I occasionally fly an old( ish) Cessna 182, and have always assumed that the fuel dipstick is calibrated as total fuel in tanks, i.e if I dip 90 litres per tank, I have 180 litres minus 9 litres per side unusable, i.e 162 litres to not only base my endurance on, but also to enter into the weight and balance calculations, as the 18 ltrs unuseable fuel is already included in the 'empty weight' that one starts with.
But ... is this likely to be so ? I was recently questioned on this point by a pilot preparing a weight and balance loadsheet.
The original dipstick that might have had some notation that would have shown the answer is long gone, the present one is like George Washingtons' axe.
I know we can drain the tanks down and start again, but that is quite an undertaking, and can one guarantee to get down to the true unuseable figure anyway before starting to refill ?
Is there a precedent, or protocol, that is usually followed when Cessna sell a new aeroplane ? Do manufacturers even sell a dipstick with a new aeroplane, after all, it would be an indication that the fuel gauges that they provide with their product are not to be relied on - but then we all know that !
If one assumes that the fuel we measure is total fuel, then we are on the safe side so far as endurance is concerned, but 18 ltrs overweight if we load up to MUAW for take off, hardly very significant at altitudes and temperatures that we are working with, but a smart lawyer will prove that the engine failure 3 hours into the flight, that caused the crash, was due to pilot error in being overweight for take-off.
And it would be nice to know what is the true situation, too.
( OK we set to and measure it again, but any thoughts ? )