Questions about mixture leaning

[chemgeek]

It is indeed amazing that many primary instructors provide little or no information about leaning to prospective pilots. I've known more than one owner who has never touched the red knob, for fear something terrible will happen to the engine. Bottom line (assuming no fancy engine monitor):

• You cannot possibly hurt your engine by aggressively leaning for ground ops. And you will do a good job of preventing lead fouling this way. As a bonus, you will know if you skipped the full rich checklist item when you try to take off, because the engine will barf when you shove the throttle in if you forget.
• Anytime you are below 75% power, feel free to lean. (Straight from Lycoming, their definitions for two procedure choices below.)
• "Best economy" is lean until the engine stumbles then enrichen until the engine smooths out. This will typically be just lean of peak, and is a pretty safe setting, and what you would typically set for cruise for best fuel consumption.
• "Best power" is lean to maximum rpm. This is typically rich of peak, and is what you would probably choose for climb when below 75% power, or for high density altitude ops.

The Lycoming engine operation manual is actually quite helpful about when you should lean the engine for various ops.

 

[Will Kumley]

If only there was a checklist to make sure they don’t do this...

Problem is, CFI's are people and people are different. Case in point. Some CFI's at my school would almost preach to lean to a stumble, then enrich to smooth during taxi. Other instructors got real nervous anytime I touched the red knob while sitting at idle at the FBO. The Cherokee I flew would allow me to pull the knob a fair amount before stumbling, and one instructor in particular would never let me get the dang thing pulled out more than an inch. If I followed the other guidance I could easily pull it out 2-3 inches before the stumble occurred. The wary instructor also hated it when I would lean for cruise. I think he thought I was going to stall the engine mid flight. Sure its possible, but I wasn't yanking the thing out at mach 5. Its always a slow/gradual pull to get where I want. I've also noticed many of their checklists no longer have "lean for taxi" which is why I generally create my own.

 

[flyingron]

That’s why on some of our checklists it says “as req’d.” For example engine anti ice. Under certain conditions it will be on and other times it will be off. Some items are not always required.

Do you run the MD88 lean of peak?

 

[flyingron]

You cannot possibly hurt your engine by aggressively leaning for ground ops. And you will do a good job of preventing lead fouling this way. As a bonus, you will know if you skipped the full rich checklist item when you try to take off, because the engine will barf when you shove the throttle in if you forget

And at idle, you're not changing the mixture at all unless it's leaned that much. Pulling the knob out an inch or at idle doesn't do anything.

Anytime you are below 75% power, feel free to lean. (Straight from Lycoming, their definitions for two procedure choices below.)

Continental pretty much says the same thing: at max recommended cruise, you can run from anywhere from full rich to so lean the engine is misfiring without worry.
In fact, Lycoming's 'red zone' is on the rich side of peak

"Best economy" is lean until the engine stumbles then enrichen until the engine smooths out. This will typically be just lean of peak, and is a pretty safe setting, and what you would typically set for cruise for best fuel consumption.

That's a stab in the dark without an engine monitor. Best Economy is at peak.

 

[crash7]

The wary instructor also hated it when I would lean for cruise. I think he thought I was going to stall the engine mid flight. Sure its possible, but I wasn't yanking the thing out at mach 5.

Not long ago I had a gal ask me what would happen if we pulled the mixture back too far when leaning after leveling off for cruise. I said, “this...” and proceeded to quickly pull it back to idle-cutoff. In less than 1/4 second, she smacked my hand away and pushed it back in. Some learning occurred there.


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[Sinistar]

I know in our plane if you don't lean for all ground ops you'll be fouling plugs and wondering why fuel burn for 1hr is 14gph instead of 10.5gph.

 

[bobmrg]

Not long ago I had a gal ask me what would happen if we pulled the mixture back too far when leaning after leveling off for cruise. I said, “this...” and proceeded to quickly pull it back to idle-cutoff. In less than 1/4 second, she smacked my hand away and pushed it back in. Some learning occurred there.


