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I've experienced spark plug fouling before but it was during the pre-flight checks and my instructor fixed it for me (by using max RPM and leaning the mixture a couple of times). But now that I have to fly all by myself, I'd rather know what to do and what not to do, especially during flight. What happens if the plugs become fouled in the air, and how do I clean them in flight?

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  • $\begingroup$ When I was first starting to fly, I often had fouled plugs. After a few expensive plug cleanings, I learned about aggressive leaning. When on the ground lean very aggressively—so much that if you give the plane full power that it will stall. I also do my mag check leaned out. When I am ready to make my call to the tower (or CTAF) I put my hand on the mixture and don’t move it until I get cleared for takeoff. $\endgroup$ – JScarry Nov 21 '16 at 20:56
  • $\begingroup$ The only time I have had fouled plugs in the 15 years since then is one time when I had a slow 2 hour flight in a Cherokee and did not lean enough. I did not notice any problems in the air, but when I did my mag check before shutting down, I noticed that the plugs were fouled. $\endgroup$ – JScarry Nov 21 '16 at 20:56
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You should read up here but generally speaking plugs are less likely to foul in flight (and is why its not talked about much). Plugs often foul due to the mix of low engine temps and high led content during warm up at full rich settings. This is why a leaned out warm up is advised for some planes, you should talk with your instructor about the plane you are flying and what the proper warm up procedures are.

Please consult your POH for any airframe specific emergency checklists that may apply to your aircraft. Generally speaking if you depart with a fouled plug and it is causing a rough running issue you should cycle the magnetos untill you are running on the good set of plugs and get the plane down safely. This is most likely the result of a poor or non-existant run up check. Unless you are sitting in line for a long time after your runup, you should generally be doing runup close to departure time.

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The best answer here is don't use an excessively rich mixture. That will avoid a lot of fouling problems on the plugs. Leaning during taxiing also helps. During flight betweeen SL and approx 4000 FT ASL, at full throttle lean the engine until the fuel flow indicator is at the top of the green arc. Above that level, do not operate the engine richer than 70 deg Rich Of Peak.

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  • $\begingroup$ This isn't good advice unless you have a fuel flow meter and EGT gauge. Leaning without them can be problematic so a lot of schools don't teach it, but it's an important skill to learn. $\endgroup$ – GdD Nov 21 '16 at 21:46
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    $\begingroup$ You can actually lean an engine pretty well by sound and Bu feel. In addition most modern light aircraft are equipped with an EGT gauge. $\endgroup$ – Carlo Felicione Nov 21 '16 at 22:04
  • $\begingroup$ @CarloFelicione is right. In fact, some Cessna manuals for Lycoming O-320s call for leaning the engine till it gets rough and then enriching the mixture till the roughness goes away. Fancy instrumentation is not necessarily required to run an engine well. $\endgroup$ – J Walters Nov 22 '16 at 2:29
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    $\begingroup$ I agree that leaning is a good idea, and that it can be done by listening for an rpm drop. This is how I do it because I rarely ever see an airplane with a working EGT. $\endgroup$ – GdD Nov 22 '16 at 8:53

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