Don't skip that runup!

[Roger Long]

I picked took over our 172N from another club member who I know to be one of
our most conscientious pilots who follows all leaning and other
recommendations carefully. This plane almost never has rough mags and I
watched as he went through the plug clearing shut down procedure. He told me
the engine was running beautifully.
We fueled the plane and I taxied about 100 yards to the run up area leaned
to the max for ground operations. On run up, I had 220 RPM drop on the left
mag and the engine was so rough the plane bounced on the gear. I cleared it
but it took a lot of hard running.

A plug can foul that fast. It isn't something that always builds up slowly.
Don't skip that run up and always do it as close to the takeoff point as the
taxiway layout permits.

--
Roger Long

[Roger Halstead]

On Mon, 05 Jul 2004 11:50:50 GMT, "Roger Long" >
wrote:

>I picked took over our 172N from another club member who I know to be one of
>our most conscientious pilots who follows all leaning and other
>recommendations carefully. This plane almost never has rough mags and I
>watched as he went through the plug clearing shut down procedure. He told me
>the engine was running beautifully.
>We fueled the plane and I taxied about 100 yards to the run up area leaned
>to the max for ground operations. On run up, I had 220 RPM drop on the left
>mag and the engine was so rough the plane bounced on the gear. I cleared it
>but it took a lot of hard running.
>
>A plug can foul that fast. It isn't something that always builds up slowly.
>Don't skip that run up and always do it as close to the takeoff point as the
>taxiway layout permits.

Twice while I was flying the Cherokee 180 I had a very rough mag
check. Both times I parked it and found the creamic near the tip had
broken off and dropped into the gap.

I do a run up even If I only was on the ground long enough to go to
the can.

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com

[Jim Fisher]

"Roger Long" > wrote in message
> A plug can foul that fast. It isn't something that always builds up
slowly.
> Don't skip that run up and always do it as close to the takeoff point as
the
> taxiway layout permits.

I flew back from the beach with the family a couple of years ago. The two
hour flight was uneventful and I parked the plane in the hangar for the
night.

The next morning, I went to fly again and had similar symptoms as you did
except I couldn't clear the plugs. Had to replace the left mag.

So, either the mag failed during my flight back from the beach and I didn't
notice or it failed during the last crank . . . Or perhaps gremlins sneaked
in the hangar overnight. It was a little unnerving to think it might have
failed during flight and I didn't notice.

So, yeah, just 'cause you had no run-up problems during the previous flight
doesn't mean you won't have them on the next.

--
Jim Fisher

[Robert M. Gary]

"Jim Fisher" > wrote in message >...
> So, either the mag failed during my flight back from the beach and I didn't
> notice or it failed during the last crank . . . Or perhaps gremlins sneaked
> in the hangar overnight. It was a little unnerving to think it might have
> failed during flight and I didn't notice.
>
> So, yeah, just 'cause you had no run-up problems during the previous flight
> doesn't mean you won't have them on the next.

I just had a Cherokee fail a runup a couple weeks ago with a student.
I showed him how to clear the plugs but the mag was clearly dead. My
concern was that he was going to switch back to the other mag before
pulling the throttle down. If you get next to nothing on a mag during
runup, don't switch back to the good mag or you may blow out your
exhaust. Let the engine die or pull the power back to idle before you
switch back to a working mag.

-Robert

[Teacherjh]

>>
If you get next to nothing on a mag during
runup, don't switch back to the good mag or you may blow out your
exhaust. Let the engine die or pull the power back to idle before you
switch back to a working mag.
<<

Why? How? What about switching to "both"?

Jose

--
(for Email, make the obvious changes in my address)

[Peter Duniho]

"Teacherjh" > wrote in message
...
> >>
> If you get next to nothing on a mag during
> runup, don't switch back to the good mag or you may blow out your
> exhaust. Let the engine die or pull the power back to idle before you
> switch back to a working mag.
> <<
>
> Why? How? What about switching to "both"?

Switching to "both" would constitute switching back to a good mag.

As for the reason, it's because without either mag operating (one shorted
out on purpose, the other dead), a bunch of unburned fuel can accumulate
very quickly. Restart the fire by switching the good mag back on, and you
could wind up with a big fireball or exhaust fire.

That said, even having heard that advice before, the time that I found a
dead mag on a run-up, I completely forgot the advice and flipped the mag
switch back to both. It was a straight-forward reflex: if you don't like
what happened when you do something, restore back to the previous
configuration.

Fortunately, nothing bad happened.

Pete

[Jay Honeck]

> > Why? How? What about switching to "both"?
>
> Switching to "both" would constitute switching back to a good mag.
>
> As for the reason, it's because without either mag operating (one shorted
> out on purpose, the other dead), a bunch of unburned fuel can accumulate
> very quickly. Restart the fire by switching the good mag back on, and you
> could wind up with a big fireball or exhaust fire.

Good stuff -- I learned something vital here today. Gotta store that one in
the old brain bank.

Thanks, guys.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

[Dan Luke]

"Robert M. Gary" wrote:
> If you get next to nothing on a mag during runup,
> don't switch back to the good mag or you may blow
> out your exhaust. Let the engine die or pull the
> power back to idle before you
> switch back to a working mag.

Thanks, Robert; I never thought about that. I hope I have enough on the
ball to remember stay on the bad mag if it ever happens to me.
--
Dan
C172RG at BFM

[Paul Sengupta]

"Robert M. Gary" > wrote in message
om...
> If you get next to nothing on a mag during
> runup, don't switch back to the good mag or you may blow out your
> exhaust. Let the engine die or pull the power back to idle before you
> switch back to a working mag.

That's how "youths" make cars backfire. Switch off the ignition then
turn it on again after a short while. Makes a big bang.

Paul