108-3 - How much useable fuel?

Gord:

Soon after we bought our 108-2 I wanted to also know exactly how much fuel the tanks held.  I did not trust any documentation.  Also I wanted to calibrate a fuel dip stick as well.  

My goal was to have accurate fuel readings on the dip stick as well as the inaccurate fuel gauges.  However, looking back I wished I had also checked for the amount of usable fuel as well.  But I will share with you what I did and also what I wished I would have done to determine the amount of usable fuel.

I drained one tank completely through the gasolator.  Then I poured fuel in one gallon at a time.  After pouring each gallon in, I then checked the fuel gauge and the fuel dip stick inputing each reading into a spreadsheet.  This was done with the tail setting on the floor and standard tires.  (I have since installed larger tires so I need to do this test all over again! Yes the diameter of the tires will make a difference!)  Once I had enough fuel in the tank to be measured by the dip stick, I then knew how many gallons it took to be measured by the fuel dip stick.  At that point, I began adding five gallons at a time and repeating taking readings on the fuel dip stick and the fuel gauge.  The end result is an accurate calibration of the fuel dip stick and somewhat of an indication where the fuel gauge reads at that amount of fuel.  

This test was better than nothing, but below is what I wished I had done and will do next time:

• Lift the tail and set the it on something so the airplane is level for flight which should put the fuel outlet at the lowest level.
• Turn the fuel valve OFF
• Turn the fuel valve on to the tank being drained.
• Drain all of the fuel out of the tank via the gasolator (This should simulate level flight conditions where all usable is used)
• Once empty, leave the fuel valve ON (open to the tank being tested)
• Fill the tank one quart at a time carefully observing for fuel to come out of the gasolator.  Once fuel begins to run out, the amount of fuel poured in is the amount of NON-USABLE fuel.
• Then I would repeat the above process with the tail on the ground to calibrate the fuel dip-stick and the gauges.

It has been suggested to me that a quicker method is to just run a tank dry, then switch to the other tank.  Then land, fuel up and read the fuel pump for the amount of USABLE fuel.  Although this makes sense, I have never done this for fear that something could go wrong, like the engine not restarting! ;-(  

One other tip Gord.  I never, ever trust the old fuel gauges.  I use an app on my iPhone called "Tank Timer."  After it is calibrated a few times by inputting fuel usage and hours flown, it is extremely accurate.  It calculates fuel burn differently for taxing, climb out and cruise.  It has never failed me and is within a fraction of a gallon.  However I input some safety factor, so I typically have more fuel remaining than it shows. 

Please let us know how you proceed.

Good luck Gord!

Bret C

N190C

108-2 

Thanks Bret for the tip - makes sense - and yes always a good idea to verify documentation.  If you (or anyone) happen to come across an 'official' reference to 46 gallons of useable fuel please let me know.

I did notice on pages 5 & 15 of the type certificate --> NOTE 1 implies the unusable fuel supply and undrainable oil is 24 lbs --> which would imply max unusable fuel would be 4 gallons (depending on how much undrainable oil there is).

First, what Bret did is a good idea, calibrating a dip stick. Obviously, you'd want to get the tank empty before doing this just so you know where you're starting from.

The term "unusable fuel" is a regulatory one. It has nothing to do with how much fuel can be used! If you're interested in all the conditions that go into determining that quantity, look at CAR 3, para 3.437 (mainly). The short version is that its NOT based on level flight. It takes into account maneuvering and a minimum flow rate, etc. So in the case of my 172A, I just drained the fuel and literally every drop came out through the gascolator, but it has 2.5 gallons unusable fuel per side. 

The fact that Note 1 says that the unusable fuel and oil are at Sta 22 tells me its all fuel, so 4 gallons total. Also, just like it says in the TCDS, your original weight and balance form from Stinson will have the usable fuel listed on it. In the case of my -0, the weight and balance form lists 36 gallons, because the other four gallons are included in the empty weight.

If you don't have the original weight and balance, its on the records disk you can order from the FAA.

The "Owner's Operating Manual" from Univair for my -3 says that the net usable fuel in each tank is 23 gallons. 

Nothing illegal or wrong about flying one tank dry, switching on first cough, and then filling up find usable fuel.  I would be in good VFR, near an airport, and be ready on the fuel switch right away, like hand on the valve.  If carrying passengers, be sure to tell them what you are doing.  Also, be aware, you are sucking gas out of a tank in ideal flight conditions with no account for pitch, slip or roll, so YMMV.  Unusable fuel is a good thing to know to take into account for slipped approaches and so on.

Good luck,

Again, flying until the engine coughs has nothing to do with unusable fuel. Flying along straight and level will use pretty much every drop of fuel out of the tank. Just like if you jack the tail up on the ground until level and drain the tank. Unusable fuel is a quantity the CAA and FAA require the manufacturer to demonstrate through various attitudes, power settings, etc to the point where an engine will be starved of fuel. Its a safety factor if you will for the manufacturer so the airplane will do what its supposed to do up to the point where fuel level gets to the unusable fuel quantity. At that point, it MAY still run, but its not guaranteed. 

I forgot to add that when you buy a fuel dipstick, its marked in USABLE fuel. So if you're going to make your own, put the two unusable gallons in an empty tank, then start adding fuel to make your dipstick

 The model 108 looks like it has less dihedral than the 105, but the 105 has around 8 gal that you cannot measure with a dipstick.