Gas gauge calibration

2723D · Post by **2723D** » Sun Apr 06, 2003 10:11 pm

Today I tried to calibrate one of those handy-dandy "universal fuel gauge" sticks. I was hoping to get a better idea of the exact amount of gas in the tanks than the built in gauges. I haven't been able to find a 170 calibrated fuel stick. So I emptied the left tank of gas (put the gas switch on right tank only so they wouldn't cross feed) and began adding one gallon at a time and measuring off the dipstick what the value was. Everything was going great until I got to the top of the tank and I was only at 19 gallons. I thought they were 21 gallon tanks? So I have a number of questions, could there still have been gas in the tank when I started (I emptied it through the quick drain), or is it impossible to really top the tanks off given the angle? Oh, another thing, I just put 180 gear on my plane so it sits a bit higher now (greater angle), but would that make a 2 gallon difference? I also felt I might be getting some crossfeeding into the other tank once i got near the top as well (through the vent system?).

Anyone tried doing this? Any ideas? I'm also thinking that the gas station's meter might be off, but in my favor? If so, I'm staying quiet!
Shane

hsjrev · Post by **hsjrev** » Sun Apr 06, 2003 11:32 pm

My manual says 37 gallons useable, that would be 18.5 gals per side. I assume that the "unuseable" fuel in the tank can't be drained by normal means.
(this is a 170A with 20 gallon tanks)

Post by **GAHorn** » Mon Apr 07, 2003 3:41 am

Correct. All you'll be able to measure is approx 19 gals. of useable fuel. A FuelHawk brand clear measuring tube (available from Sporty's or Aircraft Spruce) for a 172 is perfect, if you use a tubing cutter and cut off the two lower gallon indices (below the zero). Then, use a wire wheel on a bench grinder to polish the cut end to a smooth radiused finish like previously. Place the tube at the aft end of the fuel-cap opening, and the bottom of the FuelHawk onto the bottom of the tank exactly against the baffle at that mid-tank line. (Easy to find,....just put it on the bottom and try to move the tube aft. It'll run into the baffle.) It will read correctly when so modified and used.
Otherwise, any paint store will give you a paint-stir stick which can be marked/calibrated with an ordinary ball point pen.

2723D · Post by **2723D** » Mon Apr 07, 2003 4:47 pm

Thanks for the feedback, that sounds about right.. I used the fuelhawk brand gauge, but not the one for a 172, you have to calibrate this one and then use a conversion factor to get the gallons. So I guess the take home message is that even when I drain the tanks via the quick drain there must be about 2-3 gallons still in there; George is this correct? I understand the unuseable fuel from the engine perspective (the exit tube is at a low point when the plane is level and flying but higher than the quick drain when on the ground), but the quick drain is further back and my mechanic assured me it would drain all the gas... I plan on flying more this summer on lower tanks (short field work) and want to be light

-- but also make it back home

.

Shane

Post by **GAHorn** » Wed Apr 09, 2003 4:24 pm

Think about this. Why would you be interested in measuring unuseable fuel?
Unuseable fuel is that fuel which is not available to you in all normal flight attitudes.
It's unuseable, so therefore it's of no importance to you. (The only time unuseable fuel is of interest is when computing certain wt and balance information. Don't bother trying to make a dipstick to measure it...... You can't use it or plan to get to it in flight.)
You are only interested in the fuel you have available for flight. Right?

(In the three-point position, the wing drains ordinarily will drain all the fuel from the tanks. Some of this fuel is unuseable fuel and not available to you inflight.)

2723D · Post by **2723D** » Wed Apr 09, 2003 5:52 pm

Actually, it is very important whether that unuseable fuel amount (3 gallons) was still in my tanks when I started because that is my starting point, or conversly my ending point (when I run out of gas and fall out of the air

). I drained the tanks, and then filled it up but only got 19 gallons into it, which means that 2 gallons were either still in there (i.e. didn't drain) or ??? I'm concerned about exactly how much gas I have at the low end, when my dipstick measures 10 gallons in the tank and I have 45 minutes to fly home (at 9 gal/hr = 6.75 gallons used) that leaves 3.25 gallons (0.25 useable). Yes that is very close, just an example to illustrate why it could be a concern, it becomes very important where those three gallons are. When I tried to calibrate the fuelhawk stick I didn't get my first reading until I had pumped 4 gallons in the tank (which sounded right, given that 3 would be unuseable (not available say on landing in a 3-pt config.), and the next gallon would read -- my only consternation was that when I got to the top it only was 19 gallons instead of 21. Which might mean that my first measurement was really 6 gallons (I started with 2 in there, plus the four) - so that mark would either mean that I have 1 gallon useable (4 I put in minus the 3 of that which is unuseable) or 3 gallons (4 I put in, plus the two stuck in there minus the three unuseable) - a substantial difference when you count both tanks thats a difference of 4 gallons, or another 1/2 hour flight time.

