RV-9A Unusable fuel

[Build9A]

My DAR sent me his inspection checklist and one item says " fuel gauge each tank has been calibrated and calibration is in records showing unusable fuel". What experience have others had with this issue? How did you calculate unusable fuel? I read that some have completely run the tanks dry during the flight test. I'm not sure I want to do it that way. Also, the DAR appears to want it before 1st flight. Any comments would be appreaciated. thanks, jack

 

[N523RV]

RE: RV9A Unusable fuel

I haven't got this far yet.. but I would get the airplane level and in flight attitude. Add 5 gallons of gas. Run the aux. fuel pump and pump out the gas back into the same container. Whatever the difference between where you started and what you ended up with would be your unusable.

From what I hear.. there isn't a whole lot of unusable gas...

 

[akarmy]

That's a fine method to compute it...

and yes, there is not much left when you are straight and level... I have heard some stories of people running low and having things go queit when banking towards the selected tank. Remember that those pickups are at the low point of the root of the tank.

Guess I'm just chicken... but I start thinking long and hard about gas at 10gal left...

 

[az_gila]

Isn't the definition of unusable fuel defined in a climb attitude, not in straight and level flight?
I thought that idea idea was you would have fuel to do a go-around, and not sputter out in the climb after a balked landing at minimum fuel.
Perhaps you should perform the 5 gallon pump test with the tail held down?

Am I mistaken? gil in Tucson

 

[N523RV]

re: unusable fuel

Gil,

I would think testing in both situations would be good and might be enlightening. It would be easy to just tie the tail down and run the same test in the other wing.

If someone does this, let us know the results. I'll do this when the time comes but that won't be for serveral more months.

 

[gvgoff99]

FAR Part 23 Sect. 23.959

To Quote the FAA:
Sec. 23.959

Unusable fuel supply.

[(a) The unusable fuel supply for each tank must be established as not less than that quantity at which the first evidence of malfunctioning occurs under the most adverse fuel feed condition occurring under each intended operation and flight maneuver involving that tank. Fuel system component failures need not be considered.
(b) The effect on the usable fuel quantity as a result of a failure of any pump shall be determined.]

Amdt. 23-51, Eff. 03/11/96

Which sounds like you can decide on how level (or unleveled) to make the plane when you are measuring the supply of fuel that is "usable" or "unusable." In fact this is so vague that an aerobatic plane with an inverted oil system but without an inverted fuel system would seem to have virtually all of its fuel as "unusable."

 

[N523RV]

Far

George.. thanks for the FAR post.. I figured this info was out there somewhere but didn't have the gumption to go find it. I'll measure mine in both level flight and climb attitude just to see if there is a difference.

 

[n2prise]

Switch tanks when entering the landing pattern.

That discussion tells me when the fuel supply is low, be sure to switch to the tank that will be on the high side when making pattern turns for a landing. Of course this assumes that one tank is not completely dry.

Food for thought.

Jerry K. Thorne
East Ridge, TN
90622 - Windshield ON!

 

[Ben Beaird]

That discussion tells me when the fuel supply is low, be sure to switch to the tank that will be on the high side when making pattern turns for a landing. Of course this assumes that one tank is not completely dry.

Food for thought.

Jerry K. Thorne
East Ridge, TN
90622 - Windshield ON!

Jerry,

That would only be a problem in you're flying VERY uncoordinated. If the ball is reasonably centered in a turn, the fuel behaves as if you're staight-and-level.

Ben Beaird
Plymouth, WI
-6A

 

[kentb]

slips effect fuel

Banking the plane will not effect fuel location in the tank, but if you do a slip (uncordinated flight) the fuel might move outboard.

Kent