Aero-Tips!
A good pilot is always learning -- how many times have you heard
this old standard throughout your flying career? There is no truer
statement in all of flying (well, with the possible exception of
"there are no old, bold pilots.")
Aero-News has called upon the expertise of Thomas P. Turner,
master CFI and all-around-good-guy, to bring our readers -- and us
-- daily tips to improve our skills as aviators. Some of them, you
may have heard before... but for each of us, there will also be
something we might never have considered before, or something that
didn't "stick" the way it should have the first time we memorized
it for the practical test.
Look for our daily Aero-Tips segments, coming each day to you
through the Aero-News Network.
Aero-Tips 12.26.06
It's depressing how many airplanes run out of fuel just short of
the pilot's intended destination. Part of the reason for fuel
starvation and fuel exhaustion mishaps may be that fuel gauges are
notoriously inaccurate.
Most lightplane fuel gauges are electrical-mechanical devices. A
common installation will have float sensors in the fuel tanks,
sometimes multiple sensors within a singe tank. As fuel level
decreases the float descends in the tank. Pivoting on an arm, this
drives the other end of the arm upward as the float drops. This
other end of the arm makes contact with a wire or strip of metal; a
small amount of electricity flows through this part of the sensor
and, the farther up the arm rises (as the float drops), the greater
amount of wire or metal electricity must flow through in the
system.
The senor measures the drop in electrical power on the other end
of this wire or metal strip. The amount of electricity sensed is
directly proportional to the resistance encountered by electricity
through the sensor, which in turn is a direct function of the
distance the electricity must flow. Hence, as the float drops and
the other end of the arm rises, there's more electricity lost in
the system. Circuits in the cockpit change this electrical reading
to an indicated fuel level.
Anything that affects the system-fuel sloshing in turbulence,
uncoordinated flight, low electrical power, old wiring, stuck
floats or bad circuit boards-results in an inaccurate fuel
indication.
In fact, fuel gauges in our aircraft are required to real
accurately only when the tank is empty... like they do us a lot of
good at that point. Fuel gauges are a good trend instrument, to see
if the fuel load is about what you expect, and is burning off an a
predictable rate, but they need to be verified by visually checking
fuel level in the tanks, comparison to other fuel indicators like
totalizers, and cross-checking against known amounts of fuel put
into the tanks before flight, indicated fuel burn, and a good old
clock.
Aero-tip of the day: Understand the use and
limitations of cockpit fuel indicators.
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