So my real question for anyone who has ever worked on these gas tanks, is that when you drain them via the quick drain, does all the gas come out, or is there some left in there that you have to drain in a different way? This is a different question than the "useable" one for flight.

Post by **GAHorn** » Wed Apr 09, 2003 6:14 pm

Well, perhaps I don't understand your question, but it seems to me that you're getting all wrapped around the axle over something that is of neither consequence nor concern.
Unuseable/undrainable fuel is not what you want to measure when you're cutting it as close as you apparently feel the need to do. In fact, that is the fuel you really don't want to consider whatsoever,...so there's no reason I can see to calibrate a dipstick for it, especially when that dipstick can't get to it for the fuel to register anyway.

If you put your dipstick/fuelhawk into a tank and it measures zero, then you top it off by putting in 19 gals and the dipstick/fuelhawk measures 19 gals, ....then it's correct.
For purposes of flight, there's no valid reason that I can see to concern oneself with what's trapped elsewhere in the system.
Personally, I only want my dipstick to measure the fuel I can use. But maybe I just don't get it.
Perhaps you might alleviate your concerns by first topping off your tanks, then draining out a measured amount and calibrate your dipstick/fuelhawk in reverse order. There's actually 5 gals of unuseable fuel, consisting of approximately 2.5 gals in each wing tank including the plumbing system.

2723D · Post by **2723D** » Wed Apr 09, 2003 7:12 pm

The difference is that your method, using the fuelhawk, is reasonably accurate if you measure from the top and then you drain down.. i.e., fill the tanks, fly for two hours and then measure and you'll be pretty close. I'm looking the other way, go in as light as is reasonable to a short strip, so I would only have enough fuel for the trip plus a fudge factor for the 5 unuseable, and safety (another 1/2 to 1 hour fuel). My goal is not to do what I have been, which is have so much fudge factor fuel that the tanks are almost topped off. Having an extra 8 gallons of gas in there amounts to another 50-100 feet of runway needed for takeoff, on a 500-600 ft gravel bar, that gets important up here in Alaska. The differnce is really easy to tell, light with just me the plane is off in about 300 ft, add a passenger and/or fuel and you are up to 500 ft or more. Two people and 3/4 tanks and i'm talking 600-700 ft depending upon conditions.

N3243A · Post by **N3243A** » Wed Apr 09, 2003 9:44 pm

gahorn wrote:(In the three-point position, the wing drains ordinarily will drain all the fuel from the tanks. Some of this fuel is unuseable fuel and not available to you inflight.)

Well, I must confess that I am reluctant to step in this morass, but I feel Shane's concern is justified. As you stated George, if you drain the fuel from the sumps as the plane sits on the ramp, you should have empty or nearly empty tanks. So a measured refill should amount to about 20.5 or maybe even 21 gallons if you rock the wings a little. Why did his refill only take 19 gallons? Maybe his tanks are a little under spec in size or whatever? He simply wants the numbers to make sense (as would I).

But on the practical side Shane, I find that you really don't need to cut fuel that close when practicing short gravel bar work. I rarely go out without 1/2 tanks, even for just an hours flight up the Knik River. 8 extra gallons (or an hour) is only 48 lbs which 2.1% of the airplanes max gross weight. In my opinion, you shouldn't be on any gravel bar where 50 lbs of weight makes or breaks the takeoff safety factor. Better you get experience in analyzing the gravel bar for length and suitability before landing. And be Conservative! Risking Helicopter recovery and expensive sheet metal repairs are not worth even a 90 lb King Salmon. (You did budget some useful load for fish on takeoff didn't you?)

My two cents

Bruce

2723D · Post by **2723D** » Wed Apr 09, 2003 10:42 pm

Yes Bruce, I was being overly dramatic to make a point... As I said in the email, right now I almost always go with full tanks, I'm trying to convince myself to go with less.. to do that I want to get better at measuring fuel burn, to do that I need an accurate method for measuring gas without filling the tanks every time. But just for my knowledge, and god forbid, someday I get in a tight spot, strong headwind or something and use more fuel than I plan on, I would like to know what is left, and 4 gallons might mean the difference between making an airport or choosing to do something else before the engine dies. For me, the more I know about my airplane, especially at the margins, makes me a safer pilot. I'm not choosing to go to the edges, but I sure like knowing where they are, thats part of the fun of seeing just what this 170b will do at different weights.

zero.one.victor · Post by **zero.one.victor** » Thu Apr 10, 2003 4:53 am

Like George said,why not fill one tank to the brim,then drain from the gascolator til empty. Use four 5-gallon cans,drain 5 gallons at a time,and mark your paint-stick,er Fuel-Hawg stick,accordingly.Since usable's 37 gals,you should get 18.5 out,or a little more. Maybe put the tailwheel up on a stand to approximate level flight.
I recently calibrated my paint-stick,the first 5 gallon can I poured into my ragwing didn't even register on the stick with the airplane on all three wheels.

