This question is for Piper Tomahawk, but I guess applies to any small aircraft.

Why do we need to use the fuel mixture to starve the engine before turning it off with the key? Why can't we just turn it off with the key like in a car?

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    $\begingroup$ In 15 yrs of flying I'd never even given this much thought. Good question. $\endgroup$
    – Jamiec
    Nov 20, 2018 at 11:48
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    $\begingroup$ Car experience only, I was always told this was to leave the cylinders dry for a easier start next time. $\endgroup$
    – KalleMP
    Nov 20, 2018 at 16:38
  • $\begingroup$ Just a remark: The Aquila A 210 I fly as my trainer for PPL does not have a mixture control. We actually just turn the key as you would in a car. However my flight school added to the checklist that you first turn the ignition to the right magneto only, to slow down the engine as much as possible before shutting it down entirely (at least that was what I have been told). $\endgroup$ Nov 21, 2018 at 23:36
  • $\begingroup$ @geisterfurz007 The magnetos are a redundancy, the engine should still run on either, which is why the pre-flight check includes removing each in turn. A lean mix will prevent unconsumed fuel in the system, and allow spark retardation to also prevent knock-back and dieseling. $\endgroup$
    – mckenzm
    Nov 22, 2018 at 2:34
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    $\begingroup$ Any recent car does that too, only without you noticing. $\endgroup$ Nov 23, 2018 at 8:34

5 Answers 5


The reason is the large spinning thing on the front. Residual fuel in the engine has been known to auto-ignite (i.e. combust without a spark), causing the prop to spin, causing serious injuries and deaths. A lean cutoff reduces the risk that someone handling the prop will get maimed or killed.

In a car when you turn the engine off usually it is in park or neutral, so if the engine turns a few revs the car goes nowhere, even if it is in gear the car may lurch but that's generally it. Modern car engines have electronic fuel injection and electric fuel pumps, some modern push-button gasoline engines are stopped by performing a lean cut-off, turning off the injectors before the plugs, most work by just stopping ignition. Diesels don't have spark plugs, they rely on compression alone, so cutting off fuel is the only way to stop them.

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    $\begingroup$ In cars, this is usually called dieseling $\endgroup$
    – costrom
    Nov 20, 2018 at 12:46
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    $\begingroup$ @AEhere Well, explosion and detonation are different - detonation is a supersonic explosion. You can have an explosion which isn't a detonation. With that said, I take the rest of it back. I guess it does produce a true detonation (ie: no flame front burning progressively from an ignition point, but near spontaneous (supersonic) reaction of the entire fuel/air volume). TIL. $\endgroup$
    – J...
    Nov 20, 2018 at 14:48
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    $\begingroup$ Surely there is manufacturer info related to this that can be added as a citation? I think the largest reason when comparing to cars is that small airplane engines have ancient technology by comparison. There is no fuel mixture control in electronically controlled engines. Most of the cars since 70s or 80s have had what could be essentially regarded as “FADEC” in the airplane world. $\endgroup$
    – rkantos
    Nov 20, 2018 at 18:26
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    $\begingroup$ @MasonWheeler Most piston airplanes don't have a transmission at all -- the engine connects directly to the propeller. Neutral in a car basically tells the transmission to not connect the engine to the wheels. Without a transmission, there can't be a neutral. $\endgroup$
    – yshavit
    Nov 23, 2018 at 20:32
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    $\begingroup$ A clutch is weight and expense you don't need. What problem does it solve that isn't already fixed by using the correct shutdown procedure? $\endgroup$
    – GdD
    Nov 24, 2018 at 9:24

Combustion in a gasoline internal combustion engine for most aircraft, requires four things: fuel, oxygen, compression and ignition.

If the engine is starved of fuel, accidental combustion (and an accidental spinning prop) will not happen. So shutting off the fuel is one way to prevent accidental "start" if even for one stroke.

Oxygen is ubiquitous and is not practical to eliminate from the engine environment.

Compression can happen when someone intentionally or unintentionally moves the prop, which coupled to the crankshaft may cause a cylinder to go through a compression stroke. Since frequently props are moved to place cowl plugs, attach tow bars, etc. the risk of a partially rotating prop is non-zero.

Ignition happens in many forms. Hot carbon deposits, hot spark plugs, nicks in pistons are all sources of continued ignition when trying to shut down an engine. Removing fuel eliminates these sources of ignition from causing continued rotation of the engine.

