Mike:

The purpose of a run-up (among other things) is to check the health of the ignition system.  The more stress the ignition system is placed under, the better the test.  In order of value are the following mag checks:

Most valuable
1) high power, inflight LOP.
2) high power, inflight ROP.
3) ground run-up LOP (aggressively leaned).
4) ground run-up ROP.
Least valuable

The last one is all but worthless.  It will only tell you that the mag is working or whether or not you have an already DEAD plug.   I find weak/non-working plugs during the in-flight LOP check that will not show as bad for many more hours during a ROP ground check.  As a result, I many times do not do run-up mag checks unless I'm coming out of maintenance.  I do the in-flight LOP mag check before the descent on every flight.  If I find something, I'm about to land and can get it fixed.  If I find it working properly, it will darned sure pass a run-up mag check, so I don't fool with them.  The only thing I do is switch to left and right mags while taxiing out to confirm that both mags are on line.  We find 2-3% of NEW plugs will not pass an inflight, LOP mag check.  I toss those.

It's very common to have a bad/weak plug that acts up in flight and will pass a run-up and mechanic's bomb test.  That's one source of engine roughness that's a head-scratcher for the pilot and mechanic.  Frequently a high power, inflight mag check will find the problem.  We have a significant number of pilots now doing inflight mag checks as a matter of routine.  They are they guys whose engines run really well! <g>

Walter


On Mar 2, 2005, at 10:33 AM, Mike S. Reinath wrote:


<<< I do the runup at this leaned
mixture setting as it is more diagnostic.  >>>

Walter, or anyone else, please explain why it would be more diagnostic to runup leaned as you describe.

Mike Reinath
LNC2 at 460 hrs TT
San Jose, CA (RHV)

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