Fuel Calibration Question

[Shawn]

Just got my first new to me plane and picked up a universal fuel dipstick and have a calibration question. It has standard tanks of 65 gallons/60 gallons usable. That would be 30 gallons per side usable.

My last check came up with a reading of 6.5 on the dipstick on the left tank then I added 9.8 gallons to top it off.

My question is what do I subtract that number from to find out fuel remaining in the tank...the useable or unusable number?...is there 20.2 gallons remaining or 22.7 gallons remaining at a reading of 6.5 on the dipstick?

Commons sense says 20.2 but wanna make sure I am thinking about it correctly as I build a chart for the dipstick calibration. Unfortunately I haven't been able to find a pre marked stick for my tanks.

 

[docmirror]

I hate to say your're doing it wrong, but you're doing it wrong.

Calibration has to be - well, calibrated.

Drain one tank completely. If that means burning it off in the air, make sure you have adequate fuel to land on the other tank. park on a level surface, stick the tank, make sure it is completely dry. Now, add 5 gallons, and stick the tank and record the location, add 5 more, and stick and record. Keep doing this in 5 gallon increments until full.

Lather, rinse, repeat for the other side. In most cases you will 'find' gas that you didn't know you were hauling. In some case you will 'lose' gas you thought you had.

 

[bnt83]

I hate to say your're doing it wrong, but you're doing it wrong.

Calibration has to be - well, calibrated.

Drain one tank completely. If that means burning it off in the air, make sure you have adequate fuel to land on the other tank. park on a level surface, stick the tank, make sure it is completely dry. Now, add 5 gallons, and stick the tank and record the location, add 5 more, and stick and record. Keep doing this in 5 gallon increments until full.

Lather, rinse, repeat for the other side. In most cases you will 'find' gas that you didn't know you were hauling. In some case you will 'lose' gas you thought you had.

I disagree with drained completely. Load the airplane typical and run one tank dry in flight. Everything else, is spot on.

 

[Silvaire]

I'm not sure I follow the confusion. You have 30 gallons usable, you added 9.8 to fill it so that means you had 20.2 usable when the stick read 6.5. Your total, including unusable would be 2.5 gallons more than that.

As for calibrating the stick, because of the odd shape of fuel tanks and/or ground attitude of some aircraft you cannot depend on the dip readings being linear (although they probably wouldn't vary that much) The only way to get a verified calibration of the stick is to start empty and add a known amount such as 1-5 gallons at a time and read it.

 

[BillTIZ]

That 6.5 must have been a wrong reading. Need to be careful when dipping the T-41B tank, it is possible to have the false bottom of being on a higher step in the tank and not on the true bottom.

I think that's what happened to you.

As another noted, measure 6.5 and add 9.8 = 16.3? Does not total the 32.5 per tank 65 total.

Edit: is your 6.5 reading supposed to be a gallons reading? Or is it an arbitrary scale that needs to be checked against a calibrated sheet, 6.5 scale = 20 gallons?

Most dip sticks are to be calibrated in gallons.

 

[Shawn]

The 6.5 is just an arbitrary number on the dipstick since it is a universal stick. No relation to gallons, just a marker number. There is not a stick that is pre calibrated to my tanks.

Starting from an empty tank makes a lot more sense...hence the reason I asked.. but flying to a forced engine failure in flight sounds like not a lot of fun!

 

[flyingron]

You don't really know how to drain the tanks to zero usable so that is fairly pointless. What you need to do is to get them down fairly empty (below where you'd ever care to think about flying without fueling). Then add fuel in increments (marking the stick) until you're at the tabs or filler neck or whatever gives you the rated capacity on the tank. Yes this ends up being "backwards" but you can number it down from full. Just use the nearest full gallon, most aircraft tanks are so flat and shallow that dipsticking it isn't that accurate anyhow.

 

[weirdjim]

For those of you suggesting that you run one dry in flight and then switch tanks ...

Keep my number handy. I make money with used airplane parts I pick up from salvage aircraft.

Not that a fuel valve has ever frozen up during a flight ...

Jim

 

[Silvaire]

I'm with you Jim. Why would you ever want to put yourself in a position that if you switched tanks and something went wonky you couldn't switch back to the one that was working?

 

[docmirror]

The 6.5 is just an arbitrary number on the dipstick since it is a universal stick. No relation to gallons, just a marker number. There is not a stick that is pre calibrated to my tanks.

Starting from an empty tank makes a lot more sense...hence the reason I asked.. but flying to a forced engine failure in flight sounds like not a lot of fun!

If your aircraft was certified after 1938, it will continue to run after one tank is run dry when switched to a tank with fuel in it. That is a certification requirement from CAR3 that was carried forward. Every aircraft type tested must pass the empty tank switchover test before certification.

