Things That Go ‘Bump’ While Flying

by R.J. Reilly
Published in Midwest Flyer – April/May 2017

Airplanes can play with your brain. Think about your engine running rough over water or inhospitable terrain.

It was a short flight to re-position my Cessna 170 from a rural maintenance shop to its home hangar in a major metropolitan area. I gave the preflight inspection a bit more attention than usual…not that I didn’t trust the work done by the shop, but I’m always a bit edgy after my airplane has been seriously disassembled and put back together. The run-up at the end of the active gave assurance that all was well and the surrounding farm fields provided a ready ‘place to go’ should something be amiss just after lift off.

The weather was truly beautiful. A few scattered, fair-weather cumulus clouds lingered at about 5000 feet, backlighted by a low angled sun that had given up its work for the day. My southeasterly track put the sun over my shoulder, eliminating the annoyance of peering into a setting sun and easing the sighting of conflicting traffic when entering the more congested area.

Climb-out was routine, and I leveled at just under 3000 feet MSL, roughly splitting the altitude difference between the surface and the cloud base. There remained a bit of instability in the atmosphere that produced a gentle, rolling disturbance as I passed beneath the scattered woollies, just about enough lift to keep a good sailplane aloft until nearly sunset. I settled in on course for home and was thoroughly enjoying the end of the day.

After a lazy interlude, I tuned the home-base ATIS to get the latest atmospherics and traffic pattern:

“…landing runways one four left and one four right. On initial contact, report you have information Lima ….”

WHAT WAS THAT? As I passed under a cloud, the gentle disturbance produces a distinct, clunk-clunk, ‘oil-canning’ sound of a skin panel being displaced and returning to a resting position. Did I leave a panel unsecured, miss a screw or more than one? Very gently I induced a short pitch disturbance and a return to level flight, sort of an almost-nothing roller coaster. All was quiet. I tried a similar experiment in the roll axis – same result. I forgot about the ATIS and contemplated whether to continue or return to the maintenance facility. Another try at the little roller coaster maneuver produced no noise and my position placed me closer to home than a return to the rural port.

Tuning to the destination tower frequency and listening for a few minutes revealed only light traffic for the time of day, so there would likely be no diversions and little maneuvering on arrival. The active runways were on an almost on-course heading that might accommodate a straight-in approach if traffic allowed.

Returning my attention to the ATIS, the destination particulars hadn’t changed, but I harbored some residual unease over the potential problem.

Another cloud appeared and I elected to continue on-course and pass beneath it. Another atmospheric burble and a gentle roll easily corrected; no accompanying oilcan sound. Whoops, there it was again, not quite synchronized with the atmospherics, but close enough to renew concern about structural anomaly. Now thoroughly focused on the ATIS, I listened carefully to every word. At the finish of the taped loop, I was beneath a cloudless piece of sky, and after about a five-second delay following the verbiage, there was a repeat of the ‘oilcan.’ Although nearing the Class B penetration call radius, I was compelled to listen to the ATIS loop once more. Sure enough, after a short delay following the traffic advisory, the ‘oilcan’ sounded again, courtesy of the ATIS.

A relieved call to the tower produced clearance for a straight-in to 14 Right. The ‘chirp chirp’ of rubber on asphalt was especially welcomed. I parked in front of my hangar for a time, listening to the ATIS loop. A call to Ground Control couldn’t explain the ‘oil can,’ but to me it sounded as though a heavy coffee mug deposited carelessly on a metal table.

An old aviation aphorism says: ‘You learn something on every flight…’ if you pay attention.

Richard J. (Dick) Reilly of Hokah, Minnesota, holds 16 patents in the general areas of aerodynamics, fluid power, control systems and computing devices. He is the author of “Tell Me A Story,” a book about his experiences working for 50 years on five continents. (

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