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I used to make it a practice to kill the engine in cruise with the mixture to show the student that it was a non-event. I also shut off the master switch, to prove to non-believers that doing so would not, in fact, kill the engine. It always surprised me to learn how many students are afraid to touch something for fear that the engine will quit.

Bob Gardner

 

[Martin Pauly]

I used to make it a practice to kill the engine in cruise with the mixture to show the student that it was a non-event. I also shut off the master switch, to prove to non-believers that doing so would not, in fact, kill the engine. It always surprised me to learn how many students are afraid to touch something for fear that the engine will quit.

That is such an important thing to see for students, and should really be a confidence builder - instead, some people think of it as a scary thing.

So many pilots believe the engine will stop if you pull the mixture to cut-off in flight, when in fact the engine and prop will continue to turn just fine, driving all the accessories, and even the sound/noise doesn't change all that much - at least with my constant speed prop. Meanwhile, the airspeed decreases (or the airplane descends), the EGTs are down to nothing, and the CHTs are cooling down - but the aircraft keeps flying just fine.

- Martin

 

[bobmrg]

That is such an important thing to see for students, and should really be a confidence builder - instead, some people think of it as a scary thing.

So many pilots believe the engine will stop if you pull the mixture to cut-off in flight, when in fact the engine and prop will continue to turn just fine, driving all the accessories, and even the sound/noise doesn't change all that much - at least with my constant speed prop. Meanwhile, the airspeed decreases (or the airplane descends), the EGTs are down to nothing, and the CHTs are cooling down - but the aircraft keeps flying just fine.

- Martin

For more fun, ask the student to stop the prop. They will be amazed at how difficult it is, and amazed again at the pitch attitude and airspeed required for it to windmill on its own for an air start without using the starter.

Bob

 

[William Pete Hodges]

- Martin[/QUOTE]

Your instructor may have other virtues, but it seems mixture management isn't one of his strengths.

A very good read, if you'd like to understand this topic better is John Deakin's "Pelican's Perch" articles on AvWeb: https://www.avweb.com/features/avweb-classics/pelicans-perch/pelicans-perch-index/
Maybe start with #18 from the series, and then see what interests you from There.

I just finished reading #18 on fuel mixture. I enjoyed reading it very much. It is complex but all of the conclusions make sense. Thank you for sharing the that

 

[LongRoadBob]

I just finished reading #18 on fuel mixture. I enjoyed reading it very much. It is complex but all of the conclusions make sense. Thank you for sharing the that

yeah, from me too.
I haven’t weighed in after posting, but only because lots of good advice here, and reading material, and solid answers, so no need to follow up more. As always, thanks guys!

 

[PaulR035]

Not to hijack the thread, but I've been trying to learn a lot about leaning since I recently bought my own plane and it's getting a new engine I'd like to keep in good shape. (My CFI was also in the "don't touch the red knob" camp.) Does anyone have any specific advice for using an engine monitor to lean? Is it really as simple as, "Keep the CHTs at 400*F or below?"

 

[denverpilot]

Not to hijack the thread, but I've been trying to learn a lot about leaning since I recently bought my own plane and it's getting a new engine I'd like to keep in good shape. (My CFI was also in the "don't touch the red knob" camp.) Does anyone have any specific advice for using an engine monitor to lean? Is it really as simple as, "Keep the CHTs at 400*F or below?"

Most folks point to the old Deacon articles and of course Mike Bush’s stuff originally published ages ago on AvWeb. Seen those?

 

[Martin Pauly]

Is it really as simple as, "Keep the CHTs at 400*F or below?"

Well, yes and no. You DO want to keep the CHTs under 400 degrees F, but the engine monitor only tells you what the CHTs are, not what you can do to make them lower or higher.

Start with John Deakin's articles (link in a post above). My LOP video on YouTube may also give you some helpful ideas, but John covers this topic a lot more broadly.

