Cessna 172N - Page 39

CESSNA
MODEL
172N
SECTION 3
EMERGENCY
PROCEDURES
duce
engine
power
immediately
and
select a suitable forced
landing field.
Use only
the minimum
power
required
to
reach
the
desired
touchdown
spot.
ELECTR.ICAT
POWER SUPPLY
SYSTEM
MALFUNCTIONS
Malfunctions in
the electrical
power
supply
system
can be
detected
by
periodic
monitoring
of the ammeter
and over-voltage warning
light; how-
ever, the cause of
these malfunctions
is usually difficult to
determine.
A
broken alternator drive
belt or wiring is most
likely
the
cause of
alterna-
tor failures, although
other factors
cotrld
cause
the
problem.
A
damaged
or
improperly
adjusted
voltage
regulator
can also
cause maUunctions.
Problems
of this
nafure
constitute an
electrical emergency
and
should
be
dealt
with
immediately.
Electrical
power
malfunctions
usually
fall into
two
categories:
excessive rate
of
charge and
insufficient
rate
of charge.
The following
paragraphs
describe the recommended
remedy
for each
situation.
EXCESSIVE RATE OF
CHAR,GE
After
engine
starting and heavy
electrical
usage
at
low engine
speeds
(such
as
extended
tarciing) the
battery
condition
will be
low enough to ac-
cept above
normal
charging
during
the
initial
part
of
a
flight. However,
after thirty
minutes
of
cnrising
flight,
the
ammeter
should be indicating
Iess
than two needle
widths of charging
current.
If the
charging
rate
were
to remain above
this
value
on
a
long
flight,
the battery
would overheat and
evaporate
the electrolyte
at an excessive
rate. Electronic
components in
the
electrical
system
could
be
adversely affected
by
higher
than normal
voltage
if a faulty
voltage regulator
setting is causing the
overchar$ng.
To
preclude
these
possibilities,
an
over-voltage sensor will
automatically
shut down
the
alternator
and
the over-voltage
warning
light will illuminate
if the
charge voltage reaches approximately 16
volts.
Assuming
that
the
maUunction
was only momentary, an
attempt
should
be made to
reactivate
the alternator system.
To
do
this,
turn both sides
of
the master switch
off and
then
on again.
If the
problem
no longer exists, normal
alternator
charging
will
resume
and the warning
light will
go
off.
If the light
comes
on again,
a
malfunction
is
confirmed.
In
this
event,
the
flight
should be
terminated and/or
the
current drain on the battery
minimized
because the
battery can
supply
the
electrical system for
only
a
limited
period
of time.
If the
emergency occurs
at night,
power
must
be
conserved
for later use
of
landing
lights
and
flaps
during landing.
INSUFFICIENI
RAIE
OF
CHARGE
If the ammeter
indicates a
continuous discharge
rate
in flight,
the
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