George Patterson wrote:
Ice blonde wrote:
It can be fun getting the gauges on some airplanes to read anywhere near
accurately. Even replacing gauges or senders or both will often not get
them any more accurate.

Why?

Aircraft fuel tanks tend to be fairly short, compared to auto tanks. They also
tend to be wider and longer than a comparably-sized fuel tank in a car. It's not
economical to manufacture special sending units for aircraft, however, so
outfits like Cessna and Piper use units made for cars.


There are other reasons they're inaccurate, too. The Cessna spec is
for the float to not touch the top of the tank (the float's tapping on
an aluminum tank can wear a hole in it), and so the thing reads full,
or even overfull, when the float reaches the upper limit of its travel.
That's before another significant amount of fuel goes into the tank and
submerges the float. The tank will therefore read full (or overfull)
for some time before it begins to drop.
The float is usually hinged to the sender near the top of the
tank. The movement of the rheostat in the sender is linear but the
hinge's location creates a sine function to the indication, with the
top travel being rather slow and the lower travel moving much more
quickly. It implies that the tank has more volume in the top than in
the bottom, which is usually the opposite of the actual tank shape.
Odd-shaped tanks create their own accuracy problems. The fuel
level's descent in the tank is not linear, while consumption is, and
the gauges reflect level drop, not volume drop.
Senders are often located in the inboard end of wing tanks, and
the dihedral keeps the sender up until the fuel is well down. More
inaccuracies.
And we pay big bucks for this sort of thing!

Dan