Before Takeoff; Warm Up; Magneto Check; Alternator Check - Cessna 172R Pilot Operating Handbook

SECTION 4
CESSNA
NORMAL PROCEDURES
MODEL 172R
BEFORE TAKEOFF
WARM UP
If the engine accelerates smoothly, the airplane is ready for
takeoff. Since the engine is closely cowled for efficient in-flight
engine cooling, precautions should be taken to avoid overheating
during prolonged engine operation on the ground. Also, long periods
of idling may cause fouled spark plugs.
MAGNETO CHECK
The magneto check should be made at 1800 RPM as follows.
Move ignition switch first to R position and note RPM. Next move
switch back to BOTH to clear the other set of plugs. Then move
switch to the L position, note RPM and return the switch to the
BOTH position. RPM drop should not exceed 150 RPM on either
magneto or show greater than 50 RPM differential
between
magnetos. If there is a doubt concerning operation of the ignition
system, RPM checks at higher engine speeds will usually confirm
whether a deficiency exists.
An absence of RPM drop may be an indication
of faulty
grounding of one side of the ignition system or should be cause for
suspicion that the magneto timing is set in advance of the setting
specified.
ALTERNATOR CHECK
Prior
to
flights
where verification
of
proper
alternator and
alternator control unit operation
is essential (such as night or
instrument flights), a positive verification can be made by loading
the electrical system momentarily (3 to 5 seconds) with the landing
light or by operating the wing flaps during the engine runup (1800
RPM). The ammeter will remain within a needle width of its initial
reading if the alternator and alternator control unit are operating
properly.
4-22 Dec 2/96