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On my first intro flight. I noticed the instructor twisting the mixture knob. I didn't think of asking, but now Im curious.

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    $\begingroup$ Apparently it is designed to be turned, so it probably wasn't just idle twiddling. $\endgroup$ Commented Oct 17, 2021 at 0:51
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    $\begingroup$ What make and model airplane? Maybe we can look it up. $\endgroup$ Commented Oct 17, 2021 at 0:52
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    $\begingroup$ John K has the correct answer below, but this is pretty common on GA planes like Cessnas $\endgroup$
    – Cullub
    Commented Oct 19, 2021 at 18:18
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    $\begingroup$ Ever see a vise with one of those levers that you hold and then you can quickly pull the vise open a large way in a short time? $\endgroup$
    – Caius Jard
    Commented Oct 19, 2021 at 19:20

2 Answers 2

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It's called a Vernier Control. It has an internal screw that is part of the fixed housing, with threaded jaws that engage the threads around the outside and are part of the knob.

When you turn the knob, you are turning the jaws with internal threads like a nut, and the whole thing moves in when you turn clockwise (richer) and out when counterclockwise (leaner), moving the teleflex cable running to the carb or fuel injection controller with it. When you push the center button, it opens the jaws, disengaging the threads and now the knob can be pushed straight in and out like the throttle.

It makes it easy to make very fine adjustments by turning the knob, but when you need to make large changes, you don't have to furiously spin the knob, you just push the center button in with your thumb and you can pull the whole thing in or out. In an emergency when you need to go "balls to the wall", just shoving the knob forward with the heel of your hand pushes the button in automatically and lets you move it in one motion.

You usually see mixture and propeller controls done this way, because most of the time you are making fine adjustments until those times you need to shove them all the way in or all the way out. You'll sometimes see Vernier throttles as well, but it's less common.

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Probably because the actuation mechanism contains a screw drive which allows fine adjustment of the mixture- either that, or the screw drive locks the pull knob firmly in position so it will not vibrate out of the desired position.

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    $\begingroup$ the screw drive locks the pull knob ... so it will not vibrate out of the desired position - This was my first guess too, but why do this for the mixture and not for the throttle? $\endgroup$ Commented Oct 17, 2021 at 17:19
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    $\begingroup$ @A.I.Breveleri IIRC the throttle has a similar mechanism, see the reference to "Throttle Friction Lock" page 4-7 on longislandaviators.com/wp-content/uploads/2018/08/… $\endgroup$ Commented Oct 17, 2021 at 17:54
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    $\begingroup$ @A.I.Breveleri the mixture knob is locked in place by the screw drive, but can be released for quick adjusment by pressing the center button on the knob, which releases the half nut holding on to the threads. The throttle, on the other hand, has a friction lock, which can be adjusted to find a suitable "stickyness". In the picture, the friction is adjusted by the corrugated aluminium finger screw with "do not lubricate" stanced on it. Lube would obviously spoil the system, so that vibration would make the throttle settin creep. $\endgroup$
    – Jpe61
    Commented Oct 17, 2021 at 18:25

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