Diagnose the Shakes

Heres how to decide if that new vibration is your imagination, an imminent engine failure or something else.


by Marc Cook

There you are, cruising serenely above a solid cloud deck in the sunshine. Stronger-than-forecast tailwinds are pushing you right along. ATC is cheerful and cooperative, and your passengers have settled in for the flight. Weather at the destination is perfect and youve got enough fuel to land with generous reserves. Music is softly playing through your stereo headset and your engine monitor is showing exactly what you expect. Its a perfect day to fly your perfect airplane as you wonder at the grandeur of it all.

And then you sense the vibration. Its nothing earth-shattering, just an offbeat rumble here and there. So far, the passengers havent even noticed. Now youre on full alert.

The fine art of in-flight troubleshooting in cases of onset vibration inevitably comes back to this question: Whats different now? As much as wed all love to use our X-ray vision to see into the engine, were left to discover the truth only through secondary indications, the temperature and pressures of various fluids going into and out of the engine.

Start Looking Around
As such, your first task is to take a quick look at all the instrumentation, paying particular attention to the engine monitor. (For the purposes of this discussion, well assume you have an all-channel monitoring system.) Start with the gross-deviation stuff: Youve got four or six bars showing on the engine monitor so, generally speaking, youve got all cylinders doing something. Oil pressure and temperature are as expected. You double-check the power and mixture settings, which are where they should be for the altitude and phase of flight.

This is when its most helpful to recall and understand the normal indications for your airplane. If you cant remember that, say, the number-two cylinders CHT is always the hottest or that the EGT indications for numbers four and five always sit a bar below the others, its time to take notes and make yourself a quick reference guide to what is your airplanes baseline. If you dont know what the graphical engine picture looks like when all is well, you wont have a particularly good chance sussing out the problems.

One word on alarms before we begin. Learn your engines habits and set the analyzers alarms with a relatively narrow margin to the typical maximums. If you routinely see maximum CHTs of 400 deg. F at top of climb, set the alarm for 410 or 420. Leaving the high-CHT alarm at the nominal redline value gives you no warning; it only tells you when youve reached the too late zone. At the same time, resolve yourself to take every alarm seriously. Weve seen pilots treat alarms like an annoyance, and dismiss them with a push of the button and an Oh, it always does that. One day, the alarm will be for real and the pilot will just have missed his first, best clue. Set alarms so they do not trigger during normal events, and take them seriously when they do.

Root Causes of Vibration
Not finding anything obvious, now youre really curious about what, if anything, is going on and looking at the engine monitor more closely. Fortunately, youve just been through three days of drinking from a fire hose in the form of the Advanced Pilot Seminars (APS) <www.advancedpilot.com>, in which all sorts of in-flight maladies have been discussed. (Yes, this is a plug for the course, which every serious operator of a piston engine owes himself. Several of the following ideas are presented and explored in the course.)

At the heart of it, in-flight engine vibration-excluding the massive, cant-see-the-instruments type resulting from losing part of the prop or spinner, or the connecting rod now sawing through the case-can be traced to a change in the relative combustion pressures. That is, one or more cylinders is suddenly subject to higher or lower combustion pressures than it was before you noticed the vibration. In other words, one or more cylinders is now putting out noticeably more or less power than when the engine was smooth-this power-pulse imbalance makes the engine shake, simple as that. And this same kind of imbalance is why most unmodified, horizontally-opposed engines will not run smoothly during lean-of-peak-EGT operation. The cylinder-to-cylinder air/fuel ratios are not sufficiently well-balanced to allow smooth operation when lean of peak.

Back to our flight. The slight rumble is still there, and looking at the EGT indications more closely, you see that, say, the number-four cylinders EGT is higher than normal. Before you go reaching for knobs to pull, watch the indication carefully. Youre looking to see if it changes, or if it just sits there. In this instance, its running three or four bars higher than the rest, and staying there.

Now look at the CHT. Why? Your task is to take all the information you have and determine the vibrations cause. That cylinders CHT is at its normal value, perhaps a bit lower but not unusually out of range. Youre already rich of peak EGT, but decide to enrichen the mixture slightly to see what happens. All EGT indications come down, but number-four is still high.

Your next step, after appropriately warning your passengers, would be an in-flight magneto check. Why? Lets look at the indications. The two most common reasons for a cylinder to show long-term, slightly high EGT indications is either a clogged fuel injector or the lack of ignition from one spark plug in the cylinder. The check for a clogged injector is to run what the APS folks call a lean-cycle test, where the mixture is leaned and the EGTs noted. A clogged injector in an otherwise well-balanced installation will make that cylinder peak early; the engine run rough at or very near the lean side of peak when it would normally run quite smooth right to idle cutoff.

So why wouldnt you do a lean-cycle test first? Look at the CHT again. Youve been running rich of peak, yet the CHT on the suspect cylinder hasnt changed much. With a clogged injector, the EGT rise would have taken the CHT up as well; contrary to popular belief, maximum CHT occurs not at peak EGT but at a mixture setting some 40 degrees richer. Running well-rich, a clogged injector would bring that cylinder closer to that 40-degrees-rich mark and increase CHTs.

Gingerly you turn the mag switch from Both to Right. The EGTs for all the other cylinders rise slightly, just as you have noticed during a mag check in the preflight run-up. But EGT on our problem cylinder doesnt change, indicating that it was only operating on the spark plug connected to the left magneto. Ah ha! Now, if youre the sensitive type and dont want to bother the passengers, you return the switch to Both, look for a nice lunch stop along the way and have that plug replaced. (Thanks to your engine monitor, you can tell the technician to change the spark plug on the number-four cylinder operated by the left magneto.) But youre the curious type, so you venture the key over to Left. The other cylinders EGTs remain where they are, slightly higher than in cruise, but now the engine is really rough and the number-four EGT starts to fall. There you go: confirmation that you have one spark plug doing nothing more than filling a hole.

