Fuel quantity indication, air vs. ground

N

Neoflyer

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Feb 10, 2020
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Just purchased a ‘76 7ECA. The manual says the fuel fuel quantity gauges are only accurate, if there is such a thing, in level flight pitch attitude. But it doesn’t give any direction for how they read on the ground. Would they indicate more or less fuel when sitting on the ground than in level flight?
 
You can make one yourself. Get a couple of paint stirring sticks and a sharpie. As a quick and dirty solution, fill your tanks to the bottom of the neck, dip the stick and mark as full, then divide the length below into 1/4 increments. Bob has probably done this a million times, so I bet he knows the best way to find more accurate markings.
 
Big Ed said:
You can make one yourself. Get a couple of paint stirring sticks and a sharpie. As a quick and dirty solution, fill your tanks to the bottom of the neck, dip the stick and mark as full, then divide the length below into 1/4 increments. Bob has probably done this a million times, so I bet he knows the best way to find more accurate markings.
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I've already made mine, following the exact approach given prior. However, I wonder why we're all making our own. If the fuel tanks are identical (are they?) then the calibrated sticks will all be identical. At that point, it certainly makes a lot of sense to have one canonical dipstick.

Although this doesn't really answer the OP's question about whether the tank gauge indicates higher on the ground or in the air. I *believe* I read somewhere it indicates higher in the air, but I can test it on my 7ECA next time I take it off the lift.
 
The gauge will read lower quantity on the ground. When I get a new to me plane, I drain the fuel out, and add the fuel back in at 5 gal increments and mark on a stick. I like to use 1/2 hardwood dowels and burn the markings onto the stick when finished.

I also like to drain and fill to ensure that the tank actually holds what it should hold. When we bought our 7GCBC, I found is weird that it only took 15 or16 gals out of the advertised 18. Years later that tank sprang a leak, so we had to take it out to repair it. When I cut the fabric back in the tank bay, I was amazed to see that the tank was completely crushed in. looked like a beer can that had been stomped. Needless to say, had to replace the tank.

Point is, I like to make sure what actually fits. I don't like to run low on gas so I like 1 hour of fuel when I land. If I was that legal 30 min type (4 gals), I'd be sadly disappointed have the engine so quiet on final when I thought I had 4 gals, but in reality I had 1... maybe.
 
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If you don't want to drain completely, just mark the stick every two gallons. Then assume your tank holds what it is supposed to, mark that on the top, and count down on your tic marks.
 
Tangogawd said:
The gauge will read lower quantity on the ground. When I get a new to me plane, I drain the fuel out, and add the fuel back in at 5 gal increments and mark on a stick. I like to use 1/2 hardwood dowels and burn the markings onto the stick when finished.

I also like to drain and fill to ensure that the tank actually holds what it should hold. When we bought our 7GCBC, I found is weird that it only took 15 or16 gals out of the advertised 18. Years later that tank sprang a leak, so we had to take it out to repair it. When I cut the fabric back in the tank bay, I was amazed to see that the tank was completely crushed in. looked like a beer can that had been stomped. Needless to say, had to replace the tank.

Point is, I like to make sure what actually fits. I don't like to run low on gas so I like 1 hour of fuel when I land. If I was that legal 30 min type (4 gals), I'd be sadly disappointed have the engine so quiet on final when I thought I had 4 gals, but in reality I had 1... maybe.
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Thak you. Finally an answer to my question, higher or lower indication when on ground versus flight.
 
Big Ed said:
You can make one yourself. Get a couple of paint stirring sticks and a sharpie. As a quick and dirty solution, fill your tanks to the bottom of the neck, dip the stick and mark as full, then divide the length below into 1/4 increments. Bob has probably done this a million times, so I bet he knows the best way to find more accurate markings.
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That doesn't work. With the tank tipped at an angle the marks are not at all linear.
Bob Turner said:
If you don't want to drain completely, just mark the stick every two gallons. Then assume your tank holds what it is supposed to, mark that on the top, and count down on your tic marks.
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Sigh. You people work way too hard. Who's got time to add two gallons at a time and make marks? At my airport I would be drawn and quartered for parking at the pump for that long. Here's the lazy man way.

Start with a random amount of fuel in each tank. Unless your plane is parked on super flat ground and left to crossfeed for a while they will have different amounts (yay!). Dip the tank. Make a mark. Fill the tank. Subtract the amount you just added from your tank capacity. Just added 7 gallons to an 18 gallon tank? Write "11" next to your mark. Easy, right? You made two marks, right? Now you have two numbers. Who cares if one of them is 6.5? It's an accurate number.

Fly the plane. Some days you run the tank down low and others you just fly for 45minutes. In 3-5 flying sessions you'll have a calibrated dip stick for your plane. That last couple of gallons is a fools game. By the time you hit 3 gallons that is about the last mark you can make because the remaining fuel will be at the back of the tank and your stick will be hitting dry steel.

Once your stick is sufficiently marked, continue to use it at every fuel stop so you can verify it is accurate. Mine was within 0.25 gallon for my 13 gallon tanks.

Given that the fuel gauges are a complete joke in most planes and certainly in the Citabria I put zero trust into the reading. My fuel quantity came from dipping the tanks, using the Hobbs and knowing the fuel burn for MY airplane.
 
Mine was done the two gallon way (one gallon for the J3s and the J4. Didn’t take more than five minutes for the initial marking. Of course, the markings are accurate at the 3 point position - I do not dip during flight.

In flight my gauge is a clock. If I see a fuel gauge do something weird, I land and dip. Happens on Cubs that use fish line floats.

Pretty sure the Dec stick is linear. Will check this afternoon.
 
Big Ed said:
It works sufficiently for my purposes. It's linear enough, except at the very top of the range.
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Ah, as Bob pointed out the decathlon tanks may be a different shape and read in more linear fashion. The rectangle tank of the Citabria is not linear at all when tipped at the three point attitude.
 
If anybody here is interested in purchasing a non specific aircraft model “Fuel Hawk “ gauge that is made for custom calibration , I think that they are about $16 , I will send you the gallon quantity/gauge calibration numbers that I made up by adding one gallon at a time to a empty fuel system all the way down to the gascolater on my 73 7GCBC ( 18 gal tanks ) . The gauge is a lot longer than needed for our tanks and can be cut to convenient length with a tubing cutter so it and the custom laminated story card fit in the door side pocket along with the 87 lbs of other shit crammed in there .
 
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When at the hangar tomorrow morn , I’ll take a pict of the calibrated story card and can text/email it .
My bird was on standard spring gear and 8:00 - 6 tires when calibrated
 
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