Magneto grounding
Keith Elkins
and there are several questions regaurding magneto grounding. It
recommends momentarily turning off the ignition switch to see if the
engine keeps running due to a broken magneto ground wire.
The problem is that my first instructor, also an A&P though I
hear that he's not a very good one, taught me that if I accidently turned
of the ingnition to just let the engine die and then restart it using the
normal procedures. He said that turning the ignition back on while it
was turning would destroy the ingnition system.
The questions then are:
1. Am I not already doing this one magneto at a time, so
what causes a different problem while turning off both.
2. That was in a C-152 then, and I am flying a piper warrior now.
Would the same damage occure in this plane.
I don't want to try this without some other input.
Thanks
keith
Mike Mladejovsky
> I have been reading through the commercial pilot test questions
>and there are several questions regaurding magneto grounding. It
>recommends momentarily turning off the ignition switch to see if the
>engine keeps running due to a broken magneto ground wire.
> The problem is that my first instructor, also an A&P though I
>hear that he's not a very good one, taught me that if I accidently turned
>of the ingnition to just let the engine die and then restart it using the
>normal procedures. He said that turning the ignition back on while it
>was turning would destroy the ingnition system.
What you are going to do is destroy the exhaust system, not the
ignition system. If you kill both mags with the key at a fast idle,
you pump the muffler full of unburned fuel-air mixture. When you turn
the mags back on, the still-burning exhaust gases from the first power
stroke ignite the unburned gases in the exhaust manifold, muffler, and
stack, causing a powerful explosion. The explosion is called a
"backfire" and can be powerful enough to open up the muffler like a
sardine can.
Since a new airplane muffler costs $500 up, this is not a good idea!
Turing off one mag at a time prevents a backfire because the cylinders
keep firing (albeit at a slightly lower power) on one spark plug, and
they still consume most of the fuel before exhausting it into the
muffler.
---------------------------------------------------------------------------
Michael Mladejovsky, Phd PPsel
University of Utah, Center for Engineering Design, Salt Lake City, Utah
Ham Radio: WA7ARK Civil Air Patrol: Uncle Mike 41 Skylane N3453R
Opinions mine, not CED's
Ron Natalie
: 1. Am I not already doing this one magneto at a time, so
There have been noted cases of defects or miswiring of the ignition switch
that allow mag drops on the L and R key switch positions but leaving a mag
hot in the OFF.
: He said that turning the ignition back on while it
: was turning would destroy the ingnition system.
I don't think it would have any effect on the ignition system, but it
generally isn't nice to the engine. If you have the engine up at 1800
RPM or whereever you do your mag checks (in my plane 2500 RPM) and you
turn off the ignition, and turn it back on again you will get a pretty
large backfire. (Rod Machado refers to this as the click-click-click-
click-boom method of mag check).
However, the hot mag check is done with the engine at or near idle.
It's usually in the shutdown checklist so that you have checked it
after most of the vibration and shock that could have caused the
grounding to fail has occured.
-Ron
Ryan Healy
KE-=>and there are several questions regaurding magneto grounding.
-SNIP- 8< 8<
KE-=>normal procedures. He said that turning the ignition back on while it
KE-=>was turning would destroy the ingnition system.
Keith, what you are performing is a P-Lead check, to see if the P-Lead
(a shielded wire between the primary coil and the capacitor to the
ignition switch)is intact and functioning. This merely provides a path
from the primary circut to a ground to prevent the plug from sparking
when the points open.
KE-=>The questions then are:
KE-=>1. Am I not already doing this one magneto at a time, so
KE-=>what causes a different problem while turning off both.
KE-=>2. That was in a C-152 then, and I am flying a piper warrior now.
KE-=>Would the same damage occure in this plane.
I have never heard of any "damage" that may occur to the Ignition system
when performing a P-Lead check. In fact I know of a number of
Airworthiness Directives pertinent to several aircraft I fly that
REQUIRE a P-Lead check at a predetermined interval.
When you ground out the mags by turning he switch off, you are
eliminating spark, but you are not cutting off the fuel/air supply to
the engine. When you turn the key back to a mag, WHOOF! There is a
great deal of uncombusted fuel and air present in the cylinders,
the exhaust pipes and the muffler, which, when ignited could leave a
large 'afterfire', perhaps damaging the exhaust system. The above that
I have described is the only possible 'damage' that you may inflict to
the aircraft when performing this check. It is for this reason that you
are admonished to perform it at an idle power setting instead of a
normal runup power setting, and then only for a brief second. This
minimizes the amount of fuel/air being sent to the engine for combustion
purposes, and further minimizes the amount of uncombusted fuel/air that
you will find built up in your cylinders and exhaust system components.
Blue Skies,
Ryan
---
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Scott Maclean
Subject: Magneto grounding
In Reply-To: ke9...@u.cc.utah.edu (Keith Elkins)
KE> I have been reading through the commercial pilot test questions
KE>and there are several questions regaurding magneto grounding. It
KE>recommends momentarily turning off the ignition switch to see if the
KE>engine keeps running due to a broken magneto ground wire.
KE> The problem is that my first instructor, also an A&P though I
KE>hear that he's not a very good one, taught me that if I accidently turned
KE>of the ingnition to just let the engine die and then restart it using the
KE>normal procedures. He said that turning the ignition back on while it
KE>was turning would destroy the ingnition system.
This has been propagating here for a few days now.
1. If you shut off BOTH mags while the engine is running, raw fuel/air
mixture will be pumped into the exhaust system. If you then switch
the mags back on, this mixture can ignite, causing severe damage to
the exhaust system.
2. When you are doing your mag checks, you only shut one off at a time.
The engine continues to run, because the other mag, operating the
other set of plugs, is still working. Therefore, there is no unburned
fuel/air mixture coming out of the engine.
