fuel system doesn't measure the weight of fuel, but the flow of fuel. If the measuring system is not correctly calibrated, or the amount of fuel loaded is not correctly reported, there shall be trouble and tribulation.

[Brasidas]

aevans, on Mon 2 Jun 2008 2327, said:

Aviation fuel tanks have pressure relief valves, don't they?

That's what the "vent the tanks" statement meant Tony.

[TSJ]

Brasidas, on Tue 3 Jun 2008 0330, said:

That's what the "vent the tanks" statement meant Tony.

We would typically put 10,000 lbs of fuel in the A-4 bird and it only took a few minutes. That's a lotta fuel in a very short time. On A-4s there is a fuel cap on top of the wing that we were supposed to unscrew before fueling the plane in case the vent became plugged up or something. The fuel bladders could be damaged if it did get plugged. Then, after fueling, the cap gets screwed back down, right?

Well, that's how it's supposed to work. So some dumb a$$ pre flights the bird before a mission and forgets to check and see if the cap has been screwed back down. :( The pilot comes out and kicks the tires, hops in and takes off. Uh, the cap didn't get screwed back down. After take off, it is duely noted by the pilot's wing man that fuel is streaming out of said pilots plane. Mission is aborted and pilot, steam running out of ears, safely lands the plane. Uh, the Dumb A$$ who did the pre-flight is very, very, sorry and will never miss that again. :( Oh, groan. The only thing that said Dumb A$$ can say in his defense is that a lot sorties were going on that day and you don't get a lot of time to do a pre-flight sometimes when things are really, really busy. :(

This post has been edited by TSJ: 03 June 2008 - 0012 AM

[Brasidas]

shep854, on Tue 3 Jun 2008 0328, said:

General aviation aircraft have vented tanks, to prevent a vacuum as fuel as used. Otherwise, the tank would (has) collapsed. The vent also allows fuel to escape if it expands.

When I mentioned the use of fuel weight due to expansion/contraction due to temperature variations, this also has a large effect on measurement. At subzero temperatures, a given weight of fuel obviously takes up less volume. If a fuel system is measuring volume, it will be in error at other than the temperature it was calibrated. Weight of a given volume obviously does not change with temperature. The fuel system doesn't measure the weight of fuel, but the flow of fuel. If the measuring system is not correctly calibrated, or the amount of fuel loaded is not correctly reported, there shall be trouble and tribulation.

Actually the newest FMS will auto-sense fuel mass on the ground and do volumetric flow measurement in the air. You can do a manual override fuel mass entry, or an airborne "auto-sense" if your burn is not matching up with mass left on board (ie a leak). Of course, that particular FMS is on a $51 mill bird, but it's going to become more common as more airframes get them installed. I saw them being implemented on some of the smaller $ 8 mill jets at one time four years back. Garmin might make a good GAMA FMS but they don't do a lot of the nice bells & whistles of the more integrated suites.

[shep854]

Brasidas, on Tue 3 Jun 2008 0536, said:

Actually the newest FMS will auto-sense fuel mass on the ground and do volumetric flow measurement in the air. You can do a manual override fuel mass entry, or an airborne "auto-sense" if your burn is not matching up with mass left on board (ie a leak). Of course, that particular FMS is on a $51 mill bird, but it's going to become more common as more airframes get them installed. I saw them being implemented on some of the smaller $ 8 mill jets at one time four years back. Garmin might make a good GAMA FMS but they don't do a lot of the nice bells & whistles of the more integrated suites.

Neat! Ain't progress grand?

Pilots? Who needs 'em? :unsure:

This post has been edited by shep854: 03 June 2008 - 0652 AM

[Brasidas]

shep854, on Tue 3 Jun 2008 1152, said:

Neat! Ain't progress grand?

Pilots? Who needs 'em? :unsure:

Try and convince a planeload of people to board "robo plane".

[rmgill]

Ivanhoe, on Mon 2 Jun 2008 1016, said:

Fuel is used in an exothermic chemical reaction. The amount of usable energy in the fuel tanks is dependent on mass, not volume. Recall the "golden number" of recip engines is the SFC, in lb/HP/hr.

But different fuels have more or less BTU's per unit of mass. Gasoline has less heating value than diesel.

[aevans]

Brasidas, on Tue 3 Jun 2008 0330, said:

That's what the "vent the tanks" statement meant Tony.

It sounded more like the pilot had to make a conscious decision to dump vapor/fuel.

[Brasidas]

aevans, on Tue 3 Jun 2008 1418, said:

It sounded more like the pilot had to make a conscious decision to dump vapor/fuel.

You can do it either way on some aircraft depending on various sets of parameters. Set fuel vent to auto or manual.

This post has been edited by Brasidas: 03 June 2008 - 1152 AM

[shep854]

Brasidas, on Tue 3 Jun 2008 1245, said:

Try and convince a planeload of people to board "robo plane".

"Nothing can go wrong...go wrong...go wrong..."

I'm still not thrilled with UAVs in the National Airspace System. :mellow:

[ShotMagnet]

Quote

Try and convince a planeload of people to board "robo plane".

Not a problem for me; I'll risk a software failure against human error (provided the software has been sufficiently ruggedized/safeguarded) any day of the week.

Anyone know, by the by, how many airliners have been brought down by human error as opposed to mechanical failure compounded with human error? At any rate; I'll fly with software which has been appropriately constructed, as opposed to someone who's coming off a bender, in the middle of a divorce, etc.


Shot