From: kerry@Ingres.COM (Kerry Kurasaki)
Subject: Re: how to do a mag check?
Reply-To: kerry@Ingres.COM (Kerry Kurasaki)
Organization: Ask Computer Systems Inc., Ingres Division, Alameda CA 94501
Date: 31 Aug 92 18:37:52 GMT

In article <71795@apple.Apple.COM> stearns@Apple.COM (Bryan Stearns) writes:
>ginsberg@t.Stanford.EDU (Matthew L. Ginsberg) writes:
>>Can someone tell me why the mag check is done both - x - both - y - both?
>>I simply do both - left - right - both.  After all, B-L-B-R-B is in
>>reality B-L-B-L-R-B because of how the switch is set up.
>
>My experience is that the switch back to "both" between tests of the
>individual mags allows the engine to restabilize at the "normal" run-up
>RPM (ie, your ears are also comparing the sound changes made by switching
>from both to individual mags).

It is best to switch to BOTH to allow the previously grounded mag's
plugs to burn clean before checking the other mag.  What has not been
covered is the "proper" way to perform a mag check.  Whether you perform
BLBRB or BRBLB is insignificant (unless contrary to the AFM/POH or engine
manufacturer's operating manual).

To "properly" check the mags, run the engine up to precisely the same
RPM each time.  If the manual say 1700 RPM, use 1700, not 1600 or 1800.
As you ground each mag, you want to check that the RPM drops are
consistent with each other and consistent over time.

That is, you want to note a history with a particular engine if you
can.  (Obviously this is much more difficult if you rent.)  That actual
amount of drop (within limits) is not as significant as its consistency
of drop over the history of the engine and the difference between each
mag.  An engine that drops 50 RPM is not necessary "better" than an
engine that drops 100 RPM.

You also should not compare RPM drops between two different makes or
models of engines.  e.g. parameters for an O-200 are not the same for
an O-360.  Consult the manual.

If there is no RPM drop, DO NOT FLY THE PLANE.  Due to the dual ignition
system of most GA airplanes, there MUST be a mag drop.  It might be as
small as 25 RPM but it should be noticable.  Zero RPM drop could be
indicative of a bad ignition switch, bad P-lead from the magneto, or
worse, mistimed magnetos.  Yes the timing of magnetos have been changed
by unscrupulous sales agents to make the runup look acceptable (minimize
RPM drop) yet it wreaks overall havoc to the engine.

If you have a sensitive EGT, GEM, or scanner, you might notice a slight
increase in EGT.  I notice a 10 degree increase on a plane I fly that
has JPI scanners.  This is due to the "late" combustion afforded by
only one mag.  The mixture is still burning a bit when the exhaust
valve opens so the EGT goes up slightly.  If the EGT does NOT go up,
then something is suspect.

Oh yes, I almost forgot.  When performing the runup, use all of your
senses.  Hearing is particularly important during the mag check and
propeller cycle.  [Seeing flames from underneath the cowling is a good
indicator too. :(  ]  "Feel" for unusual or odd vibrations.  If you
can, use the "Force".

Don't let the discussion of BOTH-LEFT/RIGHT-BOTH distract you from what
you should really be looking/listening for in a runup.

Kerry