Why were we taught to shut down the engine via leaning?

[RobertE]

I was taught, as I bet most of you were, to follow a 3 step procedure upon shut down.  First step is to confirm that there is no loose p-lead by briefly switching the ignition to off.  If everything is in order then the engine will stop firing.  But I was taught to promptly return to "both" so the engine could then be stopped via pulling the mixture to idle cut off.  Third step was to switch the ignition to off so that even if any fuel remained in a cylinder it wouldn't fire if the prop were moved. (I, by the way, routinely forget the first step.)

So my question is why were we taught three steps when one alone - turning off the ignition - would do the job in an absolutely failsafe manner?  What prompts the question is that the other day I forgot to switch the ignition off before the line guy fueled and moved the plane.  There wasn't a disaster but there sure could have been one.  Whereas simply turning the ignition to off as the only step would eliminate any possibility of disaster.

Why shouldn't that one step become my new routine?  Surely there is no concern about a little fuel in the cylinder, is there?  That fuel is a risk only if there is an ignition source, which there isn't if the ignition is off and the p leads are intact, no?

 

[gsengle]

Because if you cut off the ignition you'll leave a good deal of fuel in the cylinders that pumps in there as the engine comes to a stop. That's like leaving a gun loaded. Also the fuel can strip the oil from the cylinders. Third, you're less likely to leave a plug fouled if you lean it out as you stop the engine.

 

By cutting the fuel first you guarantee it is all burned off.

 

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[Guitarmaster]

It's a good question and I've always wondered it myself.

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[neilpilot]

8 minutes ago, RobertE said:

  Third step was to switch the ignition to off so that even if any fuel remained in a cylinder it wouldn't fire if the prop were moved. (I, by the way, routinely forget the first step.

 

Step 4 (or 3b) - remove ignition key; maybe even place it on glare shield.

[rbridges]

9 minutes ago, gsengle said:

Because if you cut off the ignition you'll leave a good deal of fuel in the cylinders that pumps in there as the engine comes to a stop. That's like leaving a gun loaded. Also the fuel can strip the oil from the cylinders. Third, you're less likely to leave a plug fouled if you lean it out as you stop the engine.

 

By cutting the fuel first you guarantee it is all burned off.

 

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didn't think about oil in the cylinders, but I figured it was to burn as much fuel as possible.

[gsengle]

Step 4 (or 3b) - remove ignition key; maybe even place it on glare shield.

Good point, good habit to get into, so you can visually confirm from outside that the ignition is off!


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[peevee]

As mentioned, I was taught that it's really a safeguard for a broken p-lead. If you have fuel in the cylinder and the impulse coupler is wound up just so, a very small bump of the prop cold fire the spark plugs and kick the engine over, like when you level the prop.

[RobertE]

1 hour ago, peevee said:

As mentioned, I was taught that it's really a safeguard for a broken p-lead. If you have fuel in the cylinder and the impulse coupler is wound up just so, a very small bump of the prop cold fire the spark plugs and kick the engine over, like when you level the prop.

But, of course, if there were a broken p-lead the engine wouldn't stop, so absolutely, positively every defective p-lead would be identified.  To recap, it seems to me that the three step procedure I was taught requires that all of them be performed without fail for safe operation.  I often forget the first step and, the truth is, probably a couple times per year I forget the third.  Whereas if we treat our aircraft engine just as we treat our cars, lawn mowers, chainsaws and, basically, every other internal combustion engine we come in contact with (many of which also use magnetos), it seems to me that the risk of inadvertent starting falls to zero.  As to stripping oil from the cylinders due to unburned fuel, that's a legitimate issue.  But every other engine seems to survive that challenge.  So I do wonder.

 

 

[xcrmckenna]

But, of course, if there were a broken p-lead the engine wouldn't stop, so absolutely, positively every defective p-lead would be identified.  To recap, it seems to me that the three step procedure I was taught requires that all of them be performed without fail for safe operation.  I often forget the first step and, the truth is, probably a couple times per year I forget the third.  Whereas if we treat our aircraft engine just as we treat our cars, lawn mowers, chainsaws and, basically, every other internal combustion engine we come in contact with (many of which also use magnetos), it seems to me that the risk of inadvertent starting falls to zero.  As to stripping oil from the cylinders due to unburned fuel, that's a legitimate issue.  But every other engine seems to survive that challenge.  So I do wonder.
 
