Cessna 172R, Operating Handbook

Page: 94 / 500

SECTION 3
EMERGENCY PROCEDURES
CESSNA
MODEL 172R
ELECTRICAL POWER
SUPPLY
SYSTEM MALFUNCTIONS
Malfunctions in the electrical
power supply
system
can be
detected by
periodic monitoring
of
the
ammeter
and low voltage
lannunciator
(VOLTS); however, the
cause
of
these malfunctions is
usually difficult to
determine. A broken alternator drive belt
or wiring
is
most
likely the cause of alternator failures,
although other factors
could
cause
the problem. A defective alternator
control unit can also
cause malfunctions.
Problems of this
nature
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
CHARGE
After engine starting and heavy
electrical usage
at
low engine
speeds
(such as extended taxiing) the battery
condition will be low
enough
to accept
above normal
charging during the initial
part of a
flight. However, after thirty minutes of
cruising flight,
the
ammeter
should be indicating less 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
can be adversely
affected by higher
than normal voltage. The alternator control unit
includes an overvoltage
sensor which normally will automatically
shut down the alternator if the
charge voltage reaches
approximately 31 .5 volts. If the overvoltage
sensor malfunctions, as
evidenced by an excessive rate
of charge shown on the
ammeter,
the
alternator
should
be
turned off, nonessential electrical
equipment
turned off and
the flight
terminated as soon as practical.
3-22
Revision 7