Listen Up

Over time, you become accustomed to the sights, sounds, and, yes, even smells of flying—when you do, patterns emerge. You learn the sound of the slipstream passing by the not-quite-sealed window at the correct approach speed. You recognize the slight flutter of the engine as you pull back to idle abeam the numbers. It all starts to make sense.

As you gain more experience, you realize the engine is talking to you the whole time. From start-up to the first time you advance the throttle fully, from the downwind until you pull the mixture to idle cutoff, it’s whispering to you. 

Sometimes it does much more than whisper—it speaks quite loudly that something’s wrong. Let’s break down the symptoms and possible reasons for abnormal behavior. 

All engines vibrate to some degree, and they do so from two primary sources. One is the combined imbalances of the rotating elements, such as the pistons, crankshaft, and propeller. These will be rhythmic and only partially related to power levels. That is, an out-of-balance propeller might shake more at certain engine speeds than others, but the essential vibration will be there.

And then we have vibration induced by the combustion events themselves. Normally, when all cylinders are producing roughly the same amount of power, combustion-induced vibration will be relatively minimal. However, it does increase when the engine is at high power, simply for the fact that the combustion events are stronger.

Because for most piston aircraft, the pilot is in control of the fuel/air mixture, this is where most of the combustion-imbalance vibration will be noticeable. Generally speaking, when the engine is running rich, the chances for a cylinder-to-cylinder imbalance are reduced. But as you lean the mixture, most engines will reach a point where the cylinder-to-cylinder variation is high and you’ll get the shakes, so to speak. This state is usually accompanied by a noticeable reduction in power.

What I’ve described applies to carbureted engines especially, but fuel-injected applications may or may not have a greater tolerance for lean mixtures before starting to shake. If the engine has so-called balanced injectors, it may run smoothly all the way to idle cutoff. Conversely, an injected engine with a partially clogged injector may be rough even with a very rich fuel mixture. Finally, understand that a grossly rich fuel mixture can also create vibration, though it’s a lot less likely.

Sudden-onset vibration—meaning you did nothing to change power or mixture settings—is most often related to ignition problems. A misbehaving magneto or a fouled spark plug can change the dynamic in one or many cylinders, which upsets the power balance across the engine. A full engine monitor can give you clues to the problem: A cylinder suddenly running on one of the two spark plugs will show a rising EGT. But even if you have just a single EGT probe (or none at all), it’s always worth testing each side of the ignition system (left mag, right mag) to try to isolate the problem. If an engine misfires badly and running on the one “good” mag cures the symptom, that’s your solution as you find the nearest airport as your new destination.

Other, less likely causes for new vibration include the propeller throwing a balance weight or some part of the spinner departing the scene. Also note that an engine that shakes vigorously on the first start of the day is probably experiencing what mechanics call “morning sickness.” This is the early stage of a valve sticking and should be remedied without delay. Typically more common with parallel-valve Lycoming engines, incipient valve sticking can happen to any aero engine.

Pilots experiencing sudden-onset abnormal vibration would do well to reduce power, if the phase of flight permits it, and begin working the emergency checklist. Check mixture, fuel quantity and tank selection, ignition, carburetor heat, and run a scan of all the instruments. 

The engine might not shout that it’s not making expected power, but the airframe will tell you in the way it performs. In this case, less-than-expected power is almost always mixture related. As you lean the mixture, the power curve is relatively shallow through full-rich and toward the best-power mixture, but as the ratio reaches best economy and even lean-of-peak-EGT settings, power drops rapidly. It’s not unusual to lose 200 rpm with a quarter-turn of the mixture control when the engine is quite lean. Of course, a creeping or misrigged throttle control also affects power.

Other reasons for reduced power include incomplete or improperly timed ignition. Magnetos whose timing is retarded relative to the spec will definitely kill power, just as those too advanced will make the engine run hotter than expected, and in rare cases could lead to destructive detonation. A carb-heat door that doesn’t close properly will also stifle power production, as will a blocked or misaligned air filter. Exhaust-system blockages can also decrease power.

Finally, cylinders with low compression can influence power, though they have to be really weak to start having a major impact on power. (You’d probably notice the very high oil consumption first.) You won’t troubleshoot any of these in flight, so an airplane that shows signs of being underpowered needs to visit the shop for a methodical checkup.

A loud pop after startup isn’t all that unusual if the engine has been overprimed. But consistent popping during taxi should be investigated immediately. Backfires (technically afterfires) are the result of fuel being burned in the exhaust system, which can come from an abundance of fuel there (normally the post-combustion mixture is not itself easily set off) or very late magneto timing. Some airplanes have a notable pop on descent at very low power. In most cases, correcting the idle mixture setting will help but sometimes it’s just what they do.

And that’s the nature of the beast. The more you fly a certain type of airplane, the more familiar you become with its quirks, its vibration signature, the sounds it makes going about its business. As you absorb that knowledge, you’ll be able to better tell what’s not normal. All part of the learning process.