The International Cessna^® 170 Association

This may be the crucial difference between what Cessna advertised back in the day "two 21 gallon tanks" and why the planes are placarded for 18.5 useable per side.

N9149A wrote:George in my case the tank was empty but the line down to the fuel valve probably still had fuel in it. I don't think that would make up the difference though.

Engineer strikes again! A piece of 3/8 OD, .035 wall tubing would have to be over 260 feet long to hold a gallon (231 cubic inches).

Miles

Miles,
What a remarkable coincidence! That happens to be the exact length of my fuel line! It makes a few ess turns in the belly before connecting to the selecter valve.

The schlit plokens.

I just went through a similar process in calibrating a fuel measuring stick (the straw type that you put your finger over). What I did was:

Put the fuel selector on Right position

Drained all the fuel from the right tank from the gascolator on the lower part of the firewall. This was performed hoping that the 'unusable' fuel only remained in the tank, while the plane was on a flat hangar surface (7.00 tires...)

I then added fuel in 5 gallon increments. When I reached 18.5, I could not get any more in. So, I think the 37 usable seems reasonable, and that is the value that I use with confidence.

Now, how much was still left in the tank after I drained it down, not sure. But I will sit here in the Montana spring weather (i.e. not snowing today..) and ponder that question.

The "FuelHawk" brand of clear, acrylic/lexan, fuel dip-tube, ... the one for the 19 gal Cessna 172...will work perfectly for a 170 A/B by using a tubing cutter and removing the lower end of the tube at the "0" mark. In this fashion, the remaining graduations are correctly indexed. (Polish the end of the tube with a wire wheel to return it to it's orginal "blunt" radiused end. Remove the "172" marking from the tube to satisfy your anal lawyer.)

In other words, the 172 tube, below the "0" mark, has additional markings below that index mark. By removing that additional portion, the Fuelhawk works perfectly. (Placing the tube down into the tank at the fillerneck, you will find an anti-slosh baffle within that tank. An accurate method to repeatedly obtain good results will be to place the lower end of the tube adjacent to that baffle, (which can easily be located by manipulating the tube.)

Realized tonight why my fuel gauges have such a cushion factored in, 8.50 tires and 180 gear legs. I raised the tail up a little to where I thought a stock 170 might be and the gauges came up considerably.

I'm going to make a fuel dip stick. Plan on draining the tanks from the wings. Will add 5 gals a wing and mark increments. Any suggestions? After draining the tanks and out of curiosity, I might raise the tail up to level, and measure how much more gas drains out, but I'll put that amount back in to start the gauging.

Don't forget to turn your fuel selector valve to one or the other tank while filling a tank for calibration purposes. This will perclude fuel cross flow, due to tank level imbalance, if left in the "BOTH" position.

Switched fuel selector to right tank.
Drained the right tank, dry, through the sump valve on the wing.
Readied my new "FuelHawk" brand of clear, acrylic/lexan, fuel dip-tube, ... the one for the 19 gal Cessna 172, but I didn't cut the first 2 tiks off George (just now dug this subject back up to see that suggestion).
Drew out a matching scale on paper to record data.
Put in 4 gals to reach zero (I gauge by sticking tube against anti-slosh baffle and back of filler neck also). Guess 4 gals would be 2 tiks if stick was modified?
5 gals reached the 1 tik and so I thought good deal (1 tik = 1 gal).

This 5 gals was indicating the middle of my "No Take Off" zone on interior fuel guage. I lifted the tail wheel axle 34 inches to simulate flight conditions. The indication went up to the top edge of "No Take Off" zone. So in flight, when I get down to the "No Take Off" zone, I have 10 gals left. Put the wheel back on the floor and filled until the needle hit same mark at 6.35 gals. So it seems there is a 1.35 gal difference in tail up or down.

Continued marking every gal - however at about 16, each gallon added jumped 2 tiks on the stick, all the way to 20. (What is this?)
20 gals filled to the brim of the filler neck.

I'm pondering the usuable / nonusuable thing this morning.

Just thought of something - wish I would've lifted that tail back up to see if I could've added 1 more gallon. Glad I didn't though, my back is pulled this morning! Also my plane is a 55' on 8.50's with 180 gear legs.

It has been my observation, through flight testing, that every ounce of fuel in the tank is available to the engine when in level flight. This has been accomplished by applying very slight rudder pressure to keep the inclinator ball about one half ball opposite to the tank in use. That is, ball to the right when on left tank, etc..
I ran these experiments when I wanted an empty tank to flight test the two kinds of Mexican Mogas.
When the engine quits, the fuel selector is switched over and the engine picks up in about two seconds.
It has always been my practice on a long trip to run the tanks on "Both" down to about one quarter and then switch to one tank until the engine quits. I then have a pretty good idea of how much is left. If I'm then coming in to land before it quits, I switch to the fullest tank.

I agree, it's been my experience that when the plane is flown out of gas on both tanks, there is no gas left in them tanks. None!
IF I strap my tanks and the guage reads 4 gals on each tank, then I have 8 gals USUABLE left. I might have to look up that Usuable definition again, when I get back in with another bucket load of firewood.

The "unuseable" fuel spec is due to the certification requirement that "useable" fuel be available in all normal flight attitudes. The technique described by BluElder is not "normal" or ordinary.
The "no take off" range red-arc/placard on the fuel gauges is neither a required placard nor an aircraft limitation (regardless of it's adviseability.)

Dave,

You may have just demonstrated what I've believed for some time now: that with larger than stock tires and/or longer than stock gear legs, the added height of the nose on the ground moves the apex of the tank forward, so that the filler is no longer at the top of the tank, and you can't get 21 gallons in it any more without raising the tail some.

Come to think of it, given that tires larger than 6.00 are approved equipment, maybe part of the unuseable fuel component is to allow for the ullage due to larger tires.

Next time I fill a wing up I might have to lift the tail and see if there is room for that last gallon. I'm thinking like you!

I was told the 2.5 unusable was because if you are coming in to land with 40 degrees flap, your nose is pitched so far down that the fuel is rolled toward the front of your tanks. The fuel line comes out of the rear of the tanks and if Cessna had put a fuel pick-up at both the front and rear of the tanks then we could count all the fuel as usable.