Switch Off!

As a young man and CAP cadet, I arrived at my unit's meeting to find a search in progress.   It was a fine late-summer evening in Lake Elmo, Minnesota.   There were several airplanes I recognized on the ramp,   Since we were already on the North side of the airport, we took a reading right where we were.   to our surprise, the signal was VERY strong AND it was behind us!   Most of the aircraft on the field were hangared on the other side of the airport, to the South of our position.

Slightly puzzled, we moved to a different part of the ramp and tried again.   We found that the bearing to the target had swung considerably, indicating that we were VERY close to the ELT.   The FBO's hanger door was open and about 8 airplanes were inside.   Taking a deep breath and knowing we were in for a reflection nightmare, we proceeded underneath the hangar door.   Still surprised, the signal centered behind us . . . that would mean the only airplane it could be is that one at the gas pumps!

The airplane parked at the pump was a black on white Cessna 150 Commuter.   I knew the owners; they were two college students who were not much older than myself at the time.   Like a few enterprising young aviators, they purchased the airplane to build time towards additional ratings.   They happened to be inside the FBO signing a fuel receipt.   I told them I thought their ELT was going off.   I got a very typical response: "Impossible!   We take good care of our airplane."   To humor me, though, they accessed their ELT and turned the switch from the ARM position to OFF.   We heard no change in the signal through our L-Per receiver.   Just to ensure that it wasn't the 150's ELT, we briefly turned the switch to the ON position.   Clear as day, we heard the ELT overpower the signal we had been receiving--a nice strong signal with a good sweep.

Well, now, it couldn't be that Cessna 150, could it!?   Stymied, we went back to DFing our scratchy-sounding ELT.   Another trip to the corners of the parking ramp turned up nothing new: switch or not, the signal was coming from that 150.

I was, at the time, still a little unsure of my DF skills.   Could I be wrong?   I had figured that, after the ELT gave considerable evidence that it was not the offender, that it must be my skills in using the DF equipment that were lacking.   The second time, however, I KNEW it was that 150.   We were a little more insistent this time.   We asked the two pilots to briefly remove the antenna lead from the ELT.   They did and to everyone's amazement, the signal stopped!   We reconnected the antenna so as not to harm the transmitter, then disconnected the battery to silence the ELT.

As you may have guessed, the arming switch had partially failed.   It allowed the transmitter to operate in either the ARM or in the OFF positions.   The change in the signal was because the electrical contact in the ON position was much better than the partial contact in ARM or OFF.   Switch failure is another common source of false alarms in ELTs.

In summary: trust your equipment and your skills.   Leave no stone unturned.