If you don't like draining it in the air, then do it on the ground, I don't care which, but it has to start dry.

 

[docmirror]

For those of you suggesting that you run one dry in flight and then switch tanks ...

Keep my number handy. I make money with used airplane parts I pick up from salvage aircraft.

Not that a fuel valve has ever frozen up during a flight ...

Jim

I'm with you Jim. Why would you ever want to put yourself in a position that if you switched tanks and something went wonky you couldn't switch back to the one that was working?

If one is worried about an in-flight fuel valve freezing, you should go buy a lotto ticket right now. the payoff is high and the odds of you hitting the lotto number are better than having an in flight fuel valve freeze.

Life has risks, minimize the ones you can. The OP is trying to get an accurate measurement of his fuel to prevent such an occurrence. Good for him. If he doesn't want to do it in flight, then don't. But all you nervous nellies might as well stay snug in your bed. Oh wait, millions of people die in bed each year. Can't do that.

 

[bnt83]

If your aircraft was certified after 1938, it will continue to run after one tank is run dry when switched to a tank with fuel in it. That is a certification requirement from CAR3 that was carried forward. Every aircraft type tested must pass the empty tank switchover test before certification.

If you don't like draining it in the air, then do it on the ground, I don't care which, but it has to start dry.

THAT BEING SAID. When I DO run one dry, I MAKE sure it feeds off the other tank PRIOR to empty on the low tank.

 

[BillTIZ]

If you want to run a tank dry for calibration.
And think your valve will freeze.
Then do it over the airport, then glide to landing if fuel does not flow.

But if you are concerned that the valve won't work, never get more than one tank away from a landing. You'll never get anywhere.

 

[bnt83]

For those of you suggesting that you run one dry in flight and then switch tanks ...

Keep my number handy. I make money with used airplane parts I pick up from salvage aircraft.

Not that a fuel valve has ever frozen up during a flight ...

Jim

You do know that it SAYS right in many fuel injected cessnas "for maximum range to run tank till fuel pressure drops off" right?

It's a non event. I still don't recommend at low AGL...

 

[flyingron]

If you want to run a tank dry for calibration.
And think your valve will freeze.

Most of us would like to calibrate to zero USABLE, not bone dry.

 

[bnt83]

Most of us would like to calibrate to zero USABLE, not bone dry.

The other way to get that would be to run the tank low then:

#1 Level the airplane on the ground

#2 Drain the low tank not from the tank sump, but from somewhere downstream like the gascolator.

 

[docmirror]

Most of us would like to calibrate to zero USABLE, not bone dry.

Not me. I want to know every drop I've got. Although once you start at bone dry, adding the unusable fuel and taking a stick dip isn't a bad idea.

 

[denverpilot]

You don't really know how to drain the tanks to zero usable so that is fairly pointless. What you need to do is to get them down fairly empty (below where you'd ever care to think about flying without fueling). Then add fuel in increments (marking the stick) until you're at the tabs or filler neck or whatever gives you the rated capacity on the tank. Yes this ends up being "backwards" but you can number it down from full. Just use the nearest full gallon, most aircraft tanks are so flat and shallow that dipsticking it isn't that accurate anyhow.

And then hope a bladder tank doesn't develop a wrinkle right user the filler cap hole. (Grumble. Grumble. Damn it.)

 

[weirdjim]

If one is worried about an in-flight fuel valve freezing, you should go buy a lotto ticket right now. the payoff is high and the odds of you hitting the lotto number are better than having an in flight fuel valve freeze.

THere is a famous quote from John Glenn taken right AFTER he got back down onto the ground safely. A reporter asked him his immediate thoughts just before they lit the fuse.

Glenn's answer: "This whole &*^%$% thing was built by the lowest bidder."

O-rings are priced to sell in market. You think a little fifty cent piece of neoprene will never bind up then you are betting your hiney on a hunch. Then again, how many fuel valves have you rebuilt looking at the condition of a seal last installed by Clyde Cessna himself? They aren't in showroom condition.

Jim

 

[flyingron]

Not me. I want to know every drop I've got. Although once you start at bone dry, adding the unusable fuel and taking a stick dip isn't a bad idea.

Not me, I don't count on burning my unusable fuel. I want to know how much USABLE I have.

 

[Silvaire]

If one is worried about an in-flight fuel valve freezing, you should go buy a lotto ticket right now....

Well it doesn't have to be that, it could be any number of things such as contamination or a blocked vent. I just don't see any legitimate reason for running a tank dry in flight. I know people do it but no one has ever convinced me that it's something I need to do. Even for calibration purposes the average unusable fuel quantity is maybe 10 minutes so what's wrong with a calibration error that's 10 minutes on the conservative side? Are you really expecting to ever cut it that close?

 

[weirdjim]

Not me, I don't count on burning my unusable fuel. I want to know how much USABLE I have.

