The number of times I was not able to fly on any chosen
day or to my desired destination for reasons stemming
from electrical problems outnumbers the scrubs for mechanical
reasons. Those black boxes crave attention. Following
are some hints for proper maintenance and indications
for what one might do to troubleshoot a problem.
Obtain a wiring diagram for your aeroplane. Does the
installation resemble the diagram? Often, when new equipment
is installed old wiring is not removed. There should be
no wires in your aeroplane which do not have a function.
Label all wires. Obtain/retain all instructions with acquired
equipment, even if you do not understand what it says.
Check the battery voltage (14.2V) - a cell may be dying.
Low voltage may keep some system from being triggered,
giving the appearance of a faulty avionics unit.
Check the charging voltage with the engine running - should
be above the battery voltage - typically 14.6V.
If you fly a homebuilt or a decertified aeroplane, install
both an ammeter and a voltmeter so that you can monitor
both alternator performance and detect any sudden increases
in current demand.
If you hear alternator whine on the speaker/phones which
varies with engine RPM, check/replace the capacitor and
its connection at the alternator. They do not last forever
and are working in a nasty environment.
If you suddenly hear the strobes firing in your headset,
and did not before, check the ground connections of the
shields. Grounds should be connected at one end only -
the end where the noise is generated. Connecting both
ends negates the effect of the shield.
If you have changed headsets and your intercom is no longer
performing, it is likely that the impedance of your new
earphones no longer matches that required by the intercom.
Many intercoms are set up for the popular David Clark
headsets. Particularly the cheaper intercoms have limited
flexibility in the type of headsets that may be used.
Impedances for headsets vary from 150 to 600 Ohms.
The phone and mic jacks do not have an infinite life.
If you find yourself having to wiggle the plug to get
it to perform, it is overdue for replacement.
The wires going to the phone and mic jacks are very thin
and fragile.
Working behind the panel may cause a wire to be inadvertently
torn loose.
Antennas die, particularly the broadband blade types.
Corrosion, crud and abrasion degrade their performance.
Be particularly fastidious when inspecting antennas mounted
under the fuselage - an area that typically accumulates
water and therefore promotes corrosion between skin and
antenna.
Corrosion under the antenna mount is death to performance.
If paint is blistering around the base, it is high time
to remove the antenna, remove the corrosion, alodine the
aluminium, install the antenna using new hardware, and
seal around the edges to prevent water from getting between
the mating surfaces. Done properly, one should use a special
conductive antenna sealer containing tiny aluminium balls,
a product superior to conductive gaskets supplied.
If the antenna connector is not the BNC 1/4 turn twist
type, but attaches the lead and shield to a screw using
a lug held by a nut, make sure the screws, washers and
nuts are free from corrosion, and as close together as
possible. Coax is like a blotter, so make sure the ends
are sealed against moisture.
Radio signals travel along the surface of antennas. Therefore,
the surface of wire antennas must be rust and dirt free
for maximum performance.
Although most installers pay attention to the proper connection
of the power supply lines (12V Plus), the other side of
that connection, the ground (Minus) is often forgotten
and is consequently the usual source of problems. These
forgotten joints often corrode and inhibit the flow of
current.
Buy a hand-held radio as a stand-by. To get maximum performance
from the hand-held, have a separate external antenna installed
for it. Its performance on 2 Watts will amaze you, and
you will not have to skip your $100 hamburger on that
day when #one radio quits.
A good working nav/com system is a pleasure you deserve.
