What the [Beep]?

Well, that was interesting.

My Condition Inspection is due. The A&P knows I have a professional-type Carbon Monoxide detector, and always asks me to take a flight and log the levels before the inspection.

Popped the detector in my shirt pocket, climbed aboard, and put on my headset. I had switched back to a standard headset, and decided to do a test transmission before starting the engine.

Odd. When I hit the Push to Talk switch, I heard three beeps: beep beep beep.

Huh. Fiddle awhile, then try with the microphone plug disconnected. Beep beep beep.

Fiddled more. Then unplugged the earphone plug as well, and hit the PTT. Beep beep beep.

What the hey?

Where is the sound even COMING from. The radio speaker itself? Shouldn't, with the headphone adaptor plugged into it, even with no headset attached.

Other than the warning buzzer for my ejection seat (which sounds quite different) there were NO other devices with speaker or beepers installed.

Got out of the plane, switched headsets, and the beeping was gone. But...shoot, it was beeping WITHOUT a headset installed.

Took the flight. Turned the detector back on (it had auto-shutoff).... CO at the panel level was a max of 15 PPM during climb, cabin heater on or off. About 6 PPM at cruise, and during glide. Maximum was a momentary 22 PPM during full-power climb, measured underneath the panel between my calves. All well within margins.

Didn't hear any beeping. I tucked the meter back in my shirt and flew home.

Back home to land, park in front of the hangar and shut the engine down.

When I heard a beep. It was the CO detector doing an auto shutoff.

Then it hit. I pulled out the detector and watched its display as I hit the PTT switch. Reading jumped to ~300 PPM...and the three-beep high-CO warning sounded.

The CO detector apparently has some sensitive circuitry; stray RF from the mighty Icom was messing with it. It had stopped when I'd switched headsets because it did its auto-shutdown while I was changing over, and I didn't turn it on again until I was flying.

Geeze, that was weird.

What the [Beep] is that Beep?