Pounds vs. Gallons

rmciottijr

rmciottijr

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rmciottijr
I was talking to someone about fuel and its weight is 6 lb per gallon. Then I was wondering why does the military, commercial and others use pounds instead of gallons. Do they have load cells that send weight back to the cockpit? If not why add another step to ordering fuel.

Thanks,
Bob
 
Pounds are a more accurate measure of how much fuel you have available because fuel is burned by mass (effectively, weight), not volume, and fuel density varies with fuel type (jets may use different fuels of different density, e.g., JP-5/8 at 6.8 lb/gal versus JP-4 at 6.5 lb/gal). However, light plane fuel measurement systems measure volume, not mass, and we only use one density fuel (avgas at 6 lb/gal), so volume is what we get.
 
Ron Levy said:
Pounds are a more accurate measure of how much fuel you have available because fuel is burned by mass (effectively, weight), not volume, and fuel density varies with fuel type (jets may use different fuels of different density, e.g., JP-5/8 at 6.8 lb/gal versus JP-4 at 6.5 lb/gal). However, light plane fuel measurement systems measure volume, not mass, and we only use one density fuel (avgas at 6 lb/gal), so volume is what we get.
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What he said. Our fuel probes are capacitance type probes that account for both volume and density. Everything in the cockpit is read in LBS and LBS/HR, and the gas truck operates in LBS. The Beech read in gallons, so we had to convert from gal to LBS when ordering from the gas man, but all the transport category planes I've seen use LBS for everything.
 
I've only seen gallons on the jet-fuel truck meters. It would be great if they used #'s instead, and would have prevented quite a few accidents (or red faces) over the years. Some of the fuel ordering conversations, especially those which take place late at night after long duty days, are great comedy. Or would be if the subject weren't so serious.

All the crews know that the conversion multiplier is 1.5 going one way (#'s to gallons, drop the zero) and .67 the other way. Many use only one gage and multiply X2 because the arithmetic is easier, and every time you fly with a different guy it seems that he has his own formula.

I almost got trapped by bad arithmetic one cold night as we were departing Midway for Dallas. The only reason we didn't have to shut down and add more fuel was that the extra fuel we (thought) we added was padded sufficiently that the screwup didn't eat all the reserve.



Teller1900 said:
What he said. Our fuel probes are capacitance type probes that account for both volume and density. Everything in the cockpit is read in LBS and LBS/HR, and the gas truck operates in LBS. The Beech read in gallons, so we had to convert from gal to LBS when ordering from the gas man, but all the transport category planes I've seen use LBS for everything.
Click to expand...
 
wabower said:
I've only seen gallons on the jet-fuel truck meters. It would be great if they used #'s instead, and would have prevented quite a few accidents (or red faces) over the years. Some of the fuel ordering conversations, especially those which take place late at night after long duty days, are great comedy. Or would be if the subject weren't so serious.

All the crews know that the conversion multiplier is 1.5 going one way (#'s to gallons, drop the zero) and .67 the other way. Many use only one gage and multiply X2 because the arithmetic is easier, and every time you fly with a different guy it seems that he has his own formula.

I almost got trapped by bad arithmetic one cold night as we were departing Midway for Dallas. The only reason we didn't have to shut down and add more fuel was that the extra fuel we (thought) we added was padded sufficiently that the screwup didn't eat all the reserve.
Click to expand...

I should have phrased that better. We order it in LBS from them because the gas gauges on our pressure fueling panel are in LBS, which is what they use to judge how much they give us. Thinking back, that makes more sense as to why we had to convert our order to GAL from LBS on the Beech. The Beech did measure in lbs, but there were no external gauges for the gas man, so they had to use the truck gauge. So I got it backwards.
 
You weren't the first, won't be the last. It's an insidious mistake waiting to happen every time we put fuel in a jet. Or even talk about it here. :D

Teller1900 said:
I should have phrased that better. We order it in LBS from them because the gas gauges on our pressure fueling panel are in LBS, which is what they use to judge how much they give us. Thinking back, that makes more sense as to why we had to convert our order to GAL from LBS on the Beech. The Beech did measure in lbs, but there were no external gauges for the gas man, so they had to use the truck gauge. So I got it backwards.
Click to expand...
 
Teller1900 said:
Thinking back, that makes more sense as to why we had to convert our order to GAL from LBS on the Beech. The Beech did measure in lbs, but there were no external gauges for the gas man, so they had to use the truck gauge. So I got it backwards.
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That makes more sense because I know the King Air measured in lbs and it's just a baby Beech.
 
I wish the trucks would use pounds. I'm used to pounds from jet and KA flying. The 210 has fuel gauges marked in pounds and gallons both (pounds are easier to read) and the fuel flow meter is in pounds. The JPI EDM 700 will display in either pounds or gallons, depending on how you set it. I'd about as soon be consistent and do everything in pounds but haven't decided whether to change the 700 setting or not. Converting to gallons for fuel ordering is not a big issue. When I want to take a given amount of gallons to meet a landing fee minimum I can just order the gallons and multiply by 6 to set the 700. Otherwise, all planning would be in pounds. Oh, well. I'll see.
 
jnmeade said:
I wish the trucks would use pounds.
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That would be pretty hard to do. Flowmeters measure volume, not mass, and creating and calibrating a system to measure the change in weight of the truck's fuel tank before/after refueling would be rather expensive.
 
I decided it was easier to stay with gallons whenever possible, just to eliminate the potential mistakes. Even so, I found myself second-guessing the calculation from time to time. Maybe because it was too easy.

jnmeade said:
I wish the trucks would use pounds. I'm used to pounds from jet and KA flying. The 210 has fuel gauges marked in pounds and gallons both (pounds are easier to read) and the fuel flow meter is in pounds. The JPI EDM 700 will display in either pounds or gallons, depending on how you set it. I'd about as soon be consistent and do everything in pounds but haven't decided whether to change the 700 setting or not. Converting to gallons for fuel ordering is not a big issue. When I want to take a given amount of gallons to meet a landing fee minimum I can just order the gallons and multiply by 6 to set the 700. Otherwise, all planning would be in pounds. Oh, well. I'll see.
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My old mooney, had pounds on the fuel gauge.

Not adding anything just wanted to hear myself type.
 
Ron Levy said:
That would be pretty hard to do. Flowmeters measure volume, not mass, and creating and calibrating a system to measure the change in weight of the truck's fuel tank before/after refueling would be rather expensive.
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Actually with today's technology it would be easy to dispense in pounds. It would also be fairer since as you pointed out earlier, fuel energy is directly related to mass not volume. Even the avgas we buy varies a small amount with temperature so when you buy fuel from an above ground tank in the summer you're getting less energy per gallon (and $$) than you'd get from cooler fuel stored underground (rare these days for environmental reasons).
 
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