Ignition switch, what happen when Left or Right is selected

"Peter Duniho" wrote in message
...
"OtisWinslow" wrote in message
m...
That's a good thing to pay attention to. You can also check it at
idle before shut down by briefly turning the switch to "OFF" and
see if the engine starts to shut down.

Emphasis on "at idle", of course.


Yes. At idle. And I should have added that if it starts to shudder and
die don't turn it back on. When I do it it's just a brief switch to the
OFF position and back on.

Yes. At idle. And I should have added that if it starts to
shudder and
die don't turn it back on.

It WILL start to shudder and die if you turn it off, unless there's a
bad P-lead connection or mag switch. You want it back on so you can
shut the engine off with mixture, not ignition.
I get really annoyed when students go to "Off" during the runup
and mag testing. We try to get them to hold the key firmly, not by its
edges, so that it won't inadvertently snap over to "Off" when they
select "R." They almost always immediately turn it on again (instead of
letting it spool to a stop and restarting) and the exhaust makes an
almighty BANG and I have to look closely at that muffler and piping
during inspections to find the cracks that often result.
That was the big joke when we were young, right? Turn of the
auto's ignition and back on again to get the big boom from the exhaust
system. Doesn't work anymore with fuel injection. Saves exhaust
systems, no doubt.

Dan

I get annoyder when they go the other way and grind the starter bendix. I'm
not sure which one is more expensive to fix.

{;-)

Jim


I get really annoyed when students go to "Off" during the runup
and mag testing.

My point was that if it gets past the point where it's going to keep
running, just leave it off. A quick flick of the switch to OFF and back ON
it
will certainly quit firing and lose a few rpm, but it won't die. I've been
doing these as long as I've been flying (the 70s) and I think it's
important to know the mags aren't hot. It's part of my shut down.
You should simply use any method you feel comfortable with to
check the P leads.


wrote in message
ups.com...

It WILL start to shudder and die if you turn it off, unless there's a
bad P-lead connection or mag switch. You want it back on so you can
shut the engine off with mixture, not ignition.

Dan

Exactly. I learned to fly in the early '70s and they didn't
teach us to look for a hot mag. I learned that when upgrading some
years ago. Doing this, I found that one of our 172's mag switches would
allow the mags to keep firing if the key was forced against the "Off"
stop, and upon investigation found an AD against any of those switches
that did that (76-07-12). Bet there are still a lot of them out there.
As far as the discussion about a single impulse mag rather than
two: Lycoming may have recommended a change to one to get rid of the
impulse coupling on one mag. The springs in these things have been
known to fail, and with two mags one might fail and not be readily
noticed if the pilot is a careless twit who doesn't do a decent runup;
if the other spring then failed as well the timing on both would go to
near zero BTDC. Power output would be low indeed. Corrosion has been a
problem in engines seldom flown or flown on very short flights, and the
springs, being a high-carbon steel, corrode quickly. The Bendix dual
mag (one housing, one gear and impulse coupling, two mags) had an AD
against it to replace that spring with a better one, since its failure
could and did cause accidents.

Dan

DT - Good explanation. I'd think a broken impulse spring would cause
the timing to lag so much that the runnup would make it obvious
something is grossly wrong. Maybe not?

wrote in message
ups.com...
Exactly. I learned to fly in the early '70s and they didn't
teach us to look for a hot mag. I learned that when upgrading some
years ago. Doing this, I found that one of our 172's mag switches would
allow the mags to keep firing if the key was forced against the "Off"
stop, and upon investigation found an AD against any of those switches
that did that (76-07-12). Bet there are still a lot of them out there.


Well, I certainly had occasion to turn the key to "OFF" on a 172 and hand it
to my instructor in the right seat while the prop kept spinning. We
reported it (and it was returned "fixed") at least 3 times on that aircraft
before somebody finally managed to isolate and *really* fix the problem.

Otherwise the RPM drop for each mag was consistent, so both mags were
"grounding", but not in the OFF position.