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Scenario: While demonstrating a simulated engine failure emergency landing a flight instructor puts on carburettor heat and reduces the throttle to idle of a small single engine Cessna. After a couple of minutes descending to the simulated emergency landing site, the CFI adds throttle "to clear the plugs" for a few seconds, then brings it back down to idle to complete the demonstration.

Is this is a legitimate thing that needs to be done for safety?

I've seen several pilots do this, and to me it always seems like cheating, because they are coming in a little short of their landing target.

  1. How much plug fouling can occur in 2 or 3 minutes of idle with carburettor heat at low altitude?
  2. How much good can 3 to 4 seconds of full power do?
  3. Is there something else about this that I'm missing? (I couldn't find anything on this after a brief Google search).
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    $\begingroup$ My flight instructor made it no secret that I could stretch the engine warm a few seconds longer if I was going to be short. Of course, how prepared I am for a real emergency is another question. $\endgroup$
    – canadianer
    Commented Nov 24, 2017 at 4:11

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straight out of the Airplane Flying Handbook, pp 9-4

Operating the engine at idle speed for any prolonged period during the glide may result in excessive engine cooling, spark plug fouling, or carburetor ice. To assist in avoiding these issues, the throttle should be periodically advanced to normal cruise power and sustained for a few seconds.

This procedure is common during emergency descents or steep spirals to prevent the engine from cooling off too much. If the engine mixture is excessively rich, spark plug fouling can occur. Fouling can be cleaned off the plugs by running the engine at Cruise power settings with a properly leaned mixture in about 10 or so seconds.

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  • $\begingroup$ Excellent. Thank you for the reference. I wonder if there is any authority on what the Flying Handbook means by "prolonged periods." $\endgroup$
    – Devil07
    Commented Nov 22, 2017 at 20:47
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    $\begingroup$ @Devil07 FWIW, I've personally experienced plug fouling when taxiing for a couple of minutes with a rich mix. It doesn't take long. After running it up to 2k RPMs for ~30 seconds and testing again, all was fine. $\endgroup$
    – reirab
    Commented Nov 23, 2017 at 5:09
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    $\begingroup$ Also, if the engine actually fails for some reason, you may not note that due to propeller windmilling. In that case, the simulated emergency turns in a real one, and the actions taken by the instructor/pilot may change. Giving momentary power to the engine ensures that it will be able to climb out in a go-around. $\endgroup$
    – ricardomenzer
    Commented Nov 23, 2017 at 15:46
  • $\begingroup$ @ricardomenzer I think what you said (confirming power availability) is probably the most persuasive reasons for doing this. I've talked to mechanics and "shock cooling" is a real thing, but unlikely to be encountered in actual idle gliding descent, carb ice isn't really an issue if you have carb heat on or its fuel injected, and plug fouling isn't an issue if you're leaned properly (plus mixture just continues to lean as you descend). $\endgroup$
    – Devil07
    Commented Nov 1, 2020 at 16:37
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This practice is often referred to as "clearing the engine" and serves two purposes. The temporary change in power has a positive effect on clearing potential fouling of spark plugs, which are susceptible to lead-fouling, carbon-fouling and oil-fouling.

The second effect is to provide feedback to the pilot on the status of the power plant. Should the engine not respond favorably to a clearing power change, then the pilot is immediately made aware of that, and can either take corrective measures, such as increasing the power and leaning the engine to burn off or dislodge deposits. Alternatively, if low to the ground on approach, the pilot is aware that there is a potential power plant issue and can plan accordingly should there be a need to alter his path.

This maneuver is best practice in a light piston powered aircraft during certain training maneuvers, such as simulated engine out procedures.

Furthermore, while notions of best practice vary, there is general consensus that extended low or idle power glides of one minute or longer should have an clearing power change. This power change may only last a second, and if so executed, has a negligible impact on the glide path of the aircraft.

Because aircraft such as helicopters, tend to run at high power settings in all phases of flight, this maneuver does not apply except in situations such as extended auto-rotation practices. Also, this maneuver has little benefit to turbine powered aircraft, and is not indicated for extended glide in aircraft so equipped.

So bottom line, clearing the engine has a preventative effect in that it provides a moment of power to alter power plant conditions which could lead to unreliable operation. Secondarily, if there is a degradation in the performance of the power plant, it provides the pilot notice of that, so that the pilot can situationally evaluate options and act accordingly.

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  • $\begingroup$ I think you're a word in "simulated emergency out procedures", but I'm not sure if "emergency" should be replaced by "engine" or if "emergency out" should be "emergency engine-out". $\endgroup$
    – user
    Commented Nov 23, 2017 at 14:34
  • $\begingroup$ @MichaelKjörling, good spot, and I have edited accordingly. Thank you. $\endgroup$
    – mongo
    Commented Nov 23, 2017 at 19:26

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