Electrical Power Supply System Malfunctions; Excessive Rate Of Charge; Insufficient Rate Of Charge - Cessna SKYHAWK 1976 Pilot Operating Handbook

"
CESSNA SECTION 3
MODEL 172M EMERGENCY PROCEDURES
ELECTRICAL 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 could cause the problem. A damaged
or improperly adjusted voltage regulator 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 ac­
cept above normal cruirging 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 could be adversely affected by higher than normal
voltage if a faulty voltage regulator setting is causing the overcharging.
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
malfunction 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.
INSUFFICIENT RATE OF CHARGE
If
the ammeter indicates a continuous discharge rate in flight, the
alternator is not supplying power to the system and should be shut down
since the alternator field circuit may be placing an unnecessary load on
the system. All nonessential equipment should be turned off and the
flight terminated as soon as practical.
3-15/(3-16 blank)