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for at least 30 minutes). To get an
idea of your aircraft’s capabilities,
add up the number of devices that
use power (check the circuit
breakers) and divide them into the
amp/hour rating. So, if your devices
collectively use 45 amps (see below),
and your battery supplies 45
amps/hour, you should be able to
get an hour out of it. You could also
use half the devices and get two
hours. When faced with such an
emergency, it is usual to use the
navaids, for example, to get a
position fix, then turn them off until
you start feeling a little lost, then
turn them on again until you are
once more certain of your position.
The same with radios. This will get a
little extra time out of your battery.
There are two types of battery used
in aircraft, lead acid, as found in cars,
and NiCad, as found in portable
computers. People who use both will
already understand the difference,
but just in case, the lead acid’s
output tends to fall off with
discharge, whereas a NiCad can
pump out power at a constant rate
until it can do no more, as well as
recovering more quickly. The trouble
is that NiCads have short memories,
in that if you keep charging them up
when they have only discharged a
little way, they will begin to think
they have a lesser power rating, so to
stop them causing hot starts they
need regular deep cycling to keep them
awake. So, although it’s good
practice to start a helicopter, for
example, from a battery cart, to
preserve the ship's battery for better
reliability in remote places,
occasionally a battery start is good
for the system.
Another problem with NiCads is
that they can spontaneously combust
when too much current is drawn and
then replaced (actually called Thermal
Runaway). This is why some
helicopters have a Battery Temp
caution light on the warning panel
which means you must land
immediately, before the battery catches
fire and takes other stuff with it, if it
doesn’t actually burn its way through
the airframe and fall out.
If you spill any electrolyte from a
NiCad, you can neutralise it with
dilute boric acid.
The Battery Master Switch controls the
power to all circuits, and there will
be other switches to control smaller
groups of equipment, such as the
Avionics Master Switch, for the radios
and navaids. The Battery switch may
well be in two parts, one for the
battery itself, and the other for the
alternator circuit. Circuits will be
protected by fuses or circuit
breakers, which should only be reset
once, since there is a reason for them
blowing the first place.
A fuse is a deliberately weak part of
a circuit that is designed to fail if a
problem should happen, thus
protecting the rest of the circuit and
saving the trouble of replacing
wiring in odd places.
A more modern replacement is the
circuit breaker, which is a button that
pops out when a fuse would
otherwise break. A trip-free circuit
breaker is one that will trip even if it
is held in.
The voltage regulator is there to stop
the battery being overcharged or the
system being overloaded by the
generator or alternator. There will be
Electricity & Radio 117
a warning light in the cockpit to
indicate that this is happening, and
that you are getting battery power
only. The over voltage sensor may be
reset with the Master Switch. It may
be tested by turning the ALT half of
the battery switch off for a moment.
When starting, the starter switch will
activate a solenoid, which is just a
bigger switch that can handle more
current, to actually turn the engine.
Since the current is large (60 amps),
there is no fuse protection, which is
why there is a starter light.
Amps
The flow of electrons in a conductor
(i.e. the current) is expressed in terms
of amperes, or amps, which are defined
as the movement of 1 coulomb per
second (a coulomb is the
accumulated charge of a large
number of electrons, actually 6.28 x
1018). In an aircraft, amps would be
measured with an ammeter, or
loadmeter, a useful device for checking
if your battery is being charged. An
ammeter needle should always be
showing in the + side of the gauge,
showing a positive charge.
Volts
The work done to add electrons to
an atom is expressed in volts, which
move from high to low pressure, like
air does. Once a body is charged this
way, it is "pressurised" (for want of a
　
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