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Chapter 4 - Electrical Systems: Knowing What’s Watt
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Problems With Brains THE LOW- OR HIGH-
Most airplanes have a warning light VOLTAGE WARNING THE VOLTAGE WARNING
LIGHT
to alert pilots about electrical prob- LIGHT
lems. This light is commonly known Illumination of the high- or low-
as either a high- or low-voltage warn- voltage warning light indicates
ing light (Figure 20). When illuminat- the alternator isn’t supplying ade-
ed, it informs you that the alternator quate system voltage. Most air-
isn’t supplying appropriate voltage to plane electrical systems have
self-protection circuitry that
meet the system’s electrical needs. takes the alternator off line in the
If your airplane has a low-voltage event it produces excessive volt-
light, it can illuminate during low age. This results in the illumina-
engine idle when there isn’t enough tion of the high- or low-voltage
RPM to develop even minimal volt- warning light. Follow your POH’s
age (pressure). Increasing the RPM recommended procedure for
raises the water pressure (voltage) bringing the alternator back on Fig. 20
and usually shuts off the light. line.
A low-voltage light can also illumi-
nate if the alternator has been taken produces the same result. Manually ied out to the runway and forgot his
taking the alternator off line becomes tail number (which is usually posted
off line because of excessive voltage an important consideration if an inside the airplane). Hoping to catch
production. This usually follows a overvoltage condition occurs and the a glimpse of the number, he leaned
popped alternator circuit breaker or
circuitry doesn’t automatically iso- out and slipped off his seat. He was
alternator field circuit breaker. As
late the alternator. Figure 21 shows hanging inverted, half outside, half
far as the electrical system is con-
three ways of manually taking the inside the airplane, suspended by a
cerned, no voltage is interpreted as
alternator off line. loose shoulder harness. The funniest
low voltage.
Occasionally, a benign overvoltage part was that he held onto the micro-
If your airplane has a high-voltage
condition exists and the voltage regu- phone and continued talking to the
warning light, it can activate when lator’s brain automatically takes the tower. I couldn’t quite hear what he
an overvoltage condition occurs. The alternator off line. This is sometimes said, but I think it was, “Ahhh,
alternator field current (the prime) is called a nuisance tripout. Even tower... aghhh... I’ve fallen out and I
automatically removed, which takes
the alternator off line. This keeps the human brains experience nuisance can’t get back in!” I ran over and
circuits from being damaged by tripouts. It happened to a student helped this guy back into his air-
excessive system voltage (pressure). pilot in a Cessna 150. This fellow tax- plane. I’m just glad he didn’t forget
In either case, the activation of a
high- or low-voltage warning light
may follow a popped alternator or Fig. 21
alternator field circuit breaker. When
any circuit breaker pops, it is always
worth paying attention. Personally, if
I’m airborne and a circuit breaker
pops, I don’t reset it if the equipment
it’s protecting isn’t essential.
Resetting a circuit breaker is playing
with fire—literally, and an in-flight
fire is considerably less comforting
than a campfire. If I need to use the
equipment, I reset the circuit breaker
once and only once, but only after let-
ting it cool for several minutes. If it
pops again, I leave it alone. In other
words, it’s not a good idea to reset a
circuit breaker a second time.
If a popped circuit breaker (alter-
nator CB or alternator field CB)
takes the alternator off line, then if
you manually pop either of these, it