Your connection with the sky

“Automatic rough” – Bumps in the Night

I know I'm supposed to write about sailplanes and soaring. However, I spent a lot of my flying career driving single engine light planes just about everywhere and in just about every weather it's possible to do so. I also read all the blogs on Lets Go Flying which led me to think this subject might interest some readers.

This is about piston engines - you know, those noisy things on airplanes that blow cool air over the pilot affecting his emotional well being. Most young folks today, having better things to do, aren't spending their weekends taking car engines apart and putting them back together like I did in my misspent youth. This episode in my life taught me that piston engines are basically transducers which efficiently convert money into noise, heat and noxious fumes. Fortunately, there's a useful 'side effect' called horsepower.

Why is this important? Well, a piston airplane engine converts money into noise pretty much the same way a 1950's hot rod did. They work on the same 4-stroke "suck-squeeze-bang-blow" principal.

This is about the squeeze-bang part. To squeeze (compress) the air/fuel mixture properly, and get the power (bang) we expect, the pistons have to seal to the cylinders well enough to prevent leaks. If they leak excessively, your mechanic will wag his finger at you and your wallet will get lighter.

To help keep things air tight, pistons have "compression rings". (And one called an "oil ring" but that's not important here.) They really are metal rings about 5 1/2 inches in diameter and very thin. Two of them fit very precisely into grooves cut into each of the aluminum pistons. There is a roughly 1/8th inch gap in each ring so a mechanic can expand them in order to slip them onto a piston and to allow room for thermal expansion. Keep these gaps in mind.

Rings are designed to rotate in their grooves once every few minutes or so to keep them and the cylinder bore wearing evenly. We know they rotate because cylinders do, in fact, wear more or less evenly. Unfortunately, top and bottom compression rings don't necessarily rotate at the same rate which creates a problem. Not a serious problem mind you, just a low level psychological warfare kind of thing inanimate objects are wont to indulge in.

It seems that every so often ring gaps on one piston will line up and your normally smooth running engine will run noticeably less smoothly. Eventually, the ring gaps will rotate past each other and, with full squeeze-bang restored, the engine will again run smoothly - as if by magic. Each set of rings has its own mischief schedule so the frequency and duration are unpredictable - sort of like slot machine symbols. To be fair, light engine roughness can be caused my many things, but the cryptic "come and go" kind is often caused by rotating ring gaps.

It always happens at night, over water or any time you are beyond gliding range of an airport with nowhere to land. In short, anytime a pilot is really paying attention to the engine. It will always end just as you cross a shore line, the sun rises or flat Kansas wheat fields appear.

Generations of aviators are familiar with the phenomena and even gave it a name, "Automatic Rough". When the AR gremlin does its thing, idle conversations stop and pilots eyes will nervously scan engine gauges - which, naturally, will say nothing is wrong. Nonetheless, maps will come out and nearby airports will be identified. The course may be altered to pass near them.

A really nervous pilot may land and consult a mechanic who will run the engine then shrug while looking at the pilot suspiciously - AR gremlins hide when a mechanic is present. This pilot will go on his way a little less trusting in piston engines - which some will say is not necessarily a bad thing.

So, back to my usual subject, flying gliders - intentionally. If you know how to land without an engine, "Automatic Rough" will seem a little less threatening and you may deal with it more confidently - and mechanics may stop looking at you sideways.

4 Responses to ““Automatic rough” – Bumps in the Night”

  1. Gordon Feliciano Says:
    June 25th, 2009 at 2:25 pm

    I enjoyed your blog on "AR" and, I have to say, I have just learned something new and practical. When I first started flying back in the late 80's, AR was a common problem for me. On one particular cross country flight, I flew a Cessna 172 from NAS Moffett Field (when I was still in the Navy and stationed there) that I had rented from a Navy flying club, to Mendocino, California and back. On the way back to base, the sun began to set and as it got darker, AR began to creep into the picture. My engine instruments were telling me everything was fine, but my gut was telling me a different story. With nightfall approaching, I decided to pay extra attention. That's when I noticed that the RPM needle was bouncing around a little and my ear picked up roughness in the engine. I leaned the mixture a bit to see if I could make the AR disappear, but nothing seemed to work. Eventually, I decided to cut my flight short and land at the nearest airport, which was Santa Rosa. When I got down, I called my flight instructor and explained to him that I thought the 172 was having engine problems and I requested that he come pick me up and bring along a mechanic to check things out. Well, long story short, an inspection revealed that there really wasn't anything wrong with the aircraft accept for a small bit of lead deposits that had built up on one of the plugs, but we managed to fly the plane back that night without incident. Instead of being laughed at, my instructor looked at me and told me that what I had done was demonstrated good judgment. I've only managed to cut short one other flight and that was one where I was about to go from VMC to IMC conditions had I chose to continue. This was a learning experience that I will never forget.

  2. Vince Jones Says:
    June 27th, 2009 at 10:39 am

    Bill, this isn't really germane to flying or soaring - this comment - but I do enjoy your writing style. I found it to be refreshingly clever, witty and full of pithy little references and humorisms. So, I wish to thank you for your efforts in creating a perfectly enjoyable essay (on AR). I look forward to reading your past and future articles.

  3. Principal Principia Says:
    August 25th, 2010 at 10:45 pm

    Principals have principles.

  4. Freddy the Flyer Says:
    February 19th, 2012 at 6:21 pm

    Hello to all. This so called AR is nothing more than someone starting to notice items that are not there in reality,99.99 % of the time.

    Most of the So Called AR reports are no more than air shifting over the prop at a Different angle. One has to remind one self that there are Two or Three top rings and one oil ring.
    Even if all gaps were lined up for a split second as all rings rotate at a Different rates as the piston is moving the amount of air that goes past the piston ring end gap would not make the engine run ruff for you to notice. But being safe and checking out any problem real or not is the Wise thing to do.

    If you think there is a problem while flying your piston Engine Airplane Change Course, Change Altitude, Check for Carb Icing, and check your Mixture and Mags. I personalty would put the Carb heat on first that is if you have that option.

    I will bet other than a real Mechanical Failure
    one of the above will sort out the problem of that Fluke AR.

    Next your Flight Instructor should have gone over this type of Issue with you prior to you flying solo in a piston powered airplane.

    Gordon did not mention if the AR stopped when he reduced power or while landing. Gordon did not check his Mags as he should have. How much time does Gordon have Total and in type. Gordon did not mention if the AR was gone on the returned flight or if the airplane was allow to be used upon return to the home airport.

    I do not understand why Gordon said that AR was a common Problem for him?? Makes me Wonder Why!!

    Gordon did the right thing and a good lesson was learned but not explained to him.

    Air Cooled Piston Engine are in of and by there self a very very reliable source of power and not the reason for most downed Aircraft. Hence Two Mags and Two fuel pumps.

    Typos and Grammar do not count

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