Advancing the throttle on shutdown

labbadabba

labbadabba

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labbadabba
So, I noticed something weird that a buddy picked up from his CFI.

During shutdown with the throttle at idle, he pulled the mixture. Just as the engine started to sputter he advanced the throttle to the firewall. The engine for just a moment came back to life and then shuttered to a stop.

I asked him why he pushed the throttle in and he said that his CFI told him that it burned out the remaining fuel in the cylinders.

I guess I can kind of see that, but is there any real benefit to this? More importantly, is there harm done here?
 
Never heard of that technique in particular but it makes sense. I was taught to simply idle a little higher prior to pulling mixture. I will increase throttle to get 1100-1200 rpm and then pull mixture. I think that accomplishes the same thing with less juggling.
 
I alway increase to 1500 for shutdown


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The boost pump keeps head pressure in the fuel system (make sure it is off first).
The mixture control is a valve that allows fuel to flow from fuel tanks to fuel system.
The throttle controls the mass flow-rate of air (in fuel-injected engines) or air/fuel mixture (in carburetted engines) delivered to the cylinders.

Once you pull the mixture control to cutoff you are stopping fuel flow into the fuel system. Then once you pull the throttle to idle the only fuel that can be left by the time you turn the ignition off will be in the fuel lines not the cylinders.
 
citizen5000 said:
The boost pump keeps head pressure in the fuel system (make sure it is off first).
The mixture control is a valve that allows fuel to flow from fuel tanks to fuel system.
The throttle controls the mass flow-rate of air (in fuel-injected engines) or air/fuel mixture (in carburetted engines) delivered to the cylinders.

Once you pull the mixture control to cutoff you are stopping fuel flow into the fuel system. Then once you pull the throttle to idle the only fuel that can be left by the time you turn the ignition off will be in the fuel lines not the cylinders.
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So, in this case, it was a carburetted engine, the only thing advancing the throttle would do is clear out the fuel lines?
 
labbadabba said:
So, in this case, it was a carburetted engine, the only thing advancing the throttle would do is clear out the fuel lines?
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Well, if carb'd the fuel getting to the cylinders is coming from the carb bowl(s) once the mixture set full cutoff (and boost pump is off). So revving the engine is likely not accomplishing the imagined "clear the cylinders" as some assume. In a fuel injected system you have a better chance of clearing fuel from the system if you run the engine with the mixture at full cutoff and wait for the engine to die but I don't know an A&P would say it's necessary or advantageous to waste fuel in that way.
 
citizen5000 said:
Well, if carb'd the fuel getting to the cylinders is coming from the carb bowl(s) once set the mixture to full cutoff (and boost pump is off). So revving the engine is likely not accomplishing the imagined "clear the cylinders" as some assume. In a fuel injected system you have a better chance of clearing fuel from the system if you run the engine with the mixture at full cutoff and wait for the engine to die but I don't know an A&P would say it's necessary or advantageous to waste fuel in that way.
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K, that's basically what I thought. The fuel in the cyliders would evaporate pretty quickly anyway I would think. So it's just a waste of fuel.

What about the technique that others have mentioned? Where you increase the RPM to 1000-1500 and then cut the mixture. Doesn't this net the same result?
 
I believe it's in my AFM


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labbadabba said:
K, that's basically what I thought. The fuel in the cyliders would evaporate pretty quickly anyway I would think. So it's just a waste of fuel.

What about the technique that others have mentioned? Where you increase the RPM to 1000-1500 and then cut the mixture. Doesn't this net the same result?
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Yea, the cylinders are hot and will evap any fuel remnants, that's also a good point. Not sure why a Mooney Ovation AFM says to rev the engine on shutdown since you are running a Continental, correct? (@gsengle). Most AFMs are more concerned with the order you shutdown. For example mine says Boost Pump off, Mixture full cutoff, throttle idle, turn off ignition.
 
It doesn't say rev it, but I think it recommends an rpm for shutdown that is higher than idle. I'll have to look.


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My engine would sometimes keep running by firing occasionally after pulling the mixture to idle cut off. Advancing the throttle stops this.
 
I just go idle then ICO, engine runs like s champ and also starts within a blade or two
 
citizen5000 said:
The mixture control is a valve that allows fuel to flow from fuel tanks to fuel system.
Click to expand...

Huh? That's not right at all. The mixture control is internal to the carburetor, and controls the amount of fuel delivered for a given throttle position. If it controlled the fuel coming from the tanks, all it would do would be to starve the engine, and it would quit only once the carburetor float bowl was empty... which can take quite awhile.

Opening the throttle after you pull the mixture makes the engine demand more fuel and thus quit sooner, instead of lurching along for awhile at idle on the minimal fuel flow.
 