Mixture management is one of those topics that can be daunting and intimidating, until you understand it and then it becomes super simple.

- Martin

 

[WannFly]

Not to hijack the thread, but I've been trying to learn a lot about leaning since I recently bought my own plane and it's getting a new engine I'd like to keep in good shape. (My CFI was also in the "don't touch the red knob" camp.) Does anyone have any specific advice for using an engine monitor to lean? Is it really as simple as, "Keep the CHTs at 400*F or below?"

most engine monitors have their procedures to document how to Lean using their instruments
if i am tooling around for no reason, i sometimes fly at 50% power, i keep my CHT under 350 and lean until stumble/ rich to make smooth

going somewhere i usually fly at 65% power in cruise, again, from what i have read, you cant damage the engine with that power, so keep CHT happy

going at 70% or more power, i lean using JPI LeanFind and do 100-125 ROP with carb-ed engine. same on a hot day before take off with DA close to 3000

 

[LongRoadBob]

Not to hijack the thread, but I've been trying to learn a lot about leaning since I recently bought my own plane and it's getting a new engine I'd like to keep in good shape. (My CFI was also in the "don't touch the red knob" camp.) Does anyone have any specific advice for using an engine monitor to lean? Is it really as simple as, "Keep the CHTs at 400*F or below?"

hijack away, it’s all of interest!

 

[flyingron]

Not to hijack the thread, but I've been trying to learn a lot about leaning since I recently bought my own plane and it's getting a new engine I'd like to keep in good shape. (My CFI was also in the "don't touch the red knob" camp.) Does anyone have any specific advice for using an engine monitor to lean? Is it really as simple as, "Keep the CHTs at 400*F or below?"

Read the instructions on your engine monitor to lean the engine. If you're at the recommended cruise power settings (and there's nothing wrong with your baffling) you will not have much to worry about either from CHTs and certainly not from detonation.

Running rich all the time is NOT good for the engine.

 

[PaulS]

I used to make it a practice to kill the engine in cruise with the mixture to show the student that it was a non-event. I also shut off the master switch, to prove to non-believers that doing so would not, in fact, kill the engine. It always surprised me to learn how many students are afraid to touch something for fear that the engine will quit.

Bob Gardner

This is good thing to do for students. I flew a basic trainer where the lean procedure was to lean until the engine stumbles, the enrich until it smooths out, no egt gauge. Pull too hard you get a short stumble followed by less noise and no power. Fixing it just involved pushing the lever forward.

 

[Dan Thomas]

That would be fantastic
For what it's worth the method on the AN-2, at least as my ex Ukraine AF pilot friend told me, uses a sealed balloon which contracts or expands with density changes (much like an altimeter works) which is then mechanically linked to a mixture control. There *is* a mixture knob, but you are instructed not to touch it. And this is tech dating back almost 100 yrs.

Lycoming's fuel injection system:


See that thing in the lower left corner? An automatic mixture option. Uses an aneroid capsule.

Continental:

Far right, under the vapor return line, we see an aneroid for automatic mix control ("Auto-Lean"). This is an option for naturally aspirated engines. Turboed engine systems also use an aneroid.

The problem with these is that they only sense ambient air pressure. They don't make any compensation for temperature, so density doesn't get compensated for. And they make no adjustment for high power, which needs a richer mixture for cooling and to avoid detonation. The pilot still has to think. One could design a system that used the aneroid and a thermostat and some throttle cams to get the mixture to go where we want it, but all that stuff increases the failure points and cost, and nobody wants more risk and expense.

 

[Tantalum]

density doesn't get compensated for. And they make no adjustment for high power, which needs a richer mixture for cooling and to avoid detonation

Good point.. although some basic camming off the throttle and an analog temp sensor (like you note) could probably compensate for some of that.. but that's probably why the Twin Bonanza, AN-2, etc. still have a mixture knob

all that stuff increases the failure points

..and I think that's where the philosophical debate lies.. the pilot is far from fallible, and represents a fairly large pandora's box of potential failure points, so, to your point

The pilot still has to think

..