The most significant source of ignition in most aircraft gasoline engines is the magnetos which are used to energize the spark plugs. Magnetos are effective at creating energy even at low rotational velocities. Many aircraft engines also have "impulse" magnetos, which are spring loaded, and trigger on a very low rotational velocity. They have the advantage of being effective starting aids. Furthermore, magnetos, while switchable, are normally "grounded" to disable them. There is significant history that broken or intermittent magneto switches, broken or intermittent wires and other causes are responsible for unintended engine starts or undesired engine firings.

To relate a story which happened at a local airport several years ago. A ferry pilot brought a plane to town at night, tied it down at the rural airport, and was to meet the prospective buyer the next morning. The buyer arrived at the strip early the next morning, and while waiting for the ferry pilot, poked around the locked plane. Eventually something possessed him to rotate the prop through manually. Normally that would not have been a problem, except the magneto switch in the aircraft was "open" in the off position, which meant that both magnetos were hot. This was likely not noticed because not everyone does a magneto "ground" check in the off position during their run-up process. However, if the mixture idle-cutoff were in the cutoff position, there would not be fuel available in the carburetor to ignite, except for some reason (such as the ferry pilot extracting his RON bag in the dark) the mixture was nearly full rich. Normally this might have resulted in the engine firing, or even starting and idling. Again, this morning, things were not normal. The RON bag, extracted in darkness by the ferry pilot not only had bumped the mixture control, but also the throttle, so the aircraft engine faithfully roared to full power. Fortunately the startled prospective buyer did not get hit by the spinning prop. However, again, this was an unusual morning. As the engine roared to life, the tiedown on the right wing broke, and the plane pivoted on the left tiedown and swung approximately 180 degrees until the prop was buried into the fuel tank of an adjacent Cessna. Av gas was all over the place. One thing went right that morning, in that the avgas did not ignite and there was no fireball. Unfortunately, the day was not over, yet. After missing the spinning prop and not getting butchered by the surprise of his life, and then dodging a partially freed airplane, the buyer watched in horror as 40 gallons of avgas was sprayed over a running engine, as it came to a somewhat abrupt stop, the prop buried in someone else's wing. At that point he started running up the hill to the FBO to get help, and collapsed to the ground and died of a heart attack.

True story, and this is one that my students, private, commercial and certainly CFI, have all heard. If it helps reinforce engine safety, and understanding shutdown and operational procedures, it is worth the time telling it.

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    $\begingroup$ You could use this as tangential reference: en.wikipedia.org/wiki/Dieseling Engines sometimes keep running even with key off. $\endgroup$
    – Agent_L
    Nov 21, 2018 at 9:24
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    $\begingroup$ A remarkable story - does anyone have a source (or a debunking)? $\endgroup$ Nov 21, 2018 at 10:28
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    $\begingroup$ @Joe Lee-Moyet, it happened at a local rural field. I was part owner of the plane which was hit, and had a wing replacement. I knew the potential buyer who had a nearby grass strip at his house, and owned several aircraft. There was no NTSB report. There was a FAA investigation, and a state police investigation, both brief. Probably the only tangible "proof" I might have is the logs for the plane which was hit, and the insurance settlement, which was multiple layers of complexity due to the many actors: the potential buyer, the airport owner, the broker, the owner, the ferry pilot, etc. $\endgroup$
    – mongo
    Nov 21, 2018 at 14:26
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    $\begingroup$ To my students the story tended to hit home because the plane which started up was also repaired, and was bought by a local. So the planes were local, and there were about 12 people who were at the airport witnessing it when it happened. $\endgroup$
    – mongo
    Nov 21, 2018 at 14:26
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    $\begingroup$ This story demonstrates a couple of threads common to a great many human-made disasters: (1) multiple redundant safety systems all had to be inoperable at the same time, and (2) bad practices that seem harmless in isolation interact in odd ways in combination. I see this sort of thing all the time in both software defect analysis and in diagnosing previous-owner "homeowner wiring" in my friend's old houses. $\endgroup$ Nov 21, 2018 at 19:41

It's done for two reasons:

  1. Much better for the piston rings and cylinder to not have raw fuel left in the cylinder to wash away or dilute the oil on the cylinder walls after shut down.
  2. Carbon or lead buildup in the cylinder can retain enough heat to cook off the fuel mixture and the engine will continue to go kadunk kadunk kadunk.