I've been in several planes that would not die easily at absolute idle. They would continue to spit and sputter for many seconds before dying. Increasing the rpms a couple of hundred usually alleviates this.
 
Graueradler said:
My engine would sometimes keep running by firing occasionally after pulling the mixture to idle cut off. Advancing the throttle stops this.
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The only time my engine keeps running on shut down is when I forget to turn the boost pump off! I like to think of it as making sure it works when I do it
 
I know a lot of people that do some variation of the technique but not to clear the fuel from any part of the system. They do it to help clean the plugs with the thinking that it will make for easier starting. If you have a fouling issue when you do a prolonged taxi and have to do a lean run up to clear the plugs, this is essentially doing the same thing for the taxi back in. Cutting the mixture and advancing the throttles should create a lean condition. Not sure if anyone has proven how effective it is though.
 
Lycoming's published shut down procedure. Have never owned a Continental powered airplane so do not know if it has something similar.

Excerpted from Textron Lycoming Service Letter No 192B.
Link: https://www.lycoming.com/content/service-letter-no-l192b

"9. Prior to engine shut-down the engine speed should be maintained between 1000 and 1200 RPM until the operating temperatures have stabilized. At this time the engine speed should be increased to approximately 1800 RPM for 15 to 20 seconds, then reduced to 1000-1200 RPM and shut-down immediately using the mixture control."​
 
That is pretty much the procedure at the place I rent for all their aircraft. p lead check, then 1700 rpm for 15 seconds with the mixture out about an inch, back to 1000rpm then pull the mixture to kill. I always figured it was to burn off any buildup on the plugs during taxi in.
 
Never done this, never had a fouled plug since I started staying as lean as possible on the ground.
 
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Cramming the throttle to the firewall would mess up my fuel-injected hot-start procedure which starts with setting an 1100RPM idle before pulling the mixture and not touching anything thereafter.
 
Marvel Schebler carburetors on the Continental C-90s C-85s etc had an AD requiring the mixture controls to be wired full rich. The correct procedure for shut down was idle the engine down and then simultaneously turn off the mags while pushing the throttle in (open). This leaned the mixture rapidly and assisted in preventing dieseling with a hot engine.
 
I used to do this based on advice from a CFI at an old club, reasoning was to burn all the fuel. But I stopped because it didn't make sense in my head for a carburetor engine and the checklist doesn't indicate it. For what it's worth the owner of the plane I'm flying now likes to lean aggressively on taxi

On my father's sailboat there's a diesel engine and during shut down he advances the throttle to full open to clean and blow the exhaust out. The diesel is shut by either cutting the fuel to the injectors or pulling the decompression lever...
 
midwestpa24 said:
I've been in several planes that would not die easily at absolute idle. They would continue to spit and sputter for many seconds before dying. Increasing the rpms a couple of hundred usually alleviates this.
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If you mean idle throttle, good they should die

If you mean at mixture to cut off, the planes need to go to the shop, something ain't right.
 
The whole problem with this thread is that every engine might have different requirements or technique.

If any CFI tells you to do something and you're wondering, "This seems weird..." make them show you documentation that calls for the technique. Seriously.

An awful lot of engine management garbage comes from cross-over between aircraft and engine types that need one thing, and are completely wrong for a different aircraft and engine.

It's all in the manuals. It's *supposed* to be in the aircraft manual, but once in a while you'll find it in the engine manufacturer's manual. If it isn't in the manual, it's more likely to be wrong than right.

That said, manuals usually suck at hot start procedures. THOSE seem to be passed along better from person to person than trying to find them in any manual, and that's what keeps OTHER stuff that IS covered in the manuals, floating from person to person -- even when those instructions are too often wrong -- in my humble opinion.
 
This exactly ... for my FI engine, it was set the throttle where you wanted it on start, pull mixture and don't touch the throttle until next start (if hot starting).
 
Briar Rabbit said:
Marvel Schebler carburetors on the Continental C-90s C-85s etc had an AD requiring the mixture controls to be wired full rich. The correct procedure for shut down was idle the engine down and then simultaneously turn off the mags while pushing the throttle in (open). This leaned the mixture rapidly and assisted in preventing dieseling with a hot engine.
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This is probably where it came from...standard procedure in Champs & Cubs. Be interesting to find out how many generations of "that's the way my instructor told me to do it" there are between when it was actually required and the OP. ;)

Besides adding lots of air to lean the mixture, wide open throttle also reduces the fuel through the carburetor at idle rpm.
 
iflyforfun said:
This exactly ... for my FI engine, it was set the throttle where you wanted it on start, pull mixture and don't touch the throttle until next start (if hot starting).
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I find this works well for the plane I am the sole pilot of. I know that it was 1000rpm when I shut down and will be very near that on next start. For the club planes there is more variability as to throttle position.
 
all this proves is that the accelerator pump in the carb is working.....other than that....lean forward. ;)
 
labbadabba said:
So, I noticed something weird that a buddy picked up from his CFI.