I tend to think machines make less mistakes.. I understand "pilot's failure to X" is the FAA's go to accident cause.. but unfortunately the stats do suggest that most accidents are human caused. I mean, how threads have we had here alone about leaning technique and debates on what "lean of peak" actually is? If you set this "automation" up right you basically get your own flight engineer and a lot of that guesswork of "gee, I read from Mike Busch X, but then my CFI also said this.. hmmm.. what do I do?"

 

[Dan Thomas]

Good point.. although some basic camming off the throttle and an analog temp sensor (like you note) could probably compensate for some of that.. but that's probably why the Twin Bonanza, AN-2, etc. still have a mixture knob



..and I think that's where the philosophical debate lies.. the pilot is far from fallible, and represents a fairly large pandora's box of potential failure points, so, to your point ..

I tend to think machines make less mistakes.. I understand "pilot's failure to X" is the FAA's go to accident cause.. but unfortunately the stats do suggest that most accidents are human caused. I mean, how threads have we had here alone about leaning technique and debates on what "lean of peak" actually is? If you set this "automation" up right you basically get your own flight engineer and a lot of that guesswork of "gee, I read from Mike Busch X, but then my CFI also said this.. hmmm.. what do I do?"

EFI would fix it without all the mechanical bits that wear out and corrode and don't get inspected properly, just like the rest of the airplane. Homebuilders have been using EFI on Lycs for years, but of course now we're totally dependent on electrical supply. Judging by the number of posts on this forum about alternator failures, I'm not optimistic that EFI will be the answer unless maintenance and inspection is taken a lot more seriously.

 

[Huckster79]

Mixture simply regulates how much fuel goes into the engine for a given amount of air (basically). If you aren't changing altitude (or rather, the air density isn't changing), the only real reason you'd have to adjust mixture when changing power is to manage engine temps

For climb, I usually leave it full rich until around 3K-5K and start to lean for better performance, careful to watch temps
For descent, I usually leave the red knob alone for a while until the engine seems rough.. I always go full rich though by around 3K in the club Archer and Skyhawk though

PS, I know pilots (myself included) tend to be control freaks, but honestly, given how poor most CFIs are at teaching engine management it really would be nice to have the plane regulate its own mixture. This, by the way, doesn't have to be fancy electronic technology.. the Twin Bonanza mechanically managed its own mixture as does the Antonov AN-2 (yes, the big Soviet biplane), and I'm sure many others. How many engines have been damaged or killed by incorrect mixture technique. Just let an engineer figure it out and have the engine manage itself

interesting enough my 1947 with a 30s era carb actually does just that... The old stromberg carb... often More misunderstood that appreciated. It auto leans up to around 5K... it does this by leaning not via cutting fuel but an adjustment to the bowl vent which changes the atmospheric pressure in the carb bowl... the carb settings need no manual input until ya get to 5k... folks often think it doesn't lean good or properly as it leans nothing at idle nor will it cut the engine at idle... it leans just fine just a different way...

I find this to be very true in practice and seen no real effective use of it around home at cruise but flying out west it provided noticeable increase in performance... the Marvel design won the popularity contest... like VHS vs Beta..

 

[LongRoadBob]

hijack away, it’s all of interest!

Damn, took me this long to realize I should have squawked as I said it was ok to hijack


7500 7500 7500 7500


Seriously, lots of good info here. I already really got the answers I was looking for. But lots more has been helpful to know about as well.

 

[40YearDream]

It would be interesting to hear from some additional CFIs on this subject. The fellow I've started with pretty much ignored the mixture i.e. just leave it full rich all the time. I had to ask him to try moving the mixture knob in the old 150, to see what would happen - and then he got worried when I pulled the mixture a little bit too much (in low altitude cruise).

From my newbie perspective (and as a mechanical engineer), wasting expensive avgas and fouling spark plugs, seem the biggest concerns. Seems like the safest approach would be to follow the engine manufacturer's directions.

