Everyone Knows Its Windy

I had just happened to stop by another squadron's meeting on a blustery spring evening, only to find an ELT search in progress.  

"The tower has asked us to investigate an ELT they heard for a while, but now has quit," an informative member told me.

I helped with the operation of the L-Per and was quickly shown that the familiar audible sweep could be heard for about 5 seconds once every minute or so.   Hmm.   That wasn't enough signal to DF, now, was it?   That's what I thought, anyway, until I discovered that we had reliable deflection in DF mode.   I had heard of people DFing carrier-only ELTs, but had never tried it.   Since we had nothing to lose, we used standard methods (like triangulation) and narrowed the search down to one particular aircraft sitting on the ramp.   Even at only a few feet, we never got a continuous sweep; we could only hear an occasional trill.   What could be causing such an intermittent signal?

Now that we knew which aircraft was the offender, we wanted to know what was going on.   As we were pondering the situation, a ground team member bumped into the tail of the aircraft.   BHEW BHEW BHEW!   We immediately heard a few sweeps on our receivers.   Then a gust of wind came up--with the same results.   A few broken sweeps were heard, then nothing.   Hey!   If we drum our fingers on the tail here, we get sweeps . . . and the same thing happens when the wind blows!   Weird!

That ELT had been operating for such a long period of time that its battery was very nearly dead.   Whenever the wind would blow, or a GT member would tap the empennage, it shook up the batteries enough to produce that extra millivolt of electricity and therefore we heard a sweep.

The moral of the story: You CAN DF a signal even if you can't hear the ELT sweep, as long as you have reliable deflection.   To determine whether or not you have a signal that isn't just noise, compare your deflection on one frequency to the deflection on another frequency.   If they're the same, you probably have noise.   If one is significantly different, you have a carrier-only nonaudible signal.