Those air craft plugs control both the earphones in the helmit and the
microphone. Usually, when they're in a helmit, a lip mike is used. The ones
I've seen use the tip of the plug for center to the earphones and the bottom
connection for the other lead to them.
Tom
Tom Brennan KD5VIJ, CCC-A/SLP
web page http://titan.sfasu.edu/~g_brennantg/sonicpage.html