 

[Daleandee]

So many pilots believe the engine will stop if you pull the mixture to cut-off in flight, when in fact the engine and prop will continue to turn just fine, ...

I will insert one caveat here believing that most pilots know (or should) know their airplane. On my experimental with the short, lightweight, wooden prop - if the engine quits the prop ain't gonna windmill.

 

[Dan Thomas]

interesting enough my 1947 with a 30s era carb actually does just that... The old stromberg carb... often More misunderstood that appreciated. It auto leans up to around 5K... it does this by leaning not via cutting fuel but an adjustment to the bowl vent which changes the atmospheric pressure in the carb bowl... the carb settings need no manual input until ya get to 5k... folks often think it doesn't lean good or properly as it leans nothing at idle nor will it cut the engine at idle... it leans just fine just a different way...

I find this to be very true in practice and seen no real effective use of it around home at cruise but flying out west it provided noticeable increase in performance... the Marvel design won the popularity contest... like VHS vs Beta..

I flew those carbs and didn't find any auto-leaning. The carb bowl is vented to an area behind the venturi and to the venturi itself; two sources that meet in a cavity atop the bowl and which is vented to the bowl. Air is normally drawn through that cavity, from the stagnant air behind the venturi into the venturi, the bigger source being the stagnant air, with the result that the bowl gets pretty much ambient pressure. There was an option for a few parts that fit into that cavity: two valve plates and a lever and a different cap, giving the pilot control over the mixture. Those parts are rare and expensive. I made them myself for the carb on my E-AB Jodel, and later came across a carb with the stuff in it. The parts I made worked well so I left them alone. Still have the other carb. The valve throttles the air from the stagnant-air source so that venturi vacuum begins to reduce the pressure in the bowl, reducing fuel flow. At idle, or low power settings, there's little vacuum in that venturi, so it can't act as an idle cutoff.

 

[Will Kumley]

So, has anyone accidentally killed this engine while leaning? Just curious as anytime I lean and get the washing machine feel in the engine I quickly push the mixture in a little bit to go back to smooth operation. I want to avoid two things. Scaring the crap out of myself with no engine in flight and scaring the crap out of my family. More so the family as I know what to do without an engine. Pray- scream, and tell my wife she has the controls.

 

[crash7]

So, has anyone accidentally killed this engine while leaning? Just curious as anytime I lean and get the washing machine feel in the engine I quickly push the mixture in a little bit to go back to smooth operation. I want to avoid two things. Scaring the crap out of myself with no engine in flight and scaring the crap out of my family. More so the family as I know what to do without an engine. Pray- scream, and tell my wife she has the controls.

When I’m flying with non-pilot passengers, I always tell them “I’m going to adjust something here. You may hear the engine change sound a bit. It’s normal.” People fear the unknown. Just make it known.

As far as your first question, yes you can kill the engine in flight by pulling mixture too far. It’s how you shut down on the ground after all. In flight, however, the prop will windmill (doesn’t stop turning) and you just push mixture back in.

Just be nice and slow during that pull and YOU will most likely feel/hear the stumble long before your pax do. Don’t stare at the mixture knob either. It’ll move without your eyeballs looking at it. . Keep looking outside.


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[Will Kumley]

I know I can kill the engine in flight if I yank the red knob. I was just curious if anyone has done it by accident. I would imagine the engine would give you plenty of heads up as long as you're not yanking the mixture all the way back fast. Just makes me think to some CFI's I've flown with that are super sensitive to a student pilot adjusting the mixture for fear of killing the engine.

 

[Greg Bockelman]

Just makes me think to some CFI's I've flown with that are super sensitive to a student pilot adjusting the mixture for fear of killing the engine.

Sigh. CFIs super sensitive to that might ought to get some dual on what happens when the mixture is accidentally pulled.

