Radio Static When Engine Runs

[ECB]

Fellow Flying Dudes,

I own a '63 Piper Cherokee with King KX 155s ... the radios both run
fine .... until I fire up the engine. There is a lot of popping and
static that changes a bit with adjustments in engine speed. I have
turned off all other equipment to omit other causes. I have shut down
the alternator as well via the field and armature breakers.

A fellow flyer said it could be a bad/intermittent ground between the
engine and the airframe.

Any ideas? Thanks in advance.

Ed B - N7025W - MMU

[Paul Tomblin]

In a previous article, ECB > said:
>A fellow flyer said it could be a bad/intermittent ground between the
>engine and the airframe.

Yup. This is what we found when it was happening to one of our planes:
http://xcski.com/gallery/rfc/38290SparkCbl1
http://xcski.com/gallery/rfc/38290SparkCbl2


--
Paul Tomblin > http://xcski.com/blogs/pt/
"He's overweight, uninformed, and litigious. That's an American
hat-trick" - Lewis Black

[ECB]

On Mon, 31 Oct 2005 01:58:10 +0000 (UTC),
(Paul Tomblin) wrote:

>In a previous article, ECB > said:
>>A fellow flyer said it could be a bad/intermittent ground between the
>>engine and the airframe.
>
>Yup. This is what we found when it was happening to one of our planes:
>http://xcski.com/gallery/rfc/38290SparkCbl1
>http://xcski.com/gallery/rfc/38290SparkCbl2


Paul,

Thanks for the super rapid response. I will check this out tomorrow.
I might just get back in the air tomorrow and burn some really
expensive petrol.

Thanks again.

Ed Berberich

[nrp]

ECB wrote:

> Any ideas? Thanks in advance.

You might first try switching to one mag & then the other to see which
one is making the noise.

Then - is the noise that of a popping at a single cylinder firing
frequency (which would be a bad plug lead as described by others), or
is it at the much faster engine firing frequency - which would suggest
a bad magneto capacitor or p-lead (i. e. the wire to the ignition
switch) filter. There is a remote chance the p-lead shield(s) could be
broken.

It probably is not an engine ground problem - at least not if you don't
have starting problems, or alternator noise or regulation problems.

[nrp]

It probably is not an engine ground problem - at le.........

On second thought - a bad engine ground would probably force the
starter return current thru the ignition harness shields - probably
burning it out like the pictures posted by Paul Tomblin.

[OtisWinslow]

Check that the condenser on the alternator is still grounded.


"ECB" > wrote in message
...
>
> Fellow Flying Dudes,
>
> I own a '63 Piper Cherokee with King KX 155s ... the radios both run
> fine .... until I fire up the engine. There is a lot of popping and
> static that changes a bit with adjustments in engine speed. I have
> turned off all other equipment to omit other causes. I have shut down
> the alternator as well via the field and armature breakers.
>
> A fellow flyer said it could be a bad/intermittent ground between the
> engine and the airframe.
>
> Any ideas? Thanks in advance.
>
> Ed B - N7025W - MMU

[mikem]

OtisWinslow wrote:
> Check that the condenser on the alternator is still grounded.

Why? The capacitor on the alternator is there to suppress brush-noise
Radio Frequency Interference (RFI) at ADF/AM Broadcast/Loran
frequencies (190-1500Khz). It does nothing to suppress Magneto/Spark
noise, which is the original posters problem.

btw-contrary to popular misconception, the alternator capacitor does
nothing to suppress alternator whine heard in the aircraft headphones,
either. That is usually caused by airframe ground loops resulting from
grounding various parts of the audio system (avionics, audio panel,
intercom, mic/headphone jacks, cabin speaker) in multiple, disjoint
places.

MikeM

[ECB]

Other details:

- Switching the Mags .... no change
- Cutting out the Alternator ... no change
- Turning off various equipment ... no change

I did move ignition wires thru the insulators a bit to see if there
was any change. After doing this, the noise went away for the most
part. A test flight over central NJ for about 1 hour resulted in a
burst of static that lasted about 15 seconds about 20 minutes into
the flight. There are some occasional clicks here and there. It
seems to vary with engine speed, attitude, etc. I will continue
with the search.

Thanks all.

Ed Berberich

[mikem]

Aircraft ignition leads are shielded in a coaxial cable. To suppress
RFI which would otherwise be radiated off the HV leads, the shield must
be continuous (no breaks), and the shield must be electrically
connected at both ends, plug and mag tower. Might be necessary to
disconnect each lead (one at a time) at the plug end, and then use an
Ohmmeter to make sure you have continuity from each dangling gland nut
to the airframe.

Similarly, the P leads running from the mags to the rear of the mag
switch should be shielded to prevent RFI from being radiated. Their
shields should be unbroken, and grounded to the mag body, and to the
panel very close to where the mag switch is mounted.

[John_F]

You may have worn through and broken the shield braid where it passes
through a hold down clamp or the baffles. If so the shield may not
have full electrical conunity for the full length. Disconnect the
plug end of the wire and hook a ohm meter to the plug nut and the
other end to an engine ground. Now shake and pull the plug wire and
see of the reading changes. It should read less than 0.3 ohms. If
the shield is broken or worn through anywhere this leads to EM leakage
which you can hear in the radio.

On Thu, 03 Nov 2005 03:14:14 GMT, ECB > wrote:

>
>Other details:
>
> - Switching the Mags .... no change
> - Cutting out the Alternator ... no change
> - Turning off various equipment ... no change
>
>I did move ignition wires thru the insulators a bit to see if there
>was any change. After doing this, the noise went away for the most
>part. A test flight over central NJ for about 1 hour resulted in a
>burst of static that lasted about 15 seconds about 20 minutes into
>the flight. There are some occasional clicks here and there. It
>seems to vary with engine speed, attitude, etc. I will continue
>with the search.
>
>Thanks all.
>
>Ed Berberich