Dead Cut And Live Mag. Check - Cessna 152 Training Manual

Dead Cut and Live Mag. Check

It is important to realise if the ignition is live, the engine may be started by
moving the propeller, even though the master switch is OFF. The magneto
does not require outside source of electrical energy.
Placing the ignition switch to OFF position grounds the primary winding of the
magneto system so that it no longer supplies electrical power. With a loose or
broken wire, or some other fault, switching the ignition to OFF may not ground
both magnetos.
To prevent this situation, just before shutting an engine down, a "dead-cut" of
the ignition system should be made.
The dead-cut check is made by switching the ignition momentarily to OFF and
a sudden loss of power should be apparent. This is carried out most effectively
from R, not from Both, to prevent inadvertent sticking in OFF.
On start up, a live mag check is performed, to ensure both magnetos are
working before taxi. This is not a system function check detailed below, as the
engine is still cold and plugs may be fouled, rather just a check to ensure both
magnetos are working by switching from Both to L, then R, and back to Both,
noting a small drop from Both in L and R positions. A dead-cut check may be
carried out at the same time.
The engine will run on just one magneto, but the burning is less efficient, not
as smooth as on two, and there is a slight drop in rpm.
The magneto check to confirm both magnetos and plugs are operational should
be made at 1700 rpm as follows:
Move ignition switch to R position and note the rpm;
Then move switch back to BOTH to clear the other set of plugs;
Move switch to the L position, note the rpm and return to BOTH
position.
Rpm drop should not exceed 125 rpm on either magneto or show greater than
50 rpm difference between magnetos.
An absence of rpm drop may be an indication of faulty grounding of one side of
the ignition system, a disconnected ground lead at the magneto, or possibly
the magneto timing is set too far in advance.
Excessive drop or differential normally indicates a faulty magneto.
Fouled spark plugs (lead deposits on the spark plug preventing ignition) are
indicated by rough running usually combined with a large drop in rpm (i.e. one
or more cylinders not firing). This is due to one of the two plugs becoming
fouled, normally the lower plug. Plug fouling, if not excessive, may be burnt
off.
by D. Bruckert & O. Roud © 2004
CESSNA 152 TRAINING MANUAL
Page 45