During shutdown with the throttle at idle, he pulled the mixture. Just as the engine started to sputter he advanced the throttle to the firewall. The engine for just a moment came back to life and then shuttered to a stop.

I asked him why he pushed the throttle in and he said that his CFI told him that it burned out the remaining fuel in the cylinders.

I guess I can kind of see that, but is there any real benefit to this? More importantly, is there harm done here?
Click to expand...

That might shed some light on the time our club A&P found one of the aircraft with the mixture pulled and the throttle "fire wall forward" after an aircraft checkout. In discussing this with the A&P one of the concerns was this creates a potential unsafe condition with the possibility of a full throttle start for the next person that operates the aircraft. I know, there are checklist to prevent that from happening. However, the "How do you remind yourself to do your checklist?" points out that checklists are overlooked.
 
I keep my plugs clean by lean mixture on ground. In fact after landing my throttle is half way open and then use mixture control to add power for taxi. I keep mixture leaned even during landing
 
denverpilot said:
The whole problem with this thread is that every engine might have different requirements or technique.

If any CFI tells you to do something and you're wondering, "This seems weird..." make them show you documentation that calls for the technique. Seriously.

An awful lot of engine management garbage comes from cross-over between aircraft and engine types that need one thing, and are completely wrong for a different aircraft and engine.

It's all in the manuals. It's *supposed* to be in the aircraft manual, but once in a while you'll find it in the engine manufacturer's manual. If it isn't in the manual, it's more likely to be wrong than right.

That said, manuals usually suck at hot start procedures. THOSE seem to be passed along better from person to person than trying to find them in any manual, and that's what keeps OTHER stuff that IS covered in the manuals, floating from person to person -- even when those instructions are too often wrong -- in my humble opinion.
Click to expand...
POH for the 172 that I rent says nothing about RPM, I was taught the '1500 then pull mixture' method:

SECURING AIRPLANE
1. Parking Brake -- SET.
2. Electrical Equipment, Autopilot (if installed) -- OFF.
3. Avionics Master Switch -- OFF.
4. Mixture -- IDLE CUT OFF (pulled full out).
5. Ignition Switch -- OFF.
6. Master Switch -- OFF.
7. Control Lock -- INSTALL.
8. Fuel Selector Valve -- LEFT or RIGHT to prevent cross
feeding.

Now flyforfun and ravioli's comment got me thinking, I wonder if this 1500 setting corresponds to the 1/4" that the POH recommends on the start procedures. Maybe that is where the practice originated?
 
N659HB said:
No lead fouling when you're burning mogas. :rolleyes:
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I've been running mogas about 90% of the time since I bought my plane 2 years ago. Never occurred to me this may be why in 2 years I've never had fouled plugs. It definitely should have!
 
GRG55 said:
Lycoming's published shut down procedure. Have never owned a Continental powered airplane so do not know if it has something similar.

Excerpted from Textron Lycoming Service Letter No 192B.
Link: https://www.lycoming.com/content/service-letter-no-l192b

"9. Prior to engine shut-down the engine speed should be maintained between 1000 and 1200 RPM until the operating temperatures have stabilized. At this time the engine speed should be increased to approximately 1800 RPM for 15 to 20 seconds, then reduced to 1000-1200 RPM and shut-down immediately using the mixture control."​
Click to expand...
I watched a CAP 182 pick up a chock off the ramp and fling it, while using this technique. I won't do it. Taxi and idle power won't do it, but 1800 RPM will.
 
If your mixture control doesn't absatootly posihootly shut off all of the fuel, advancing the throttle can add air which makes the mixture too lean no matter what so for what it shuts down nice and clean.

But, of course, the stuff in the POH was handed down directly by God on Mount Sinai and is to never be questioned.
 
James331 said:
If you mean idle throttle, good they should die

If you mean at mixture to cut off, the planes need to go to the shop, something ain't right.
Click to expand...

And that is what it is, an idle even a minute amount of fuel may be getting to the cylinders and sustaining a small amount of combustion, which causes the engine to spit and pop. By idling up even just slightly, moving the mixture to cutoff causes a smooth shutdown. Now I am only talking about 800-900 rpm as opposed to idle at 600 rpm. I have never revved the engine up to 1800 rpm like some of the other posters. That I don't entirely understand.
 
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