3. If you shut off one mag, and the engine dies, DON'T SWITCH IT BACK
ON! Let the engine stop. Then, shut both mags off, open the throttle,
pull the mixture full lean, and crank the engine to force the
remaining fuel/air mixture through the engine. Then, switch the mags
back on, restart the engine normally, and taxi the plane back in and
get it fixed.
Scott MacLean CASMEL-I Artificial Horizons BBS
art...@cais.com Aviation! (301) 417-9341
http://www.cais.com/arthor/www/index.html 300-28,800 bps V.FC/V.34
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Jasper Daams
> I have been reading through the commercial pilot test questions
>and there are several questions regaurding magneto grounding. It
>recommends momentarily turning off the ignition switch to see if the
>engine keeps running due to a broken magneto ground wire.
> The problem is that my first instructor, also an A&P though I
>hear that he's not a very good one, taught me that if I accidently turned
>of the ingnition to just let the engine die and then restart it using the
>normal procedures. He said that turning the ignition back on while it
>was turning would destroy the ingnition system.
> The questions then are:
> 1. Am I not already doing this one magneto at a time, so
> what causes a different problem while turning off both.
> 2. That was in a C-152 then, and I am flying a piper warrior now.
> Would the same damage occure in this plane.
One morning after snow overnight(Many years ago), I turned our Cessna
172 propellor by hand as was customary for cold mornings. I got a
good belt across my hand and arm as the engine started, luckily only
causing severe bruising. The pilot was sitting in the cockpit with the
keys in his hand in my sight. The poor bloke was totally shocked. The
Cessna was only just over one year old, but wear in the switch had
caused the copper filings to short circuit the switch. Thus always a
good idea to do some sort of check. And never trust any switch.
Regards
Jas
jas...@melbpc.org.au
Howard Jones
Mike Mladejovsky (mlad...@ced.utah.edu) wrote:
: In article c...@news.cc.utah.edu, ke9...@u.cc.utah.edu (Keith Elkins) writes:
: > I have been reading through the commercial pilot test questions
: >and there are several questions regaurding magneto grounding. It
: >recommends momentarily turning off the ignition switch to see if the
: >engine keeps running due to a broken magneto ground wire.
a friend ad I uncovered the only malfunction I've ever experienced in an
aircraft on one of these checks. on dismantling the magnetos to examin the
cause one was found to be a bundle of loose components, it had come apart.
: What you are going to do is destroy the exhaust system, not the
: ignition system. If you kill both mags with the key at a fast idle,
: you pump the muffler full of unburned fuel-air mixture. When you turn
: the mags back on, the still-burning exhaust gases from the first power
: stroke ignite the unburned gases in the exhaust manifold, muffler, and
: stack, causing a powerful explosion. The explosion is called a
: "backfire" and can be powerful enough to open up the muffler like a
: sardine can.
correct explanation but it is an "afterfire", a backfire goes back through
the inlet manifold, no less dangerous though, it can melt the plastic
components of a carby.
: Since a new airplane muffler costs $500 up, this is not a good idea!
yep, always do it at idle rpm........though the Jabiru is routinely switched
off this way because the autocompensating carby has no manual mixture control.
: Turing off one mag at a time prevents a backfire because the cylinders
: keep firing (albeit at a slightly lower power) on one spark plug, and
: they still consume most of the fuel before exhausting it into the
: muffler.
hmmmm, new one on me, I wonder what other malfunctions would be hidden by
this technique, a dead cut is a dead cut is a dead cut. I use a dead!! cut.
: Michael Mladejovsky, Phd PPsel
dont ever be lulled into believing that that phd make you immune from
human error ;-)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Howard Jones, (how...@perth.dialix.oz.au) _--_|\
66 Towton Way, Langford 6147, Western Australia / \
Toy Aeroplane Flyer VH22143. Tyre Kicker & Current Pilot! *_.--._/
Corby Starlet Plans #279....RAAF Association Flying Club Member V
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
just an unqualified bum who loves flying.......truely!
Larry A Hendrickson
JD>ke9...@u.cc.utah.edu (Keith Elkins) wrote:
JD>> I have been reading through the commercial pilot test questions
>>and there are several questions regaurding magneto grounding. It
>>recommends momentarily turning off the ignition switch to see if the
>>engine keeps running due to a broken magneto ground wire.
>> The problem is that my first instructor, also an A&P though I
>>hear that he's not a very good one, taught me that if I accidently
>turned
>>of the ingnition to just let the engine die and then restart it using
>the
>>normal procedures. He said that turning the ignition back on while it
>>was turning would destroy the ingnition system.
>> The questions then are:
JD>> 1. Am I not already doing this one magneto at a time, so
>> what causes a different problem while turning off both.
>> 2. That was in a C-152 then, and I am flying a piper warrior now.
>> Would the same damage occure in this plane.
Your first instructor was partly right. If you accidentally turn both
mags off during the mag check (1700 to 2000 RPM, depending on the
airplane), let the engine stop and restart.
The magneto grounding check that is recommended at the end of the flight
should be done with the throttle closed. Then you are not risking a
destructive backfire.
Larry A Hendrickson
ATP/CFI-ASMI
* SLMR 2.1a * Backup not found: (A)bort (R)etry (P)anic
BrianS.
any cylinders. you also should bring the switch back to 'both'
position for a few seconds before going on to check the other mag, this
will clear the cylinders and plugs.
the explosion out the exhaust when turning both mags off and then on
again is called an afterfire, a backfire is a burning of gases back
through the intake and out the carb.
...................................................................
Brian_...@ATK.COM
my views do not necessarily reflect the views of my company.