Interesting thread. As I recall, the certification requirements for the airplane require the manufacturer to put the aircraft into the most extreme attitude that will maximize the unusable fuel. In general, this is a "whee" maneuver, nose extremely high, in a skid or slip.

In most of what we fly, the "unusable fuel" generally amounts to less than a pint in a straight and level, normal or slightly nose down attitude.

But if you fly with the nose straight up...

Jim

 

[docmirror]

Not me, I don't count on burning my unusable fuel. I want to know how much USABLE I have.

Well, let me put it another way. If you start with fuel in the tank, and you have no calibrated method to determine it, how do you know you have your unusable fuel onboard? Maybe you have more, maybe you have less. If you start with the tank completely dry, you can then calibrate your dip stick to find out a measure of unusable fuel. There's no way to figure it unless you start at zero. I already said it would be a good thing to know, you just can't start at that point, because you have no reference.

 

[Dan Thomas]

The TCDS, if there is one for the airplane, should give the unusable fuel. One can deduce it from the difference between the placard at the fuel filler, which give total fuel, and that on the selector or shutoff, which gives usable fuel.

You want to start at the unusable fuel being the zero point for any dipstick. Unusable fuel can get your attention real quick if you have, say, full flap on approach, which means nose down some, and the engine quits. One of our guys had it happen in a 172.

Dan

 

[NineThreeKilo]

I hate to say your're doing it wrong, but you're doing it wrong.

Calibration has to be - well, calibrated.

Drain one tank completely. If that means burning it off in the air, make sure you have adequate fuel to land on the other tank. park on a level surface, stick the tank, make sure it is completely dry. Now, add 5 gallons, and stick the tank and record the location, add 5 more, and stick and record. Keep doing this in 5 gallon increments until full.

Lather, rinse, repeat for the other side. In most cases you will 'find' gas that you didn't know you were hauling. In some case you will 'lose' gas you thought you had.

+1

That's how I did it on mine, it's the best and most sure fire way.

I ran my tanks really low then opened the sump into a catch can, empty tank.

Then did the process doc listed out.

 

[Shawn]

That makes sense how to drain and start from dry tanks...so back to the premise of my original question, from a bone dry tank then I should be able to in theory cram 32.5 gallons in each tank (65g total capacity/60g usable), correct? 30 of which is "usable" and 2.5 "unusable"...then calibrating my "E" or 0 usable gallons mark on the dipstick once I fill 2.5 gallons...right?

Unusable, which is basically buffer in the tank to make sure that there is always fuel at the fuel line vs "undrainable" which would be fuel in the lines and system...am I thinking about that all correctly?

 

[Capt. Geoffrey Thorpe]

Just got my first new to me plane and picked up a universal fuel dipstick and have a calibration question. It has standard tanks of 65 gallons/60 gallons usable. That would be 30 gallons per side usable.

My last check came up with a reading of 6.5 on the dipstick on the left tank then I added 9.8 gallons to top it off.

My question is what do I subtract that number from to find out fuel remaining in the tank...the useable or unusable number?...is there 20.2 gallons remaining or 22.7 gallons remaining at a reading of 6.5 on the dipstick?

Commons sense says 20.2 but wanna make sure I am thinking about it correctly as I build a chart for the dipstick calibration. Unfortunately I haven't been able to find a pre marked stick for my tanks.

Which do you want to track? Useable or total fuel?

30 -9.8 gives you 20.2 useable when the stick says 6.5. Keep flying and filling and sticking and eventually you will figure out a bunch of points. Throw it into Matlab or Excel or Open Office and do a curve fit.

The other question is "what do they mean by usable". Someone will quote regulations, but in my opinion (and some may consider this heresy) regulations do not keep an airplane aloft. And in my experience (as in what actually happens in the air) what you can really get out of a tank can vary quite a bit.

 

[bnt83]

As I recall, the certification requirements for the airplane require the manufacturer to put the aircraft into the most extreme attitude that will maximize the unusable fuel.

You make a good point. Technically unusuable as it applies to an airplane already in service is

drainable unusable + trapped fuel = total unusable

So, when you weigh an airplane you drain the sumps and add in the drainable unusable ammount. I have no idea how many GA aircraft specify a drainable unusable figure but the turbine world often does.

For the sake of calibration of a dipstick on say a typical high winged cessna, leveling the airplane and draining the tanks from the sumps and adding the amount specified on the TCDS (or STC if the tanks have been modified in any way) would be a good zero method. I would also check the calibration sticker on the fuel pump used to fill the airplane with.


We take for granted that the gas pump is in calibration which may not be the case.

 

[Shawn]

In reality I want to be able to track my usable fuel...so for a 32.5 gallon tank at 2.5 gallons I would want to know that I am at "E" for planning and timing purposes.