Engines like the Continental A-65 used in Cubs and Champs in the 40s and 50s didn't have mixture controls or idle cutoffs (full rich all the time) and were shutoff by turning off the mags. If a cylinder had a lot of carbon/lead deposits, it would keep running intermittently kadunk kadunk kadunk.

When you are flying floats there ARE times when you shut down with the ignition, when approaching the dock and you need the engine to stop NOW and can't wait for the idle cutoff to work. Normally you time it with the idle cutoff so the engine has stopped before you have to climb out to step off, but sometimes you can't wait.

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    $\begingroup$ Unfortunately the whole 'clearing carbon / lead deposits' is an excuse many motorcyclists give around my area for needlessly reving their engines at unduly hours as well :/ $\endgroup$
    – Cloud
    Nov 20, 2018 at 13:53
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    $\begingroup$ Yeah I used to do it with my KZ650 although it was completely unnecessary. Old habit from the long ago days when a motorcycle engine would get fouled spark plugs after idling for any length of time. $\endgroup$
    – John K
    Nov 20, 2018 at 13:56
  • $\begingroup$ "kadunk kadunk kadunk" see this comment $\endgroup$
    – FreeMan
    Nov 20, 2018 at 15:23
  • $\begingroup$ @FreeMan did you mean to link to a specific comment? Youve just linked back to this question. $\endgroup$
    – Jamiec
    Nov 20, 2018 at 17:56
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    $\begingroup$ @Jamiec, yeah, actually, I did. I guess I didn't do it well. Try it again $\endgroup$
    – FreeMan
    Nov 20, 2018 at 19:32

Another reason to shut down an engine with the carb in an dry state is you do not know how long it will be before the engine gets started again. A dry carb can sit idle almost forever without suffering ill effects but a wet carb will be subject to fuel spoilage/jet plugging and corrosion from water.

This isn't an issue for a carb that is run daily or even once a week.

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    $\begingroup$ Normally the carb is not left dry, with a mixture idle cutoff. Rather the jet in the carb has a cam adjusting the area, and the cam moves to a position where the opening is obscured. Of course this is not the case in many vintage aircraft where the carb is run dry by deselecting any fuel routed to it. $\endgroup$
    – mongo
    Nov 20, 2018 at 18:38
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    $\begingroup$ good point, thanks for that. motorcycle and lawnmower carbs also lack that feature and need to be shut down dry, or their fuel must be doctored with preservative during periods of nonuse. $\endgroup$ Nov 20, 2018 at 19:38
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    $\begingroup$ The stale fuel fouling the carb issue has become more of an issue in recent years, well after the institution of mixture/idle-cutoff aircraft engine controls. Actually I think it related to the use of ethyl alcohol. $\endgroup$
    – mongo
    Nov 21, 2018 at 2:36

This is a basic question that should be covered by your ground school, flight instructor and your primary training. The vast majority of certified piston-engine airplanes have two (redundant) magnetos for ignition. The mag switch in OFF position grounds the magnetos so they don't produce a spark even when they are turning (like if you pull the prop through a 180 degrees.) The ground wires are called P-Leads. If the ignition switch or one or both P-lead should fail (open the ground circuit) the magneto(s) may be "hot", that is, they can produce a spark in the spark plug and ignite any fuel in the cylinders. This will kick the prop over with great force and possibly hit anyone/anything in the prop arc. By pulling the mixture to cutoff position, starving fuel from the engine to shut it down, you greatly reduce the risk of a surprise/unwanted engine start when the prop is next moved. You might just move the prop level to allow attaching your tow bar after flying, or someone may handle the prop for no reason at all (curious children or just idiots at the airport/airshow/fly-in) and if the engine makes a spark and fuel is available (shut-off carb "rich") there could be an injury, death, and resulting lawsuit. Teach your flying guests, especially children, to never touch the prop unless they are certain it is safe. The key may be left on, or the mixture rich, and accidental starting can occur. Your handy pilot's checklist should have "Mixture - idle cutoff" in the "Before Starting" and "Shutdown" sections. Lastly, never leave a nosewheel towbar connected when you are not moving the airplane. It is very easy to forget it before starting the engine, and ANY PROP STRIKE REQUIRES A PROP INSPECTION AND ENGINE TEARDOWN/INSPECTION. Add "Towbar Stowed" to your preflight checklist.


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