broken ground wire

[Matt]

Hello everyone.

I was looking under the cowling of the 152 the other day, and I noticed that
the ground wire from the left magneto was broken. The end of it looked like
it was frayed and burned. However, when I did a mag check I got the normal
RPM drop on each magneto. I also was able to kill the engine by switching
the ignition to off. It appears to be working normally.

Is it possible there is a second ground wire that to that mag? If I had not
been looking under the cowling, I would not have suspected anything.

Any ideas?

Matt

[mikem]

Matt,

The wire from the magneto to the key switch is called a "P-lead". In
the NORDO days, this used to be a single, unshielded wire which runs
from the terminal on the magneto to the magneto switch. To "kill the
mag", the switch makes electrical contact to airframe ground near the
magneto switch, therby "shorting out" the primary circuit in the
magneto. The circuit is complete because there is a path back to the
body of the magneto though the airframe, motor mount, crankcase, etc.

When they put VHF AM radios in aircraft, it became necessary to shield
(enclose in a faraday cage) the P-lead, otherwise the very sharp
risetime pulses that appear across the magneto's Primary (resonant
inductive-capacitive) circuit, the P-lead would radiate RF energy
which would be heard on the COM and NAV receivers. btw-to be effective
in preventing RFI radiation off the P-lead, the shield enclosing it
must be grounded at both ends (to the mag body, and to the airframe
close to the mag switch).

What you are seeing is a break in the RFI shielding between the p-
lead's shield and the magneto body. This break does not prevent
"killing the mag" in the usual way, however, it could be the reason
you are hearing a "popping" noise on the weaker signals on your COM
radio.

[Matt]

Thanks for the detailed reply. It was a few days before the rain finally
ended here and I was able to take another look. It appears you are correct.
The other mag has the same sort of ground to the mag body. It looks like it
has been spliced into the p-lead near where it connects to the mag.

Would the electrical interference pose any threat to the avionics? Or would
the interference just be annoying?

Matt


> What you are seeing is a break in the RFI shielding between the p-
> lead's shield and the magneto body. This break does not prevent
> "killing the mag" in the usual way, however, it could be the reason
> you are hearing a "popping" noise on the weaker signals on your COM
> radio.

[mikem]

On Apr 19, 1:59 pm, "Matt" > wrote:
....
> Would the electrical interference pose any threat to the avionics? Or would
> the interference just be annoying?
>
> Matt
>

The interference can manifest itself as a popping sound (rate follows
engine rpm) that is heard only when the squelch in the COM receiver
opens (loudness follows COM volume control). If the interference is
not strong enough to open the squelch by itself, then you will not
hear it until a real radio signal opens it, at which point you will
hear the signal contaminated by the popping. If the signal is weak
(distant), the popping can be so bad as to make the signal
unintellegable. If the real signal is stong (you are close to the
source), the interference is less noticable. If the interference alone
is strong enough to open the squelch in the COM receiver (with no
other signal being received), then you will hear the popping sound
continuously, even while the receiver would normally be silent, in
which case it gets VERY annoying.

The interference does not threaten your avionics; only your sanity...

[mikem]

On Apr 19, 7:22 pm, mikem > wrote:
> On Apr 19, 1:59 pm, "Matt" > wrote:
> ...
>
> > Would the electrical interference pose any threat to the avionics? Or would
> > the interference just be annoying?

I forgot to mention, if the interference is so bad that you are
missing calls from ATC or other aircraft, then it become a "safety of
flight" issue, and should be fixed pronto.