Interpreting checklist item

LongRoadBob

LongRoadBob

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When weather clears I will be asking the CFI about this, but trying to familiarize with our checklist. I've flown two other types (Aquila 211 and Piper Warrior III) but haven't run across this one and find it kind of cryptic. Now I'll be flying a Cessna 172 SP (I think SP...I saw it first in the description but as always, find conflicting mentions. What makes it a "P"?)

This is in the first entries in the Shutdown checklist:

Parking Brake............................SET
Throttle..................................IDLE
Magnetos.................OFF Momentarily
Throttle........................... 1000 RPM
Avionics Master/GPS/Elec.Equip......OFF

From there it goes to mixture to cutoff, magnetos off, keys on glare shield.

First, I can't figure out what the point is with throttle idle, then magnetos...not sure if they mean completely off and then back but seems so, them throttle to 1000?

Haven't yet seen anything in the POH that could explain. They don't have such a long checklist for shutdown.

Also "Elec.Equip" later in the checklist is Master Switch off. Not quite sure I understand what Elec.Equip switch refers too.

Not a burning issue, just if someone feels they know what the point is, or what they mean, let me hear?
 
Yes, completely shut off the mags for a second or two, then back to both.

If the engine shuts off for that second or two, then the mags are properly grounded. if the engine does not temporarily shut off, the mags are "hot" all the time, which is a problem.
 
Ok, thanks very much to both of you! I was wary of turning off and quickly on again, I had thought I understood that could cause problems to the starter motor, but they wouldn't have it in there if it did I guess.
It's just a quick way to check both at the same time.

Just a followup, if one checks L and R separately if one is not grounded I think I'd see no drop in rpm?
 
The reason for setting the engine to idle is so that, when you turn off the mags, you suck as little fuel/air through the engine and into the muffler as possible. Turn the mags back on and you won't get a loud, destructive afterfire which could destroy the muffler. Running it up to 1000 rpm after that is to burn off any carbon that may have formed on the plugs, although in a plane that seems to foul plugs easily I would not go full rich - I would increase the mixture to your leaned taxi position.

"Electric Equipment" switch is referring to those planes that have an Avionics Master. The thought here is that as an engine slows to a stop and kicks back a bit the alternator can produce voltage spikes which may damage your avionics. Best to shut them off before pulling the mixture.

Yikes! :yikes: when switching the mags back on, DO NOT turn it to the point that the starter motor kicks in! Unless your A&P needs some money for his boat payment!

-Skip
 
LongRoadBob said:
(I think SP...I saw it first in the description but as always, find conflicting mentions. What makes it a "P"?)
Click to expand...

The "SP" is just branding from Cessna for the 172S model. They are the same thing.

LongRoadBob said:
I had thought I understood that could cause problems to the starter motor, but they wouldn't have it in there if it did I guess.
It's just a quick way to check both at the same time.
Click to expand...

You don't engage the starter motor. You only turn it back to both. The propeller will still be turning from momentum, and the engine will restart.

LongRoadBob said:
Just a followup, if one checks L and R separately if one is not grounded I think I'd see no drop in rpm?
Click to expand...

Yes but the ignition switch could be bad too, or something could have failed in flight. Better safe than getting hit in the head by a propeller unexpectedly.
 
Skip Miller said:
The reason for setting the engine to idle is so that, when you turn off the mags, you suck as little fuel/air through the engine and into the muffler as possible. Turn the mags back on and you won't get a loud, destructive afterfire which could destroy the muffler. Running it up to 1000 rpm after that is to burn off any carbon that may have formed on the plugs, although in a plane that seems to foul plugs easily I would not go full rich - I would increase the mixture to your leaned taxi position.

"Electric Equipment" switch is referring to those planes that have an Avionics Master. The thought here is that as an engine slows to a stop and kicks back a bit the alternator can produce voltage spikes which may damage your avionics. Best to shut them off before pulling the mixture.

Yikes! :yikes: when switching the mags back on, DO NOT turn it to the point that the starter motor kicks in! Unless your A&P needs some money for his boat payment!

-Skip
Click to expand...

Thanks! That was a great answer, and made a lot of sense! And to all of the other responses too!
 
dmspilot said:
The "SP" is just branding from Cessna for the 172S model. They are the same thing.



You don't engage the starter motor. You only turn it back to both. The propeller will still be turning from momentum, and the engine will restart.



Yes but the ignition switch could be bad too, or something could have failed in flight. Better safe than getting hit in the head by a propeller unexpectedly.
Click to expand...

Got it! It all makes sense. Thanks.
 
LongRoadBob said:
Ok, thanks very much to both of you! I was wary of turning off and quickly on again, I had thought I understood that could cause problems to the starter motor, but they wouldn't have it in there if it did I guess.
It's just a quick way to check both at the same time.

Just a followup, if one checks L and R separately if one is not grounded I think I'd see no drop in rpm?
Click to expand...

Just to be clear, you turn the key switch for the mags to off and back to both, do NOT turn the key further to the START position on a Cessna while the prop is still turning. That WILL cause damage. The starter solenoid is only energized to send high current from the battery to the starter in the START position.

The point here of this Check is to test the Grounding lead that is attached to only the OFF position, and in that airplane both mags ground through the switch to that single ground wire. They’re not separate.

If you understand electrical diagrams and schematics, look over... or have your CFI go over with you... the electrical system diagram in the POH/AFM with you, showing the electrical paths for start, mags L/R/BOTH, and OFF. You’ll want a solid understanding of your particular aircraft’s electrical system and troubleshooting steps eventually, anyway. :)
 
If you turn the key off at anything other than idle, just let it die. Otherwise, you'll do what Rod Machado calls his student's click-click-click-BOOM mag check.

The KEY to OFF is of dubious merit. If the switch is screwed up, there's no guarantee that just because it starts to die when you turn it off doesn't mean it won't become hot again if the switch gets jibbled a tiny bit. I had one of those balky switches. The act of removing the key would move it enough to unground one mag. Just as with assuming a gun is always loaded, always assume the mags are hot when you do things like move the prop.
 
flyingron said:
If you turn the key off at anything other than idle, just let it die.
Click to expand...

Or just bring the throttle back to idle before turning it back on.

Either is preferable to the "boom" though.
 
Yeah, I don't do that crap, not worth the risk to my engine.

I already check that on the run up when I do my normal right left both mag check.


Some schools get like a bonus or something for making multi page checklists lol


Side note, some float operators shutdown via mags vs mixture for quick turns.
 
flyingron said:
If you turn the key off at anything other than idle, just let it die. Otherwise, you'll do what Rod Machado calls his student's click-click-click-BOOM mag check.

The KEY to OFF is of dubious merit. If the switch is screwed up, there's no guarantee that just because it starts to die when you turn it off doesn't mean it won't become hot again if the switch gets jibbled a tiny bit. I had one of those balky switches. The act of removing the key would move it enough to unground one mag. Just as with assuming a gun is always loaded, always assume the mags are hot when you do things like move the prop.
Click to expand...

Amen to this! I flew PA-28 once that would - sort of - pass the KEY OFF mag check. But, if you let go of the key in the "off" position, the engine would continue to run. Holding it firmly in the counter- clockwise position, and it would stop. We got it fixed, but from that time on, my mag check drill was key off - hand off the key - hand on key - mags on - continue with check list.

-Skip
 
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