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

[Bob - S50]

If I'm not mistaken, when we test the ignition switch by switching it to off we are not only checking the P lead but also for any short in the switch itself.  However, as others have mentioned we don't want to shut down the engine that way and I'm thinking for a couple reasons.

1.  As mentioned, fuel in the cylinder will strip away oil.

2.  That fuel may get past the piston and into the oil in the crankcase.

3.  If we shut down with the ignition, and there is a short in the ignition switch, and we forget to pull the mixture all the way out, then we not only have the potential for spark but fuel as well.  If we shut down with the mixture all the way out, if moving the prop causes the engine to start, it won't run very long for lack of fuel.  To see how long, try a hot start but leave the mixture all the way out (never push it forward) and see if it will start and how long it will run.  On a cold engine, it probably won't catch at all.

4.  As mentioned, shutting down with the ignition switch will increase fouling of the plugs.

5.  Step 5 on the procedure, if you need to move the prop by hand, only do so opposite of normal rotation because the magneto cannot fire when doing that.

[neilpilot]

14 minutes ago, Bob - S50 said:

  Step 5 on the procedure, if you need to move the prop by hand, only do so opposite of normal rotation because the magneto cannot fire when doing that.

Is it an OWT, or can this possibly shorten the life of your vacuum pump?

[1964-M20E]

Because we can.  

Of course at the risk of complicating things more than they need to be for those who are worried about doing the P lead test before shutdown and excess fuel in the cylinders when returning the mags to both lean very aggressively (100rpm drop or so) before dong the P lead test.  You will have a minimal amount of fuel in the cylinders when re-igniting the fires.

Confession:  Never having the P lead test being stressed in my primary training I do not always do it.

On small lawnmower engines when the P lead would not work to shut down the engine we would always choke it to death. 

 

 

[StevenL757]

17 hours 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.

Exactly.

[Bob_Belville]

1 hour ago, neilpilot said:

Is it an OWT, or can this possibly shorten the life of your vacuum pump?

I am assured on good authority that the vacuum pump tale is of the OWT variety. No comment on the magnetos.  

[orionflt]

8 minutes ago, Bob_Belville said:

I am assured on good authority that the vacuum pump tale is of the OWT variety. No comment on the magnetos.  

actually not, because of the vane configuration and material rotating the older style pumps backwards could break and damage the vanes. not so much anymore

[carusoam]

Consider the modern instrumentation variety...

1) Turning the ignition switch off, and the engine continues to run is a sure sign that the mags are not grounding properly.

2) alternating between L and R mags only indicates that at least one is not getting grounded at a time, but it could be two... (not good enough to know that both are grounding)

3) doing the full run-up version: b, l, r, b and watching for EGT rise between single and both mags... now you have tested both p-leads, and all plugs... adds another minute to the shut-down.

4) I'm not a big fan of raw fuel dumping into the exhaust system...

Best regards,

-a-

 

[MyNameIsNobody]

I remember while in a 152 shutting down with mixture pull.  Mixture knob came off in my hand.  19 year old instructor said "How do we shut it down now"?  He didn't know...I said "Lets just move fuel flow to off".  Plane shut down shortly thereafter.  I  don't do a P-lead check at shutdown.  Never trained to do this.  I will have to give it a try.

[jaylw314]

if you have an engine monitor, you confirm both p-leads are intact every flight during your runup--when you see the EGT's rise in all cylinders on one mag, you confirm both p-leads are properly connected.

Shutting off by ignition switch tests for failure in the ignition switch.  Most of us have the 100 hr AD requiring the ignition switch be checked by switching to the off position firmly to check the engine shuts down.  While it can be done by any pilot, I don't think this is important to check every single flight.  I just check it at annual and maybe a couple random times in between.  When I do, I don't turn the switch back to B--I've had one backfire that scared the bejesus out of me (even with the mixture aggressively leaned), and don't care to repeat it.  I just shut down, do a quick hot start, and then shut down normally by mixture.

Incidentally, with a fuel injection, the valve is before the fuel divider, meaning when you shut down, the fuel in the divider does leak out of the injectors as its heats up.  Don Maxwell commented in his hot start YouTube video that this means all hot starts are in a sense flooded starts, but this does not go into the cylinders where it can wash off the cylinder oil.

