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7 Answers

Engine Roughness In Flight Causes

Asked by: 27161 views
Aircraft Systems

Besides spark plug fouling and carburetor icing, what are the other causes for engine roughness? How can I differentiate between the engine roughness sound when the cause is the spark plug fouling vs when the cause is the carburetor icing?

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7 Answers



  1. Sam Dawson on Nov 10, 2013

    Bad magneto.
    Fuel injector clogged.
    Moisture/debris in the fuel.

    You may not be able to tell the difference which is why trouble shooting should include the different steps- such as 1. carb heat on 2. fuel fullest tank 3. mixture adjusted 4. mags check (first both, then individually check them). A mag’s timing going bad could result in bad things happening to your engine, so isolate each one. If the roughness goes away that is probably our problem.

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  2. Best Answer


    John D Collins on Nov 10, 2013

    I would not use the sound to differentiate between the two causes you listed. First, it is urgent that you check the carburetor ice first as your time may be limited before the engine quits do to it being iced over. Once it quits, you no longer have a heat source to clear the condition.

    To determine if you have carburetor icing, quickly pull the carburetor heat control all the way out. If you have ice, the engine may miss for a while during the period the ice is melting, but shortly you will see an increase in RPM as it clears. If there is no ice, then the RPM will just reduce a small amount and remain in this condition as long as the carburetor heat is applied.

    A plug is not likely to fowl in flight at normal power settings, but may easily fowl at low power settings during the taxi and runup or in flight while practicing low power maneuvers such as stalls or glides. On the ground, a fouled plug will show up in the magneto check with a rough test and excessive RPM drop. If you have a fouled plug, you will want to lean aggressively for a time period to see if you can clear the condition, then richen and repeat the magneto check. I would suggest that you maintain a high between 1700 and 2000 on most trainers while you bring the mixture back about an inch or so. Run this way for about a minute, then reduce your power, richen the mixture, and re check the magnetos. On some of the low power engines not really designed to work with 100LL, they often experience lead fowling at low power settings. To prevent this, after startup and the engine is running smoothly, pull the mixture knob out about an inch and leave it that way during taxi. Before you do your runup, push the mixture back in. It is best to be very aggressive when you pull the mixture out while on the ground, by which I mean don’t pull it out only a small amount. The reason is that even with the mixture pulled well out, you might not notice it on a runup if you forget to go back to full rich. Then when you go to takeoff power, the engine will definitely let you know you forgot to go back to full rich as it will protest mightely. On the other hand if you only pulled the mixture out a small amount, you might not even notice it on takeoff and it could overheat your engine or worse.

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  3. Sam Dawson on Nov 10, 2013

    One other thing to add. If your aircraft has an engine analysis page on an MFD or an engine analyzer you should use it during the mag check or if the engine begins to run rough. You will get much more information on engine health than by just using RPM.

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  4. John D Collins on Nov 10, 2013

    I would be very careful doing an in flight magneto check. If one of the magnetos has failed completely, then when you switch to test it, there will be an abundance of silence. The urge will be to quickly return the engine to both or where it was last running. In the mean time the cylinders can be filled with much unburned fuel and a large backfire can occur. It is best to be disciplined when you are doing this and be prepared for the engine to quit and resist the urge to immediately switch back. It is best to first retard the throttle to a lower power setting and only then switch the magnetos back to the working position.

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  5. Brian on Nov 10, 2013

    “I would suggest that you maintain a high between 1700 and 2000 on most trainers while you bring the mixture back about an inch or so. Run this way for about a minute”

    Just a small nit pick since I started doing maintenance and learning quite a bit more about tolerances. This is universal for just about any piston aircraft: set power to 2000 rpm, lean aggressively until the engine loses 100 rpm or more and keep it there for 20 seconds; absolutely no more than 30 seconds with this procedure. An aggressive lean at high power with no airflow will very quickly result in targeted hot spots that are not good for the engine. Not leaning aggressively or using lower power settings will not be as effective in clearing the plug in as short a time. Meaning you take longer and run the risk of hot spots again.

    A fouled plug can happen in the air, especially if the plug fails; though the condition is very uncommon. However, you won’t be able to recognize it without individual cht/egt guages for each cylinder. If you’re cruising along and a plug were to fail it would be notable by an increase in temp on that cylinder.

    A total mag failure is easily noticed in flight, though not because of roughness. Sound is a very poor way of identifying engine problems for the untrained ear. In this case even a trained ear may not pick up on the 50-75 rpm drop of the failed mag. However, their will be a notable rise in egt, 50 degrees or so, if a mag fails. A single egt gauge will suffice for noticing this problem.

    Anything that causes enough of a problem to make a notable audible difference in the engine is cause for worry if carb ice is not the issue. Bad fuel, the wrong type of fuel, failing cylinders, stuck valves, and a whole mess of other possibly failing internals could be the cause. If carb heat doesn’t fix it land as soon as you can and figure out what the problem was while safely on the ground. If it was just water, no big deal. However, there is no way to know this for sure in the air, and waiting to see if it happens again may very well destroy an engine that could have been saved if the problem were something other than a little water.

    —

    An interesting icing story I came across in college was told to me by a professor who knew a pilot that got induction ice in a fuel injected airplane. She was flying a twin in hard IMC at night, it was certified known ice. About halfway through the flight, and over water heading to one of the islands off the east coast the right engine started to sputter and shortly after failed. About 10 minutes later the left engine began having the same symptoms. We can imagine the pucker factor here, over the ocean, on a cool evening in the winter months, and about to become a twin glider.

    Well thankfully the heat from the secured right engine conveniently melted the induction ice and the pilot successfully restarted the right engine shortly before the left engine shut down. Roughly 15-20 minutes later the process repeated, the now again working right engine started running rough and before it failed the pilot successfully got the left engine going again. This process occurred almost every 15 minutes for the remaining 1.5-2 hours of the flight.

    After landing OEI and later speaking to some of the mechanics it was determined that the most likely cause was induction ice that melted when the warm, shut off engine, stopped pulling air through it’s induction system and allowed the heat to melt the ice.

    Moral of the story; just because you’re in a fuel injected airplane doesn’t mean you’re not susceptible to induction ice. The only difference is you don’t have a way to cure it if it happens, which thankfully is very rare with todays newer designs.

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  6. Lion on Nov 11, 2013

    Very helpful feed-backs. John’s answer is a 5 start answer.

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  7. Sam Dawson on Nov 11, 2013

    If you have a magneto whose timing has gone out it can cause a rough running engine and very serious damage to the engine if it is not isolated very quickly as the CHTs shoot through the roof- thus the recommendation to check your magneto.
    For an article by Mike Busch about this:
    http://www.sportaviationonline.org/sportaviation/may_2013?pg=34#pg30

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