Eric

2723D · Post by **2723D** » Thu Apr 10, 2003 5:41 am

Talked to my mechanic today, he says when you drain from the sump, the tank is totally empty... so all the non-useable is gone, which is good to know. He thinks, and I agree, that the two gallons we can't get in there is what is actually above the fill cap - in other words, the fill cap is about 8-10 inches down from the top, in the three pt position (especially with new 180 gear on) that should leave an air hole up there that you can't fill up, in my bird I think that is where the 2 gallon problem arises. As Eric points out, I bet if you raised the tail to level flight attitude, you could get that extra two gallons in there (except when you set it back down it would probably run out of the vents... Good to know, now instead of having 18.5 useable in my plane (per tank) best I can do even topping off is 16.5, so that extra half hour fuel I thought I had when I started with full tanks really isnt there. I plan on filling it up and draining it to make sure.

Dave Clark · Post by **Dave Clark** » Thu Apr 10, 2003 12:52 pm

I had a thread going a few years ago about this. It is probably on the old site. Since you're only interested in how much fuel you can burn in flight and assuming you want to know when the engine will quit on your inadvertent emergency on the way home I did the following.I started by running a tank dry in flight then after landing leaving the fuel selector on the one with fuel in it to prevent cross draining I made a wood dowell with marks on it as I filled the tank. it seems to me I was able to put almost 19 gallons in it so I think your theory on the air forward of the fuel cap in the three point position might be wrong.

My real purpose was to compute range in hours and minutes with a full tank and at a specific power setting for my normal 7500 or 8500 ft cross countries. The watch is the ONLY true fuel gauge. You might consider getting a fuel flow/totalizer if you want to cut it that close. After two close calls in my early flying days I always error on the side of carrying extra fuel. I'd rather do that and forsake some of the gravel bars than take unnecessary chances.

Of course now with the 180hp and Javelin tank I can more easily take the extra weight

Post by **GAHorn** » Thu Apr 10, 2003 6:30 pm

RE: ... if you drain the fuel from the sumps as the plane sits on the ramp, you should have empty or nearly empty tanks. So a measured refill should amount to about 20.5 or maybe even 21 gallons if you rock the wings a little. Why did his refill only take 19 gallons? Maybe his tanks are a little under spec in size or whatever?....

It's doubtfull (unless the tanks came from a wreck and are somewhat "squashed" inside the wing) that his tanks are any different in size that everyone elses. These tanks are made from stampings and are uniform in size. (3% or .63 gallons is all the worst-case difference allowed for certification.)
It's more likely that the dispensing equipment calibration, fuel temperature, aircraft position, etc. all add up to any differences observed.
If a gallon or so of fuel are all that important to you then I would think the only way to do it would be to fly it 'til it quits, and dipstick it, and calibrate from that point. Notice that I did not say "the only safe way" or "sensible" way. It seems to me we're discussing an inherently risky operation that needs to regularly cut it this close. It sounds like it's time to buy that Nitrous-oxide mod from Eric.

2723D · Post by **2723D** » Thu Apr 10, 2003 7:07 pm

It would be great if people wouldn't put so many of their value judgements on a simple request for information. I would like to know how my airplane works, not because I want to go out and kill myself, but because it says I have 18.5 useable but I don't think I really do, so when I figure I can fly 4.5 hours, I really can fly only 4, so that means I'll plan for about 3, giving quite a bit of a safety margin. I was intrigued by the whole process because as a new pilot, and new 170 owner (just one year) i'm learning tons about my plane and flying, i'm also a scientist and accuracy is something I strive for, and its fun too. Learing how short my plane can get off the ground, how far I can go, how the plane really works, I think makes you safer - not the other way around as I feel people are implying, and I see this in a lot of other posts as well which is why I haven't written much to this website. The information is great here, but the judgements are unnecessary.

The idea of unuseable fuel is interresting to me as well, I have read a lot of the NTSB reports of accidents, and they often note for people running out of gas that the tanks were bone dry, indicating they ran out of gas completely. If there is 2-3 unuseable in there, why is it that so often when people crash, there is nothing left in there (these are 170 reports I have read)? Maybe esoteric, but interresting as to why some people didn't make it home. My guess is that the gas is generally useable, except in high angles of attack, then the gas would be in the bottom of the tank, below the outlet. I also read the article in the 170 book that talks about milking the gas out of each tank by flying at different bank angles etc in order to get every drop out of a tank in an emergency. Not sure if anyone has tried that. I do like the idea of flying one tank until it runs out (with a lot of altitude to recovery of course), that is probably the best way to know how much flying time you have - then use that knowledge to keep you well on the safe side.