Realistically, though, I imagine we're talking about small amounts of fuel, but if you taxi with partial throttle and don't lean for taxi, it might be enough to cause problems.

Edited September 6, 2017 by jaylw314

[bradp]

On 9/4/2017 at 10:59 PM, 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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I recently had a p lead to midlfight.  It was fine on runup.  It was not on shutdown.  The lead had frayed.  Quick fix but doing checks at runup and shut down is useful. 

[Seth]

On 9/5/2017 at 8:01 AM, gsengle said:

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


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I had a broken p-lead two years ago.  Easy fix but the prop was "hot" at all times until it was fixed.

Was absolutely shocked when after all my years of step one the engine kept running.

 

-Seth

[jlunseth]

I always always do the P lead tests, and yes, I did find a broken P lead once.  Immediately notified the home base FBO and we flagged the prop until the plane could go into the shop, which it did.  The P lead was broken.

Probably twice in the time I have had my aircraft I have stopped it with the ignition rather than the idle cutoff.  If I try starting it again soon, it is difficult to start, and my engine is never difficult to start.  My surmise is that since the fuel injection system is a constant flow system, fuel keeps flowing into the cylinders as long as the engine is still turning, and it creates a flooded engine.  By constant flow I mean that the system does not "spurt" fuel into the cylinder in a short moment, rather, the injectors are constantly spraying fuel into the manifold just outside the intake port, then the valve opens and takes in fuel.

[jetdriven]

The hot air and engine under the cowl vaporize liquid fuel in the lines which expands and forces liquid fuel through the injectors and into the cylinders. Self flooding engine at shutdown. That's the source of the gurgling and sizzling noises from the engine when pushing it into the hangar. 

[kortopates]

On 9/6/2017 at 1:57 PM, jaylw314 said:

if you have an engine monitor, you confirm both p-leads are intact every flight during your runup--when you see the EGT's rise in all cylinders on one mag, you confirm both p-leads are properly connected.

Not true at all. The engine monitor doesn't tell us anything about the security of the p-leads since the mags will operate just fine with a broken or disconnected P-lead. Remember its grounding the P-lead that disables the Mag and which is why when we time the magnetos we either need to disconnect the p-lead to unground them or turn the ignition switch on to unground them so we can time them.

further: "Shutting off by ignition switch tests for failure in the ignition switch." it actually testing both, the ignition switch to ground the p-lead and the P-leads. Failure to shut down could mean either - not necessarily just the switch.   

 

[gsengle]

Not true at all. The engine monitor doesn't tell us anything about the security of the p-leads since the mags will operate just fine with a broken or disconnected P-lead. Remember its grounding the P-lead that disables the Mag and which is why when we time the magnetos we either need to disconnect the p-lead to unground them or turn the ignition switch on to unground them so we can time them.
further: "Shutting off by ignition switch tests for failure in the ignition switch." it actually testing both, the ignition switch to ground the p-lead and the P-leads. Failure to shut down could mean either - not necessarily just the switch.   
 

The engine monitor helps you to see that the bank of plugs / mag did not shut off at mag check.


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[kortopates]

Just now, gsengle said:

 

The engine monitor helps you to see that the bank of plugs / mag did not shut off at mag check.


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Yes, of course, and now I realize that's what @jaylw314 must have been saying since he specifically mentioned during the run up - my apologies as I wasn't thinking about the run-up even though that's exactly what he said until you said it again. But of course, by disabling one mag at a time you certainly are proving the P-lead is connected and the ignition switch grounds it. I was reacting to notion that the engine monitor showed us some indication of health of the p-lead when it does not but you both are so right that the mag test does show both are functioning together correctly; even if  the P-Lead was barely hanging on. That's something we'll never know till it disconnected and why its so important for us to always treat the prop as if it could be hot and to check it again at shutdown if it still grounding okay before we get out and push the plane by the prop, even if we did a Mag check on descent 10-15 minutes earlier. Its only good till the moment its not. 

Thanks for pointing that out -my mistake.

[peevee]

People do mag checks on descent?