Old 08-13-2003, 04:50 PM
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Default radio interference, flying wires, wing rigging.

I've heard the "loop" thing through the years but have never really seen any interference as a result.

The most often encountered problem with flying wires are metal to metal interference between clevis, or wire, abrading against the surface of the mounting bracket.

The last instance of this I observed on my friends extra 300. He had used the plastic jacketed wire, similar to that used for pull/pull installations, and he looped it through metal brackets attached to rudder and stab.

The wires would abrade and wear through the plastic jacket and come into contact with the metal bracket. Wala, electrical interference.

Metal to metal interference is caused by a phenomena termed "thermal EMF".

Thermoelectric EMFs are small voltages developed at the junctions of dissimilar metals. The magnitude of the voltage depends both on the type of metals used as well as the temperature differences between junctions.

The high frequency vibration levels of our gassers can cause quite a thermal rise in two surfaces that are abrading. An analogy would be similar to rubbing two sticks together to make fire, rubbing two metals together will make voltage/electrical noise.

Accompanying the temperature rise are ac voltages that are radiated from the junction(s) and picked up as electrical noise by our receivers.

This noise, because of the close proximity to our antennas, will cause the receiver to "believe" that it's receiving a very strong signal and subsequently causes the receiver to close down it's rf amplifier in response.

Thermoelectric EMFs can be eliminated by using no metal to metal connections.

Now some guys will claim that they have made similar hookups before and have had no problem(s). Others use pCM receivers and don't know they have a problem (until too late).......

This is a measure of "good practice" to avoid these kinds of problems. Just because you got away with it a couple of times doesn't mean that, given just the right set of circumstances that you won't encounter it on the next airplane.

So the motto is "never use metal/metal" in a gasser if at all possible.