 

[denverpilot]

Sigh. CFIs super sensitive to that might ought to get some dual on what happens when the mixture is accidentally pulled.

^^^^ THIS. ^^^^

FFS. Sigh.

 

[PaulS]

I know I can kill the engine in flight if I yank the red knob. I was just curious if anyone has done it by accident. I would imagine the engine would give you plenty of heads up as long as you're not yanking the mixture all the way back fast. Just makes me think to some CFI's I've flown with that are super sensitive to a student pilot adjusting the mixture for fear of killing the engine.

Yup, done it, several times, instructor would laugh at me. Pull too hard, engine dies, push it back in engine runs. Really a non event, if you are worried about it, take an instructor up and do it.

 

[Lance F]

Seeing the engine restart in the air after pulling the mixture out too much can also acclimate a student (at least one learning in a low wing) to the fact that if you let a tank run dry, the plane won't fall out of the sky. Engine quits because you've leaned too aggressively...push in the mixture knob a little. Engine quits because you've let a tank run dry...switch to one with gas in it. In both cases the restart is a non-event. Worrisome that this freaks out some CFIs. They're getting paid to know this stuff.

 

[Dan Thomas]

Seeing the engine restart in the air after pulling the mixture out too much can also acclimate a student (at least one learning in a low wing) to the fact that if you let a tank run dry, the plane won't fall out of the sky. Engine quits because you've leaned too aggressively...push in the mixture knob a little. Engine quits because you've let a tank run dry...switch to one with gas in it. In both cases the restart is a non-event. Worrisome that this freaks out some CFIs. They're getting paid to know this stuff.

The fact that this is even a discussion is concerning. It's little wonder that there is so much misinformation about leaning and carb ice and a whole lot of other stuff.

 

[crash7]

Fuel, fire, air. It takes all three for engine to make happy noises. Remove one, it gets quiet. Re-introduce it, happy noises again.

Pax do tend to get noisy when engine isn’t, though...


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[WannFly]

I know I can kill the engine in flight if I yank the red knob. I was just curious if anyone has done it by accident. I would imagine the engine would give you plenty of heads up as long as you're not yanking the mixture all the way back fast. Just makes me think to some CFI's I've flown with that are super sensitive to a student pilot adjusting the mixture for fear of killing the engine.

if you do it slowly, there will be a lot of heads up from the engine, if you yank, well not so much between the heads and and things getting quite. something i do to check my ignition and actually find out if my mags are fine or not.... less than 65% power, lean to LOP or below until the engine shudders, do mag check - first time doing this its really interesting . when i did this the first time i did it over an airport to ensure I have an option to land.

 

[Will Kumley]

if you do it slowly, there will be a lot of heads up from the engine, if you yank, well not so much between the heads and and things getting quite. something i do to check my ignition and actually find out if my mags are fine or not.... less than 65% power, lean to LOP or below until the engine shudders, do mag check - first time doing this its really interesting . when i did this the first time i did it over an airport to ensure I have an option to land.

Sounds like heck of a test. Pretty sure I won't be doing that with the wife in the right seat. She'd never want to fly again.

 

[WannFly]

Sounds like heck of a test. Pretty sure I won't be doing that with the wife in the right seat. She'd never want to fly again.

http://blog.savvymx.com/2010/03/mag-check.html
yah leave the SO on the ground

 

[Larry Vrooman]

So, has anyone accidentally killed this engine while leaning? Just curious as anytime I lean and get the washing machine feel in the engine I quickly push the mixture in a little bit to go back to smooth operation. I want to avoid two things. Scaring the crap out of myself with no engine in flight and scaring the crap out of my family. More so the family as I know what to do without an engine. Pray- scream, and tell my wife she has the controls.

I have never killed one by leaning too much, in the air at least...but keep reading.

