"a" wrote in message
...
On Oct 18, 6:39 pm, "Peter Dohm" wrote:
"a" wrote in message


I was not trying to suggest that you failed to check the mags prior to
takeoff, and I do not have an opinion as to whether an in-flight check
would
necessarily tell anything of value.

The problem that I have personally observed was a case of points which
had
gradually "closed up" on a 65 horsepower Piper Cub until the engine could
not be manually started--and then was started on the first "lave" pull
after
the points had been dressed and gapped. A second case, that was only
confirmed much later, involved a Cessna 172 which occasionally required
manual starting for an assortment of stupid reasons; but started very
reluctantly in those instances...

The salient point is that both aircraft passed all tests normally
available
to a pilot; but, based upon the number of hours that each was operated,
probably had one or both mags out internal tolerances for multiple years.
So there are failure modes that the pilot can not necessarily
overcome--including damaged insulation on a p-lead, or a shorted mag
switch,
amoung others.

By the way, what were the problems later identified on your aircraft?

Peter

My in-flight check in fact produced something of value, Peter. The
engine in cruise went a little rough and stayed that way with mixture
adjustments. When I went to a single bank of spark plugs the engine
noise went from rough to none: I was flying on half the spark plugs.
That told me two things -- to land for a repair, and what to tell the
A&E.

My suggestion in the OP was that pilots learn what their engine does
when on a single bank of plugs when at cruise. It might be
instructive, it might not be. The failure mode I experienced was in
the high voltage lead between the magneto and the distributer. The
voltage impulse found a gap more convenient than the one at the spark
plugs, this on an engine that was only about 1100 hours (on a 2000
hour engine) since last major overhaul. I continued on my trip in less
than 2 hours. Clearly the aviation gods smiled on me.

It looks like you did about the only thing that can be done for that sort of
problem. There is just no reasonable way, at least none that I have ever
seen, to inspect for or predict an impending failure of a shielded cable--or
of several other parts of magnetos and distributors. It just serves as the
remaining justification for dual ignition!

I'm glad that it worked out well.

Peter