Mag checks during runup

RyanB

RyanB

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If someone could educate me on this subject it would be much appreciated. Questions: during runup how do you know if a mag is not working at all? If the engine quits when its selected on the ignition?

If you takeoff with a bad mag what would happen?


A newbie question but there are always things i need to understand better.

Thank you!
 
Ryanb said:
...Questions: during runup how do you know if a mag is not working at all? If the engine quits when its selected on the ignition?..
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Not to be tough on you but maybe you should just read your own question and ponder if there is any other possible outcome you could think of. I mean if the magneto is not working "at all" and you select it what do you think is gonna happen? :dunno:
 
Silvaire said:
Not to be tough on you but maybe you should just read your own question and ponder if there is any other possible outcome you could think of. I mean if the magneto is not working "at all" and you select it what do you think is gonna happen? :dunno:
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I know its not gonna fire if its not working, point is i want to learn tips of how to identify certain things along these lines that are abnormal during runup.
 
The engine will quit and when you switch it back to the working mag, it will probably backfire. Cut the throttle to idle ASAP.
 
Sounds like you have just not had the pleasure of a fowled plug. You will know as the RPM drop will be outside what is in the POH. I've had that happen in a rented C172 - seemed like that sucker had more lead than any other. Sooo.... I got over 100 RMP drop on one of the mags. If you know how to clear the led (e.g. running lean/hot) - go for it (some are against the technique).

However, if you have something more serious (e.g. a failed mag) then you will definitely know as the engine will stumble and die.

Give it time - it will happen..
 
Ryanb said:
I know its not gonna fire if its not working, point is i want to learn tips of how to identify certain things along these lines that are abnormal during runup.
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If the mags drop outside of whatever the POH specifies, then you could have a bad mag. If I switch mags and the engine is running rough, I'll lean the mixture out for a few minutes until it runs smooth.
 
Ryanb said:
If someone could educate me on this subject it would be much appreciated. Questions: during runup how do you know if a mag is not working at all? If the engine quits when its selected on the ignition?

If you takeoff with a bad mag what would happen?


A newbie question but there are always things i need to understand better.

Thank you!
Click to expand...

This should answer your questions about mags...enjoy.

http://flighttraining.aopa.org/magazine/2002/January/200201_Features_The_Magneto_Check.html
 
My CFI, who was an A&P, taught me to take my hands of the keys when I switch mags during the runup. If you switch and the engine starts to quit and you switch it back quickly you could at the very least get a backfire but you could potentially blow the muffler up and/or start a fire which would be a horrible thing to happen in the engine compartment with no prop turning to help put it out.
 
You'll know when you have a bad mag, cause you will say to yourself, "That doesn't sound right"
 
Ryanb said:
I know its not gonna fire if its not working, point is i want to learn tips of how to identify certain things along these lines that are abnormal during runup.
Click to expand...


It'll be really obvious. If it's not, it'll still be outside of the specs on the checklist.

My favorite was when a plug wire fell all the way off one cylinder. That one was real obvious on an O-470. You wondered if you'd need to see a dentist to put all of your teeth back in from the shaking.

Thinking it was a fouled plug, I ran it up to higher RPM too, which turned tooth-rattling shaking into a full-blown man-made earthquake.

If it drops a normal amount and doesn't sound or feel odd, you're good. You'll know it when it doesn't sound and feel right.
 
Since you are not going to full power on the runup, you should not push the mixture full rich (of course you leaned right after start up) it's a little tricky to get the mixture right but as you advance the throttle each time it stumbles advance the mixture. That way you are getting the best performance from your engine without fouling it up or wasting fuel.
 
Ryanb said:
If someone could educate me on this subject it would be much appreciated. Questions: during runup how do you know if a mag is not working at all? If the engine quits when its selected on the ignition?

If you takeoff with a bad mag what would happen?


A newbie question but there are always things i need to understand better.

Thank you!
Click to expand...

Good questions actually. Did you get them all answered? Any more? Don't let the morons chase you away from finding answers.
 
Note, if the engine ever quits (or really is about to) on a mag check DO NOT just run the key back. Rod Machado calls this his student's "click-click-click-click-BANG" mag check. It is not a "backfire" as mentioned earlier but an ignition of raw fuel that has been pushed out into the exhaust. Either let it die or close the throttle before moving the key again.

Here's what can happen in a mag check when switched to one mag or the other:

No RPM drop or other indication: Either the switch/p-lead is not grounding the magneto OR the engine is severely mistimed.

A small drop but the engine runs smoothly... If the drop is within the tolerances stated in the POH (typically 50-125 rpm from BOTH and within a certain tolerance to the other mag running alone)...this is normal and expected...go fly.

A larger drop or misfiring ... likely one or more the plugs has a problem: bad wriing, fouled plug, etc... If you have a multipoint engine analyzer, the affected cylinder's egt will be reading lower. Note which one it is when you take the plane back to be checked. Some will suggest some engine leaning voodoo to try to clear this, but really as a student it's probably best you get help.

The engine completely dies....mag is completely dead, or the ignition switch is faulty, or you accidentally turned it to the OFF position. Other than the last one, take it back and get it fixed.
 
Geico266 said:
Good questions actually. Did you get them all answered? Any more? Don't let the morons chase you away from finding answers.
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Questions answered thanks, just needed a better understanding of the subject. Thanks
 
flyingron said:
It is not a "backfire" as mentioned earlier but an ignition of raw fuel that has been pushed out into the exhaust.
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… which is the classic definition of an exhaust backfire.

It's an embarrassing sound, but the real risk is making exhaust leaks, especially into the cabin heat.
 
MAKG1 said:
… which is the classic definition of an exhaust backfire.

It's an embarrassing sound, but the real risk is making exhaust leaks, especially into the cabin heat.
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No, it`s not a backfire. It`s an afterfire. Backfiring is the ignition of the unburnt fuel-air mix in the induction system and it goes out the intake, not out the exhaust.

http://en.wikipedia.org/wiki/Back-fire

Most of the student click-click-BANG stuff is caused by holding the key by its edges instead of the flats, and it slips in the fingers farther than intended, into the off position.

And closing the throttle right after that does nothing to prevent damage. It`s too late.

Dan
 
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