Things That Make You Go "Hmmmm".

(You'll need to click and enlarge the pic to see what I want ya to see.)
The central gauge in that picture is the fuel quantity gauge for the BK117. I love flying this helicopter almost as much as I loved flying the Huey, and oddly, they share a common fault...
Like many helicopters, they're too "short-legged", meaning they have little endurance. The Huey burned roughly 100 gallons of JP-4 or JetA per hour and carried 209 gallons of the stuff.
Do the math-
Taking off with full tanks and considering the required reserve, you knew you'd be stopping to fill 'er up again in less than two hours. The BK's two LTS-101 engines combine to burn almost 80 gallons per hour if you consider starting and hovering for takeoff. From the above gauge you may be able to see there are about 165 gallons in the main tanks, and an additional twenty gallons in the two supply tanks for a total of 185 gallons. Figuring conservatively, I plan for about 2 hours and 15 minutes before it wheezes and gets quiet.

I took the picture to illustrate one of the things that is frequently wrong with aircraft and the regulations that concern them...
Above the fuel gauge you can see a placard that reads, "Max Usable Fuel 1230lb." Usable fuel is the fuel the pumps can actually suck up and provide to the engine for burning. A certain amount of fuel in the tank(s) of most aircraft cannot be utilized, and is therefore "unusable fuel".

The placard, required by the FAA, complicates matters...
The gauge reads gallons of fuel. How many gallons in 1230 pounds?
Jet A weighs 6.84 pounds per gallon. I just grabbed my calculator and did the math-
My "Max Usable Fuel" is 179.82456 gallons, so 5-and-a-sip gallons in the tanks are unusable.
Wouldn't it be nice if the required placard said,
"Max Usable Fuel 179 Gallons"?

There's an old joke among pilots that goes,
"Hi. I'm from the FAA and I'm here to help!"
It always gets a chuckle.