Flying
By Day...
When Your
Plane Tries to Tell You Something, Listen!
By Bobby
Day
December 15 - I’ve been quite happy with the way my
plane’s engine has been running, but I recently noticed a couple of
little quirks. One was, on occasion, when cranking the Cherokee 140 it
would kick back. It is a Lycoming O-320 with an impulse coupling on the
left magneto (mag). The impulse coupling delays the spark until after
top dead center, so I speculated that maybe the switch wasn’t disabling
the right mag in the start position. The Annual was close consequently
I decided to check it then. The other thing was during run-up, the mag
drop on the left mag became noticeably less than on the right mag, some
times hardly any drop at all. That was a bit of a worry, but it was
still within the specified values. Therefore, I didn’t pursue it.
The switch check during the Annual confirmed that
the switch was properly disabling the right mag during starting. Back
to the drawing board. Later, during the AD check on Bendix switches, I
was very surprised when, after returning the switch to the OFF position,
the engine didn’t quit immediately. Well, the purpose of the AD check
is to test the switch. I concluded that the switch was bad and removed
it. However, when the switch was dis-assembled, it turned out to be in
excellent condition. Again, it was another dead end.
While discussing this problem with a friend who had
experienced something similar, I found out that his problem was caused
by a loose connection of the P-lead at the magneto. Sure enough, the
nut on the right mag P-lead connection was loose and had backed off a
little bit only making only intermittent connection. The switch
connects the P-lead to ground to disable the mag so when the
intermittent connection wasn’t making a connection, the mag would be hot
regardless of the switch position. Putting an alligator clip test lead
on the terminal while doing the ohmmeter checks had affected the
intermittent condition and made the connection solid so that testing
wouldn’t confirm problem. The mag was intermittently hot when cranking
and was occasionally firing. The right mag would fire before top dead
center causing the kick back. Also, with the engine running, the
vibration was affecting the intermittent connection so that during mag
check the left mag check wasn’t dropping as much because the supposedly
shut off right mag was helping, just not at 100%.
The plane had tried to tell me something and I had
heard it talking, just hadn’t listened.
If you have a problem that is perplexing, send the
info to us at
airmail@fly-low.com.
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