On Feb 22, 11:24*pm, WingFlaps wrote:
A recurring theme seems to be that one should should not rely in fuel
gauges. I can understand that from a safety point of view (I always
dip the tank before start), but I get the feeling that they are
considered just plain inaccurate.

The admonition not to rely on fuel gauges is half-right. The half
that's right is important; the half that's wrong is dangerous.

The half that's right is that you need another way to know how much
fuel there is, because the gauges have limited accuracy, and because a
previously working gauge can always fail suddenly. For the GA planes
I'm familiar with, a visual preflight inspection of the tanks is
required by the POH. From there, you calculate your fuel consumption.
If the gauges say you've got more fuel than you expect, trust your
calculations instead (double-checking if necessary). Aviation safety
is about redundancy.

The half that's wrong is that the admonition ignores a vital
possibility: the gauges may show way LESS fuel than you expect. (Say
you're supposed to have 3/4 of a tank left, but the gauge shows 1/8.)
In that case (if the gauges are working well enough to meet the
airworthiness requirements) you know you've got a problem--perhaps a
fuel leak, which your consumption calculations can't warn you about
without the gauges. A leak may be a very infrequent event, but
aviation safety is about being prepared for unlikely problems, rather
than unnecessarily relying on the gamble that it will never happen to
you.

The Part 91 airworthiness regs (which pilots are required to know
before being allowed to solo) mandate a gauge that indicates the fuel
level in each tank. No specific accuracy is mandated, either in Part
91 or in the aircraft-certification regs in Part 23. So it becomes a
matter of common sense: a working fuel gauge has to be accurate enough
to serve its intended purpose, which (in familiar light GA planes
anyway) is to provide a rough cross-check of the consumption
calculations, to warn of a leak or other problem.