Seeing the engine restart in the air after pulling the mixture out too much can also acclimate a student (at least one learning in a low wing) to the fact that if you let a tank run dry, the plane won't fall out of the sky. Engine quits because you've leaned too aggressively...push in the mixture knob a little. Engine quits because you've let a tank run dry...switch to one with gas in it. In both cases the restart is a non-event. Worrisome that this freaks out some CFIs. They're getting paid to know this stuff.

I had an instructor who was happy to let me screw up and cause the engine to stop - at a safe altitude.

Consequently, I was very comfortable with the concept of the engine quitting in flight and it being no big deal, if you had some altitude because you had plenty of time to troubleshoot the problem (especially if you already had an idea of what direction to fly to a potential emergency landing site) usually starting with undoing whatever you just did to make the engine stop.

That became important one day while flying a rented PA-28 that had one of the old metal handle fuel selectors with a small pointer on the short end, and a long handle on the other end. The FBO was a Piper dealer back when that was a thing and regularly took in all manner of PA-28s from various eras and put them in the rental fleet while they were listed for sale to ensure they were flown regularly. So if you flew often you got to fly a lot of different PA-28s with minor variations in cockpit layout.

Depending on the airplane, the selected tank was indicated either by the pointer on the front of the handle, or by the handle itself being positioned over the selected tank label. In this particular aircraft with the tank in use at the time, it was ambiguous which was which as the pointer pointed at the left tank and the handle pointed at the right tank. The other currently unused options on the selector were "Both" and "Off" . So of course turning it one way would either select the both, which I wanted to do prior to descending to land, or it would turn the fuel off. Turning it the other way had the same potential for either of those outcomes.

So of course I figured the best way to determine that was to turn the valve one way and see what happened. In about 30 seconds the engine started to stumble and I immediately turned the handle the other way, having successfully learned that the handle sat over the tank selected, not in front of the pointer.

However, "immediately" was not quite soon enough to prevent for my girlfriend's mother from unbuckling and practically crawling over the front seat back to assist. I apologized for not warning her that the engine might sound rough for a couple seconds, and then after she buckled in I asked her what she had been planning to do to help "fix" the problem, since she knew absolutely nothing about flying?

 

[Will Kumley]

I have never killed one by leaning too much, in the air at least...but keep reading.



I had an instructor who was happy to let me screw up and cause the engine to stop - at a safe altitude.

Consequently, I was very comfortable with the concept of the engine quitting in flight and it being no big deal, if you had some altitude because you had plenty of time to troubleshoot the problem (especially if you already had an idea of what direction to fly to a potential emergency landing site) usually starting with undoing whatever you just did to make the engine stop.

That became important one day while flying a rented PA-28 that had one of the old metal handle fuel selectors with a small pointer on the short end, and a long handle on the other end. The FBO was a Piper dealer back when that was a thing and regularly took in all manner of PA-28s from various eras and put them in the rental fleet while they were listed for sale to ensure they were flown regularly. So if you flew often you got to fly a lot of different PA-28s with minor variations in cockpit layout.

Depending on the airplane, the selected tank was indicated either by the pointer on the front of the handle, or by the handle itself being positioned over the selected tank label. In this particular aircraft with the tank in use at the time, it was ambiguous which was which as the pointer pointed at the left tank and the handle pointed at the right tank. The other currently unused options on the selector were "Both" and "Off" . So of course turning it one way would either select the both, which I wanted to do prior to descending to land, or it would turn the fuel off. Turning it the other way had the same potential for either of those outcomes.

So of course I figured the best way to determine that was to turn the valve one way and see what happened. In about 30 seconds the engine started to stumble and I immediately turned the handle the other way, having successfully learned that the handle sat over the tank selected, not in front of the pointer.

However, "immediately" was not quite soon enough to prevent for my girlfriend's mother from unbuckling and practically crawling over the front seat back to assist. I apologized for not warning her that the engine might sound rough for a couple seconds, and then after she buckled in I asked her what she had been planning to do to help "fix" the problem, since she knew absolutely nothing about flying?

Sounds like a great example of why to brief your passengers prior to doing anything that will confuse or cause concern for them.