On December 18, 2007, Pam and I decided to take an afternoon flight since the weather was so nice. It was very chilly, but otherwise nice flying weather. The sun was working its way down at around 4:45 pm as we did a preflight inspection and pulled Betsy out of the hangar. Sunset was to be at 5:17, so we had a little daylight to fly in yet.
We got airborne and flew over downtown Columbia, right by the State Capitol building with the big Christmas tree out front. Its lights were on, but there was still a little too much daylight to make them look bright from our viewpoint, but pretty none the less.
The sun was setting blindingly bright to our west so we flew eastward over Forest Acres. What an incredible view of the vast “forest” of the Forest Areas area as the low lying sunlight had shone on the trees, lighting them up intensely with an almost glowing golden hue on their westerly sides, and casting shadows beyond them to the east in a way that only the setting sun can.
After a while, we returned to Columbia Owens Downtown and landed on one-three. The light was fading fast as we taxied back toward the hangar. I asked Pam if she felt like going around the pattern once more. She was fighting a head cold and decided remain on the ground while I went back up.
I departed one-three again, and while on downwind, saw that the sun had set and the view westward was just beautiful. I grabbed my camera and attempted to take a photo or two, not really sure if I got a good one because I was more focused on flying the airplane. On downwind, I tried to shoot a photo of the downtown city lights as they were starting to really stand out in the fading sunlight.
I made my second landing and was clear of the runway when a taxing aircraft passed by me on the taxiway as I held short on the middle exit ramp. I started to hear a crackling sound in the cockpit that I hadn’t heard before. At first I thought it was my headset. Or perhaps my intercom. The taxiing aircraft passed and as I pulled off the exit ramp onto the taxiway behind it, taxing north back to the hangar, the crackling noise stopped.
An airplane that had been holding short of one-three departed so I slowed my taxi speed to watch it take off. The crackling started up again, so I stopped to try and find the cause. It was a rather loud constant crackling sound, sort of like a bad connection in stereo speakers, or how crinkling plastic wrap sounds. I took my headset off and the sound was all throughout the cockpit. It was hard to tell where the crackling sound was coming from. I turned the volume on both COMs down and made sure the cockpit speaker selections on the intercom were off for both COMs. It sounded like it was coming from everywhere. The panel, the doors, the dashboard, the windscreen. I put my ear to the panel, then the dashboard, and then the door panel. Still no idea what might be causing the mysterious crackling noise that was verberating throughout the cockpit.
I then continued to taxi further up the taxiway toward my hangar and the noise ceased. What the heck is going on? I slowed to a stop about at the on ramp to runway one-three. The crackling started up again. Was the noise coming from outside? It sounded like sleet hitting the windscreen, crackling with every impacting ice particle. And a bunch of them. But it’s a clear evening! Nothing is hitting the windscreen. I pressed my hands in several places on the wind screen and this seemed to effect the noise – subdue it perhaps – but not nearly stop it. I pulled my hands away from the windscreen and the noise then continued its normal level of crazy crackling. Hum?
I pushed the throttle in and revved the engine for a moment and the noise again ceased. I pulled back on the throttle and the engine settled to below 1000 rpms. The crackling noise started back again. I revved the engine again. Noise stopped. Backed off the throttle again. More crackling.
And then it hit me. This was perhaps the coldest day I had flown Betsy. The temperature was below 50 degrees and falling rapidly with the fading light. The windscreen had contracted with the cold air and was relatively loose in its mounting. The combination of typical engine and aircraft vibrations and prop wind at low rpm caused the windscreen to vibrate in its mount, thereby causing the mysterious crackling noise that reverberated throughout the cockpit. The crackling sound seemed to come from anywhere and everywhere because it was being created all about the perimeter of the windscreen, which is shape somewhat like a parabolic section – a megaphone – and was being amplified and transmitted throughout the cockpit.
And so there was something hitting the windscreen – wind! And with increased rpm of the prop, the resulting higher wind pressure against the windscreen stopped the vibrating – and the mysterious and crazy crackling noise. Case closed. I radioed to Columbia Downtown traffic and departed runway one-three, once again around the pattern, to enjoy the view of the sky and cityscape at dusk.