'/;;; Home> Aircraft
Magneto TroubleShoot
No rpm drop when checking magneto in Lycoming or Continental aircraft engine
Warning - see a mechanic immediately - if the propeller is moved even
slightly the engine may fire causing injury
A drop in rpm is expected when one magneto in a redundant ignition system
is shut off. Should the propeller be moved by hand (as during pre-flight
or maintenance) and a functional "hot" magneto exists, the engine may fire
and cause injury to personnel.
1. Open magneto primary (P) lead causing hot magneto.
| The P lead is the wire that goes from your starter switch
to the magneto. The P lead attaches to the magneto coil's primary windings
(hence the name P lead) and is used to ground out the magneto (turn it
off). |
Mag Check
Periodically ensuring that your magneto’s
p-leads have not broken is a good defense against unexpected starts on the
ground. These leads, which are connected to the ignition (or magneto)
switch in the cockpit, are responsible for grounding the mags to keep the
engine from running. Sometimes the wires or connections between the switch
and the magnetos break or come adrift. In this case, one or both mags may
be "hot," or ready to deliver spark whenever the prop is turned.
To test them, instead of shutting down the engine in the usual manner with
the mixture control, use the key. Allow the engine to cool normally and
idle down. Move the key slowly through both Right, Left, and then to the
Off position. The engine rpm should drop slightly at both of the
individual-mag positions and shut down completely in the Off detent. Allow
the prop to stop and then move the mixture to the idle-cutoff position. Do
not try to "catch" the engine before it comes to a stop because
a dangerous backfire might occur. If the engine does not stop when the key
is in the Off position, shut it down with the mixture, prominently mark
the prop as being "hot," and contact maintenance personnel
immediately.
There are a few other caveats to consider
when in the vicinity of the prop.
- Avoid pulling the airplane around by the
prop. Yes, this seems the perfect solution to a vexing problem of how
to change the airplane’s position without having to walk around and
get the tow bar, but it’s worthwhile to make the extra effort.
Neither the engine nor the prop particularly benefit from the loads
imposed by horsing the whole airplane around.
- Avoid pushing the airplane by the
spinner. The spinner and backing plate are built to be light, so
they’re quite fragile. Pushing on them can cause the backing plate
to crack and can lead to spinner failure.
- Avoid contact with prop deice boots and
associated wiring. If you want your hot prop to be toasty when it
counts, stay clear of the boots.
|
2. Magneto timing advanced beyond the specified setting.
3. Open in the grounding circuit of the feed through capacitor (Bendix
S-1200 series magnetos see Bendix S.B. 624).
4. Defective ignition switch.
5. Open magneto capacitor. For Bendix magnetos see Teledyne Ignition Systems
Critical Service Bulletin CSB641 or latest edition.  The
attitude of your plane determines how to leave it with safety at that instant.
Parachute Sense, US Navy 1944
|