The reason EGT increases with single-plug ignition is that the combustion event takes longer to get organized and burn than is the case with both plugs working. By the time the exhaust valve opens, the tail end of the event is still taking place, leading to a rise in EGT indication.

Incidentally, APS strongly recommends regular in-flight mag checks be performed at lean-of-peak mixtures; the philosophy is that under these conditions, getting the combustion event to light off is more difficult. Because of that, small problems with spark plugs and magnetos are more likely to show up than if the test is done rich, giving the pilot even more warning that a plug or mag is about to go belly-up.

Good sense dictates taking it easy on the engine and the exhaust system by not just switching back to the Both position. This is because the number-four cylinder is not seeing sparks but is still getting fuel and air. Once spark is returned, what is in the cylinder and the exhaust stack may light off. For a one-cylinder event, switching back to Both at cruise power will hardly be noticed. But had this then been a situation in which, say, one mag had completely flown the coop-all EGTs would drop with the ignition switched to it and the engine will wind down–pull the mixture to idle cutoff, return the mag switch to Both (or to the operating mag only), and then restore the mixture to cruise settings. Oh, and find a place to land soon; youre running on one mag.

Not So Clear-Cut
Lets back up in the flight and change the parameters a bit. Now, the number-four cylinders EGT is slightly up for a time, then returning to normal. The engine seems to smooth out and then roughens slightly. You notice the EGT moving up a bit at the same time.

What could that be? Bad probe? Intermittent magneto? An injector nozzle mysteriously clogging and unclogging itself? (You can rule out the last; once an injector gets clogged it tends to stay clogged.) An in-flight mag check reveals nothing unusual. At this point, youve ruled out a clogged injector, a bad mag and a fouled plug. Now what?

Try an exhaust valve going bad. Yes, you can tell such things from watching the analyzer data closely enough. The scenario is this: That cylinders exhaust valve guide is worn and the valve is intermittently landing on the seat cocked. When this happens, it cant properly seal so some of the combustion gas leaks past to the EGT probe. The slight vibration you feel is, again, from the mismatched power pulses among the cylinders. Left alone, the exhaust valve could fail completely and the cockpit indication would be-in addition to substantial roughness-a dramatically elevated, off-scale EGT on that cylinder. This warrants an immediate reduction in power and a precautionary landing.

Meanwhile, you will want to check that cylinders compression and do a borescope examination. And dont rely on just one compression check for the final say. Fly the airplane again and recheck. Exhaust-valve problems dont cure themselves.

Slow and Decisive
Central to the smart pilots troubleshooting abilities is the knowledge and willingness to act slowly when necessary but smoothly and decisively when called upon. Heres an example of the latter.

On our same flight, you notice the roughness but also see the number-fours CHT begin to inch up. Starting at the normal cruise figure, 360, it heads up the scale: 365, 372, 381. The rate of change is so dramatic that you first think it could be a probe failure. That first impression would be wrong: A sudden, rapid rise in CHT is a clear signal of detonation. Be spring-loaded to go to full-rich mixture and reduce power while looking for the CHT to stop climbing. Detonation is strongly influenced by internal cylinder pressures, and CHT is a good indication of them. Do everything reasonable you can to get CHTs heading downward as soon as you can.

Fortunately, a detonation event in a conforming engine-that is, properly built and maintained within the manufacturers specifications-running on the correct fuel is rare in turbocharged models and rarer still among normally aspirated installations. But misfueling and improperly set timing can erode the normally generous detonation margins.

Once back on the ground, have that cylinder inspected by borescope and its spark plugs replaced. Also, look for new cracks, especially around the spark plug holes and injector ports, if any. Detonation often damages spark plugs to the extent that the next event will be preignition, which can wreck an engine astonishingly fast.

Rumble Down Under
You, our normally studious pilot, run your normally aspirated engine at full throttle at just about all the normal cruising altitudes. (Good for you-thats the most efficient way; manage power with rpm and mixture.) But ATC has you down in the bumps at 3000 feet, and its probably not a bad idea to pull throttle back and reduce manifold pressure to maintain desired power and speed. As you double-check the mixture, you notice the EGT bars arent lining up as expected. The odd-numbered cylinders are slightly lower than the evens.

Because something in the indications changed when you deviated from your normal flight parameters, it makes sense to restore everything to the baseline and check again. With that in mind, you increase to full throttle and notice all the EGTs line up the way they should. Running a full lean-cycle test discloses that the EGTs reach peak at or near the same fuel flow, just as always. Reducing manifold pressure, you run another lean cycle test and notice that the odd-numbered cylinders peak early and the evens late.

Instantly you know whats wrong: Theres an induction leak somewhere on the odd side of the engine. At altitude, with the throttle wide open, the pressure differential between ambient and the induction system is minimal, but at low altitude with the throttle pulled back, its much higher. This increased manifold suction is allowing a leak-probably at a flexible coupling-to give those cylinders more air, which in turn leans the mixture. This is not a land-it-now scenario; check it out when you get home.

Reinforcing Good Habits
The best pilots know when to give something their complete attention and when to sit back and consider the big picture. Thats good advice with engine monitors. Learn to recognize normal indications at a glance. This way, when all is right, you can spend precious bandwidth thinking about the flight and caring for your passengers. Theyll appreciate it.

Also With This Article
“Rough-Engine Checklist”
“A Word About Probes”

-Marc Cook is a freelance editor and writer. He lives in Long Beach, Calif.


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