Unusable fuel test method

[Andy]

https://www.faa.gov/documentLibrary/.../AC_23-16A.pdf

See section 23.959 Unusable fuel supply.

[Andy]

But wait - there's more ...

https://www.faa.gov/regulations_poli...A-H-8083-1.pdf

Note the definitions for "Residual Fuel" and "Unusable Fuel" they are not the same!

Residual fuel should be used to establish the empty weight. EX builders who follow the maintenance manual procedure will get a correct empty weight. FX-3 owners who had the aircraft weighed with tanks empty and then had 30 pounds "unusable" fuel added would appear to have a weight penalty equal to the difference between "residual" and "unusable" fuel. It appears that difference could be as much as 15 pounds.

Does it matter? I think so, but each owner will have to decide for themselves.

[hawgdrvr]

Here is what I think is important. The placard in the cockpit says 5 gals unusable. This leaves 39. Pilots should plan accordingly. What fuel reserve do you want at your destination? Do you want 5 gals remaining as a safety factor then flight plan for 30-32 gals of fuel to be used. This discussion, while originated in private, ultimately is moot. It's good to know your systems such as this but if you're counting on these numbers to let you go a few extra miles or arrive for a deadstick landing, you screwed up...big time!

We had lights in the T-38 for low fuel, I think it was 1500lbs remaining. While called a "mission complete" light I never wanted to see it and as hard as the T-38 is to land you may need two swings at bat left. I don't trust those tubes in the wing root, maybe I will learn that they are valid, but proper flight planning is crucial. I've seen enough YouTube videos of guys being surprised by their tank levels maybe because of a fuel cap improperly seated, who knows. If I get a cargo pod I'll likely carry fuel bags with probably a min on board of two (10 gals).

[Andy]

The placard in the cockpit says 5 gals unusable.

There is no such placard in my FX-3.

[hawgdrvr]

Sorry, it says max fuel imbalance. Not sure what the POH says.

[turbopilot]

I think two issues are being merged here: Fuel planning and carrying around unnecessary weight.

The SS and EX/FX apparently share the same two fuel systems, the standard two tank 25 gallons system and the later modification to the four tank 44 gallon system. There is a great deal of experience with the simple 25 gallon system that has 1/2 gallon per side unusable. It has 6 pounds of dead weight. That is the dead weight penalty for carrying 25 gallons.

Then there is the 44 gallon, four tank system that may have between 15 to 30 lbs of unusable fuel based on all of the great input here. To be safe CubCrafters prudently recommends not to count on using the last 30 pounds of fuel. I think that is a good idea.

By any measure a prudent pilot will never plan to use that last 30 lbs of fuel in the 44 gallon system, just as a prudent pilot will never plan to use the last 6 lbs of fuel in the 25 gallon system.

So all the other confusion aside about regulations and definitions, the decision to move from a 25 gallon system to a 44 gallon system simply implies a "dead weight penalty" of 30 lbs. That penalty needs to be balanced against the benefit of having another 15 gallons of fuel available for use stored in the wings.

Bottom line, 15 additional gallons available costs you 30 lbs of dead weight. That represents a 2 pound penalty to useful load for every additional gallon of fuel available. KISS.