When one’s life depends on the proper operation of a complex mechanical device, it pays to be attentive.

This goes doubly so when one is responsible for the safe control & maintenance of said device. So it happens that if one is flying an airplane, one becomes attuned to its noises, vibrations, smells, pictures, so some deviations from the normal can be detected almost unconsciously.

The most powerful signal in our airplane is the vibration/noise caused by the two engines. In a light unpressurized craft, there is essentially no noise absorption and only the barest vibration isolation hardware. This way, the human payload is viscerally aware of the hundreds of miniature burning horses pulling us forward. We become used to the feel of several standard engine-RPM combinations, and let the droning put us (well, some of us) to sleep.

On a twin, we ideally want the two props/engines to spin at the same rate. That way, annoying beat frequencies can be reduced, making the ride more comfortable. Some pilots (hi, Luke!) spend minutes fine-tuning the blue knobs on these old birds to make the sinusoidal crescendo go flat. I am not quite that picky, but I like a nice matching pair of engines.

That’s ‘cause a suddenly mismatching pair of engines is the first sign of serious trouble. If one engine is suffering a malfunction, its RPM will almost immediately change (usually drop). This creates an instantly audible beat frequency, which would jolt awake any snoozing pilot, getting ready to run emergency checklists. (If an engine spins all the way down, the beat frequency disappears, but the transition is too drastic to ignore.) So let’s just say I don’t like to hear anything like that, except during deliberate engine-out practices.

So this sets the stage for the story. During a recent trip to and from our nation’s glorious capital, C-GXRP worked like a little champ. On the way home, for example, we were treated to a stupendously awesome transition from a high overcast to blue skies, as sudden as an eclipse. However, every now and then, my heart skipped a beat, because of sudden engine beat frequency sounds. It would last for a few seconds, then return to normal. I’d study the engine instruments afterwards to see if there was anything wrong; nothing. I was preparing to lunge my eyes at the RPM gauges, at the next event, but even then, nothing indicated wrong.

It took me a while to figure out what was going on. I was flying in relatively busy airspace on IFR, and was privy to much air-traffic-controller radio traffic. And it turns out that Ottawa is well-served with the Dash-8 commercial turboprops. And it turns out that pilots aboard these turboprops also speak to ATC, and with plenty of panache. And sometimes their engine noises get transmitted on radio.

So here we have little Piper Aztec, buzzing around two-bladed propellers at around 2400 RPM. We have a big Dash-8, buzzing around six-bladed propellers at around 900 RPM. That gives bladetip frequencies of 80Hz and 90Hz. 90-80 = 10. And a 10Hz beat frequency thus created is quite noticeable.

As soon as I figured out that the beat frequency came from our engines and our radio (relaying the sound of their engines), I could relax a bit. But then I started dreading hearing “Porter blah blah, Montreal centre, blah blah”, knowing that any time now I’ll get another scare.

I can only hope that my transmissions scared them a little too. Just to be even.