 

I would assume it has to do with everything else in aviation it never hurts for redundancy. Since we do forget one of them from time to time it's nice to do at least two of them. I know I was trained to never move the prop without seeing the keys on the glair shield. So it helps with making sure all three were performed. I also never grab the prop and only move it with an open hand as well. Checking the p lead is the second thing on my shut down check list right after turning the avionics off. If something doesn't feel right I always hear my instructor in the back of my head saying "use your check list".


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[peevee]

45 minutes ago, RobertE said:

But, of course, if there were a broken p-lead the engine wouldn't stop, so absolutely, positively every defective p-lead would be identified.  To recap, it seems to me that the three step procedure I was taught requires that all of them be performed without fail for safe operation.  I often forget the first step and, the truth is, probably a couple times per year I forget the third.  Whereas if we treat our aircraft engine just as we treat our cars, lawn mowers, chainsaws and, basically, every other internal combustion engine we come in contact with (many of which also use magnetos), it seems to me that the risk of inadvertent starting falls to zero.  As to stripping oil from the cylinders due to unburned fuel, that's a legitimate issue.  But every other engine seems to survive that challenge.  So I do wonder.

 

 

I was taught from the start to cycle the key off to test the p leads before pulling the mixture, which is a required check per ad on some mooneys every so often. So I would seem that there is some validity to it in the FAAs eyes, wouldn't it. Not to mention it's entirely possible to break a plead performing maintenance that could go unidentified before the prop could be bumped over towing the plane or trying to put a cowl on.

[Bob_Belville]

I replaced the Fuel Injection Servo a couple of weeks ago. I would not have known the servo was bad, or going bad, if I had not experienced difficulty stopping the engine by pulling the mixture.

(After the new servo was installed I realized that the mixture cable moved much easier but I did not notice the relative stiffness before the change.) 

[mccdeuce]

@Bob_Belville was that related to the high OilTemp?

[gsengle]

You know that you're already checking the p lead when you look for the rpm drop at run up. So the only thing I'm concerned with at shutdown is getting all the fuel out...


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[peevee]

5 minutes ago, gsengle said:

You know that you're already checking the p lead when you look for the rpm drop at run up. So the only thing I'm concerned with at shutdown is getting all the fuel out...


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It couldn't have changed during the flight.

[neilpilot]

7 minutes ago, gsengle said:

You know that you're already checking the p lead when you look for the rpm drop at run up. So the only thing I'm concerned with at shutdown is getting all the fuel out...


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Correct, except If you have a mag fail during a flight you may not discover it until the run up prior to the next flight.  Consider doing a end-of-flight mag check on shutdown just prior to pulling the mixture. That checks both the p leads and the mags.

[gsengle]

It couldn't have changed during the flight.

Sure but there are a lot of things we only check once a flight. I don't test the annunciator bulbs at shutdown earlier. And with no fuel in the engine, a bad p lead isn't gonna hurt ya.


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[carusoam]

Summary...

0) Set the MP to result in 1000rpm... helpful for uniformity during the next start-up.

1) P lead test: B, L, R, Off, return to B... (exposes failed plugs too)

2) Use up all the fuel in the carb/injection system: pull the mixture to ICO... also tests how deep/smoothly LOP the engine will go before dying..... (another sign of smooth ignition operation)

3) Shut off the mags: Key to off position. Verify, by putting up on the glare shield...

4) Secure the fuel: Turn fuel selector to off

There was a reason for #4...  somebody was afraid that a fuel leak downstream of the valve could drain a lot of fuel....(?)

5) what makes this different than other engines... somebody that is unknowing or asleep at the prop, can accidently get injured by a single plug firing...

6) Consider memories and distractions can happen here as well.  The added steps will keep you and other people safe...

