Fuel Gauge Inop VFR Day

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.

An old-fashioned sending unit is basically a rheostat with a steel wire attached
to it and a float attached to the other end of the wire. Usually they are set up
so that current is sent to the gauge and the sending unit is attached to the
ground ("earth" in GB) wire. When the float is at the top, the gauge is fully
grounded and it reads full -- float at the bottom, no current flows and the
gauge reads empty.

There are several problems with this, at least two of which are caused by the
use of automotive components. First, there's no requirement for accuracy in auto
gauges, and, in fact, the manufacturers deliberately make the systems so that
they still read 1/2 tank when the tanks are actually 3/8 full (this supposedly
gives the impression of better fuel economy). Second, a sending unit that's
designed to have the float travel over a range of perhaps 12" in an auto tank
does not perform as precisely when the wire is shortened and the float travels
over only 7".

You also may have had occasion to drive autos in which the gauges read higher or
lower if you are going uphill? This is caused by the sending unit being located
towards the front or rear of the tank. The same sort of thing occurs to a lesser
extent in some aircraft. Lastly, aircraft tend to bounce around more than autos.
Gasoline also tends to slosh back and forth more in the flatter tanks. This
tends to bounce the floats of the sending units, making the gauges more erratic
than autos.

George Patterson
Drink is the curse of the land. It makes you quarrel with your neighbor.
It makes you shoot at your landlord. And it makes you miss him.