Thanks for all the inputs guys.
To address some of the comments:
Leveling the aircraft... I find no requirement in the Mooney MM to level the aircraft when draining the tanks, and in this case the airplane was on a hard, flat (as near as I could tell) concrete floor of an avionics shop. I concede that there could be a minor slope to the floor that was not apparent and the airplane could have been tilted very slightly laterally and/or longitudinally. But given the dihedral of this wing and normal tire pressures, I can't believe that a total of 7 gallons got trapped. And when the drain valve in the bottom of each wing is removed and the tanks drained through those holes, each fuel tank should be well and truly empty. Good comments from "Piloto" on the drain holes and vent holes in the inboard rib and I'll check that, but again, I doubt that accounts for so much fuel.
Now, leveling the aircraft when filling the tanks is another issue. In fact there is a procedure in the Mooney Maintenance Manual for Fuel Tank Transmitter Adjustment which states that "AIRCRAFT MUST BE LEVEL TO CALIBRATE FUEL GAUGES". In my airplane the factory analog gauges are gone and fuel quantity is now on the JPI, but the refill procedure from empty tanks would be the same to get a 5-point calibration for the EDM-900 (for empty, 1/4, 1/2, 3/4, full). I wasn't there to observe this process but no doubt the airplane was rolled onto the ramp to get fuel from the truck, and that ramp is not nearly as level as the hangar floor. From the MM, the final step in the procedure to establish the FULL reading on the gauge is to "Finish filling tank until fuel just starts to spill over outer wing surface". Well, when I got to the airplane after the fuel calibration, my tanks were extremely full, but not that full!
So, my first target is to do a slow, methodical top-off of each tank to where the fuel starts to spill over the wing. I know from experience in this and previous Mooney's that you can squeeze a lot of fuel into those wings if you're patient. Assuming that in fact 68 gallons was loaded from the truck into assumed empty tanks, because my fuel flow meter is pretty accurate, I know how much I burned. Sure, there's the possibility of an error stack from the fuel truck meter that loaded that 68 gallons, variations in fuel temp and therefore fuel density, aircraft attitude on the ramp at the self-serve pump, etc. But it's a start.
If I still don't discover those 7 "missing" gallons, I'll dig deeper. And if this fueling technique from the MM is what it takes to actually load 75 gal into my tanks, that's not something I'm willing to do at every fuel stop so at least I'll know how much fuel I really have using more conventional techniques that don't involve spilling fuel over the wing!
Thanks again for the inputs.
Bill
