The weight of gasoline for aircraft use isn't necessarily what you think it is.

Please pass this post on to anyone who is flying with autogas in their airplane -- it will open their eyes as to the true weight of the gasoline!

I've been calculating the weight of gasoline as 6 pounds per gallon ever since I was a 19 year old newbie pilot.  

That's a critical number, used by several hundred thousand pilots frequently as they calculate weight and balance on their aircraft.  WikiAnswers is a little more precise and shows it as 6.02 pounds per gallon.

However, the bigger surprise for me has come to my attention as I've started to fly ultralights.  I don't often use 100LL anymore, having switched to 91 octane premium car gas.  I'm flying with car gas.  So, here's the pertinent question:



Q:  What does car gas weigh?
A:  It's not 6 pounds per gallon.  Not really close...   Here's an answer I found posted on the internet:

"When I worked in an automotive engine test lab - dynamometer tests - we often had  to calculate the brake specific fuel consumption (BSFC) of the test engines. We had a chart for fuel density vs temperature for our fuels that was updated monthly. As I  recall, the typical weight of gasoline at 72 degrees F was around 6.25# per gallon. As it became cooler it became more dense and thus weighed more and above this temperature it was less dense and a gallon weighed less."

Here's another answer I found posted on the internet:

Depends on the API gravity of the gasoline, which varies by grade and refinery.  Usually, regular unleaded gasoline has a gravity of around 58 and a weight per gallon of 6.216 pounds per gallon.  Premium gasoline may have a gravity of 54, or 6.35 pounds per gallon.

       (Source:   FAQ.ORG)

So I've been making errors in my weight calculations of the gas in my ultralight.  I shouldn't be using 6.0 pounds per gallon, I should be using 6.35 pounds per gallon.  That's significant, even for my little aircraft with their 5 gallon tanks.

It also means that all the guys that are flying auto gas in their STC converted airplanes (and there's a lot of them) are also likely making weight calculation errors in their aircraft.  Furthermore, most of the LSA community is running 91 octane in their Rotax powered aircraft -- they should be using 6.35 pounds per gallon as well.  This is very significant for aircraft such as my CTLS -- it holds 34 gallons of gas, and that weight calculation difference is 12 pounds (6.0 vs 6.35 pounds per gallon, calculated for 34 gallons)

But I'm not done.  What does the inclusion of ethanol mean in this equation?  The short answer is:  I'm not really sure, but the longer answer requires us to reweigh our gasoline one more time.  Ethanol's weight per gallon is far higher than gasoline, and the amount of ethanol mixed with gasoline at the pump is an uncertainty for any particular brand of gasoline -- they don't have to state that the gasoline they are selling you contains ethanol.  The gas can contain 0, 5% or 10%, and they don't have to tell you it's there.

According to Wiki Answers, 1 gallon of ethanol weighs 6.584 lbs.

I am positive that varying mixes of ethanol per gallon of gasoline will cause varying weights of the resulting mix.  But I can't prove it... at least not yet.

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