Mixture Control

 

If you fly an aircraft with a piston engine, 99% of the time youíre going to have a manual mixture control near the throttle.There are some very old airplanes and some very new airplanes that donít have mixture controls, but thatís pretty rare.

 

Most people donít think about the mixture control very much, and use it as a binary control Ė either forward (full rich) for start & run, and then all the way back (full lean) when you want to stop the engine at the end of the flight.You can operate a piston aircraft engine like that, but if you do, youíre making a lot of mistakes.

 

Immediately after starting the engine, you want to lean the mixture for maximum RPM for the entire time you are on the ground, until you are ready to do the run-up.You wonít hurt the engine by doing this Ė the temperatures are simply too low.All that can happen is that the engine quits, if you pull it back too far.Every airplane is going to be slightly different, but you will quickly learn if it wants half an inch, or an inch, etc.Remember and immediately put the mixture in that position after every start.

 

The reason that you run the mixture lean on the ground is to try to reduce the fouling of the spark plugs Ė especially the bottom ones.Lycoming engines Ė both carbureted and fuel-injected have a huge problem with this.Continental engines can be a bit more tolerant in this regard, but a lean mixture on the ground doesnít hurt them, either.

 

Time to do your run-up.Mixture rich as usual, but after the run-up, back to the lean mixture idle position.And, if you have to wait for a while before takeoff, be sure to set the RPM at 1100 or 1200.That with the lean mixture setting will really help your spark plugs from fouling.

 

Immediately before takeoff, select full rich mixture for takeoffs in low density altitude conditions.For takeoffs in high density altitude conditions, you must lean the mixture Ė see the aircraft P.O.H. or A.F.M. for information on how to do this.

 

If you are climbing to a high altitude for cruise, at some point in the climb you will want to lean the mixture, even with wide open throttle.The mixture becomes ridiculously rich as the air thins in the climb.Again, see your P.O.H. / A.F.M.

 

Regardless of what altitude you level off at, after you establish your cruise airspeed and power setting, be sure to always lean the mixture.Lots of people donít bother with this, especially at lower altitudes, but even if you donít care about the cost of gasoline, leaning the mixture will give you more fuel when you land, and someday, that extra gas might make a crucial difference on a long leg into wind.

 

Setting the proper mixture for cruise sounds simple but you simply wouldnít believe the arguments that come up about it.What I do Ė regardless of whether I am flying a carbureted or fuel-injected engine, or fixed pitch or constant-speed prop Ė is lean for maximum airspeed, which by definition is best power, which is 50F to 100F rich of peak.

Thatís not perfect, but itís a really big improvement over not bothering to lean at all.If you have an EGT, you can use it.With a fuel-injected engine and very good injectors, you can consider running ďlean of peakĒ which is outside the scope of this memo.It doesnít work very well with most carbureted engines which donít distribute the fuel-air mixture very evenly.

 

Letís talk about carburetor heat now.If you pull the carb heat knob all the way out, you will enrichen the mixture because you are now putting hot (less dense) air into the carburetor.That will cool the engine, and create less heat at the exhaust, and less carb heat.If you really want heat at the carburetor, lean the mixture some more.As you may have noticed, thatís a pet peeve of mine Ė no one bothers to lean the mixture when they apply carb heat, which means that they donít get as much carb heat as they could have Ė and they also donít get as much power.

 

Time to descend from cruise, for an approach and landing.Most people shove the mixture all the way in, and thatís not good.Especially with the reduced power, the engine is going to cool off in the descent, and the full rich mixture cools it even more.Thatís not what you want.Lean the mixture in the descent.Because of the low power setting and low temperatures, itís pretty hard to hurt the engine doing this.And a lean mixture keeps the engine warmer, and reduces spark plug and valve fouling.

 

One of the engines that I fly Ė the TCM GSTIO-520 Ė is famous for cracking cylinder heads.Everyone erroneously attributes this to shock cooling caused by the inlet air.TCM says this isnít so Ė what cracks the cylinder heads is the cold fuel squirted into engine during full-rich descents.Shock cooling is complete nonsense, by the way, but that again is outside the scope of this memo.Sometime ask the ďinch of MP a minuteĒ advocates if they take off with that procedure.Anyways, not cracking the cylinder heads is a very good reason to descend with a lean mixture.

 

When you are doing your downwind or pre-landing checks, sure, go full rich mixture.Itís not optimal, but it doesnít do that much harm, and if you have to overshoot, youíre ready to go.

 

After landing, guess what?Lean the mixture for taxiing.I do it on the rollout.Again, avoid the plugs from fouling.

 

Anyways, I hope this note helps you understand a bit more about how to operate a piston aircraft engine a bit better!

 

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acboyd@gmail.com

March 2013