Avionics News October 2013 - 79

be helpful to swap GCUs to see if
the problem moves to the other side.
The line between the generator and
GCU is critical, and while it’s heavily built, anything subject to vibration can fail.”
Dick Carmichael, head of tech
support for alternator manufacturer
Plane Power Ltd. added, “It’s not
unusual for avionics folks to be
involved with alternator problems,
either because they’re doing other
work – maybe increasing amp load
so you need more from the charging system, or a pilot comes to them
with a problem like noise in the
headset.
“When all of a sudden noise
appears from nowhere, either there’s
been a change in the aircraft – often
a grounding problem – or the alternator may have broken a stator wire
or developed a bad diode. Either
situation will cause an alternator
to make more noise than usual. All
alternators make some noise – a
14-volt alternator will commonly
create about 4 volts, peak-to-peak. If
the electrical system is healthy, that
won’t be picked up and presents no
problem.
“Sometimes you’ll get a ground
loop – unequal grounds for different devices. When alternators first
became common, the manufacturers
introduced a common grounding bus
– but there are pieces of equipment
where you won’t run aground all the
way back, strobes at the wingtip, for
example. If the owner hears an RPM
whine and tells you that taking the
alternator off-line makes the noise
goes away, you can have a perfectly
good alternator, but the ground looping picks up the noise and feeds it

… All avionics depend on electrical power, and
if that power isn’t stable, it can produce a wide
range of weird effects on audio/intercom systems,
radios, transponders and other equipment.

into the audio system.”
Evans said twin-engine airplanes
that have alternators on both sides
can develop problems when being
operated in parallel.
“About 75 to 80 percent of the
time when you have a problem with
this, you’ll have a problem in one
of the alternators,” Evans said. “It’s
essential that both alternators deliver
equal current output at full-field to
within 10 percent. If not, the one
with lower output is probably bad.
To troubleshoot this kind of problem, we use an induction ammeter
and two mechanics – one in the
cockpit and the other reading the
output. If both alternators are generating good output, we set each regulator, then vary load with things like
pitot heat to verify that both alternators operate properly under load.
If one regulator falls off, that’s the
problem – to check, swap the regulators and see if the problem reverses.
If it’s necessary to replace one, do
both – they should be matched.”
Another particularly frustrating
problem can be intermittent charging
system failure on long flights.
“If resetting the master switch
brings power back online, you may

have a bad overvoltage relay,” Evans
said. “Fly with a DC voltmeter
hooked to the buss bar. If it trips
after a rise in voltage, you have a
bad regulator. If it just trips for no
obvious reason, then you may have a
problem with worn or contaminated
alternator brushes. Another problem
you’ll see is an unsteady ammeter –
turn off all loads, and if it still does
it, you probably have a bad master
switch; corroded contacts, or bad
mechanical contact. Jump around it;
if the problem goes away, replace
the switch.”
Carmichael added some helpful background information.
“Alternators are simple devices
that just spin and depend on field
current from the regulator, which
in turn determines the output. The
regulator has only one mission – to
maintain a constant charging level,
whether 14 or 28 volts. It should
maintain that regardless of engine
RPM and electrical load, as long as
the alternator has enough capacity
to meet the load. There’s a physical
limit on field current available, and
if you exceed the rated load, you’ll
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avionics news

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october

2013

Avionics News October 2013

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