Best regards,

-a-

 

 

[Hank]

51 minutes ago, neilpilot said:

Correct, except If you have a mag fail during a flight you may not discover it until the run up prior to the next flight.  Consider doing a end-of-flight mag check on shutdown just prior to pulling the mixture. That checks both the p leads and the mags.

When my Left mag died at 9500 msl at sundown, even with only the factory single-point gages, I noticed something was wrong--the EGT was reading at redline! I richened up, and a little while later it was there again.

Then I learned a lesson:  let the wife beside me know what I'm doing; when I silently turned the key to L, the engine died and the plane tilted downwards immediately and very noticeably. She wasn't happy . . .

I'm now saving my pennies for a monitor, and debating what to remove since my panel is full. Maybe a G2 can replace my 2-1/2" Carb Temp gage, which I really like, although it's marked in °C.

[ArtVandelay]

Summary...
0) Set the MP to result in 1000rpm... helpful for uniformity during the next start-up.
1) P lead test: B, L, R, Off, return to B... (exposes failed plugs too)
2) Use up all the fuel in the carb/injection system: pull the mixture to ICO... also tests how deep/smoothly LOP the engine will go before dying..... (another sign of smooth ignition operation)
3) Shut off the mags: Key to off position. Verify, by putting up on the glare shield...
4) Secure the fuel: Turn fuel selector to off
There was a reason for #4...  somebody was afraid that a fuel leak downstream of the valve could drain a lot of fuel....(?)
5) what makes this different than other engines... somebody that is unknowing or asleep at the prop, can accidently get injured by a single plug firing...
6) Consider memories and distractions can happen here as well.  The added steps will keep you and other people safe...
Best regards,
-a-
 
 

Re:#4
I think rather know I have a fuel leak.

When I had a broken p lead I could hear noise in the radio.

[neilpilot]

4 hours ago, gsengle said:

 

Sure but there are a lot of things we only check once a flight. I don't test the annunciator bulbs at shutdown earlier. And with no fuel in the engine, a bad p lead isn't gonna hurt ya.


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When you do your next preflight's run up, a bad annunciator bulb isn't likely a no go event. No so with a bad mag. Much rather catch some things at shutdown.

[Jerry 5TJ]

Perhaps we double check magneto grounding because when engines were hand propped it was very important. 

 

[CaptainAB]

Shutting Down the Engine
You should not shut down a piston engine by turning the ignition switch to OFF. Instead, move the mixture control to the idle cutoff position to turn off the fuel supply to the cylinders. After the engine stops, turn the ignition switch to OFF. This procedure ensures that no fuel remains in the cylinders and that the engine won't start accidentally if someone turns the prop or if carbon deposits inside the cylinders create hot spots that ignite residual fuel.

[gsengle]

Question: has anyone in all their years of flying caught a problem with ignition grounding? I have not.


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[Bob_Belville]

9 hours ago, mccdeuce said:

@Bob_Belville was that related to the high OilTemp?

Mark, I don't think so but I have not flown yet. I have done 2 lengthy ground runs but will fly this week. Need to build data and log approaches.  

[orionflt]

15 hours ago, RobertE said:

I was taught, as I bet most of you were, to follow a 3 step procedure upon shut down.  First step is to confirm that there is no loose p-lead by briefly switching the ignition to off.  If everything is in order then the engine will stop firing.  But I was taught to promptly return to "both" so the engine could then be stopped via pulling the mixture to idle cut off.  

The issue I have with shutting the engine down with the ignition switch then turning it back to both right away is that when you shut the switch to off the plugs stop firing, now you have raw fuel passing thru your engine and into the exhaust system. when you reintroduce spark to the engine all that fuel ignites and the combustion pressure has to go some where. I have seen several mufflers expanded by doing this, usually because some one does it accidentally during the mag check. normally when I fly I'm shutting down at least twice, once for fuel either before of after the flight and again back at the hanger, I will occasionally shut down my engine with the ignition at one of these stops just to verify the P leads and ensure a smooth shut down even though I verify the P-leads during every mag check.

Brian