AirTalk.org Aviation discussions newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  broken ground wire          AirTalk.org Forum Index -> Owning Airplanes View previous topic :: View next topic   Author Message Matt Guest Posted: Sun Apr 15, 2007 8:30 pm    Post subject: broken ground wire 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 Back to top mikem Guest Posted: Mon Apr 16, 2007 6:12 am    Post subject: Re: broken ground wire 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. Back to top Matt Guest Posted: Fri Apr 20, 2007 12:59 am    Post subject: Re: broken ground wire 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 Quote: 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. Back to top mikem Guest Posted: Fri Apr 20, 2007 6:12 am    Post subject: Re: broken ground wire On Apr 19, 1:59 pm, "Matt" <m...@nospam.com> wrote: .... Quote: 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... Back to top mikem Guest Posted: Fri Apr 20, 2007 6:12 am    Post subject: Re: broken ground wire On Apr 19, 7:22 pm, mikem <mgm17...@utah.edu> wrote: Quote: On Apr 19, 1:59 pm, "Matt" <m...@nospam.com> 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. 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