no RPM drop on mag check

Our Mooney just came out of annual and now the tachometer needle doesn't move at
all during the magneto check as you switch through R, L, BOTH. The engine does
stop running with the switch in OFF.

I asked my my partner who manages the maintenance relationship to look into it
and he contacted the IA who did the annual. The answer I got was:

"I discussed mag drop problem with the IA: he complied with the mag AD during
the annual stating that he also timed the mag and set the points and would
expect very minimal RPM drop. He also tested the P lead and found it normal. The
only confirmation is turning the key off and the engine should stop, I guess
that was done. Looks like a non issue."

This is a Mooney M20J with the Lycoming IO-360-A3B6D with the "dual magneto",
two magnetos in a single housing.

I will check the POH to see what it says about RPM limits during the magneto
test. I'm not where I can access the POH right now.

My question is: is it OK to not have any RPM drop at all when switching from
BOTH to R or L? I'm vaguely uncomfortable with it, but I can't come up with any
reason it's a-bad-thing.

Thanks.

See the following site:
http://www.sacskyranch.com/eng101.htm

"Dave Butler" wrote in message
...

Our Mooney just came out of annual and now the tachometer needle doesn't
move at
all during the magneto check as you switch through R, L, BOTH. The engine
does
stop running with the switch in OFF.

I asked my my partner who manages the maintenance relationship to look
into it
and he contacted the IA who did the annual. The answer I got was:

"I discussed mag drop problem with the IA: he complied with the mag AD
during
the annual stating that he also timed the mag and set the points and
would
expect very minimal RPM drop. He also tested the P lead and found it
normal. The
only confirmation is turning the key off and the engine should stop, I
guess
that was done. Looks like a non issue."

This is a Mooney M20J with the Lycoming IO-360-A3B6D with the "dual
magneto",
two magnetos in a single housing.

I will check the POH to see what it says about RPM limits during the
magneto
test. I'm not where I can access the POH right now.

My question is: is it OK to not have any RPM drop at all when switching
from
BOTH to R or L? I'm vaguely uncomfortable with it, but I can't come up
with any
reason it's a-bad-thing.

Thanks.

Rich Badaracco wrote:
See the following site:
http://www.sacskyranch.com/eng101.htm

Thanks. Is there something in particular I should see there? I didn't see
anything that got to the heart of my question. The P-leads must be OK since the
engine stops when the switch is in OFF.

Dave

"Dave Butler" wrote in message
...

Our Mooney just came out of annual and now the tachometer needle doesn't

move at

all during the magneto check as you switch through R, L, BOTH. The engine

does

stop running with the switch in OFF.

I asked my my partner who manages the maintenance relationship to look

into it

and he contacted the IA who did the annual. The answer I got was:

"I discussed mag drop problem with the IA: he complied with the mag AD

during

the annual stating that he also timed the mag and set the points and

would

expect very minimal RPM drop. He also tested the P lead and found it

normal. The

only confirmation is turning the key off and the engine should stop, I

guess

that was done. Looks like a non issue."

This is a Mooney M20J with the Lycoming IO-360-A3B6D with the "dual

magneto",

two magnetos in a single housing.

I will check the POH to see what it says about RPM limits during the

magneto

test. I'm not where I can access the POH right now.

My question is: is it OK to not have any RPM drop at all when switching

from

BOTH to R or L? I'm vaguely uncomfortable with it, but I can't come up

with any

reason it's a-bad-thing.

Thanks.






--
Dave Butler, software engineer 919-392-4367

Dave,
You might try measuring the mag drop using a device like the Proptach
Digital Optical Tachometer.
www.proptach.com
Mechanical Tachometers do not have sufficient resolution to measure a small
RPM drop.
The Proptach has a resolution of 1 RPM and an update rate of 1 second and no
hook up required.
There should always be a drop on each mag if the engine stops in the OFF
position of the switch.
Another possible failure mode could be a short between the Right and Left P
leads but not to ground.
That failure could be in the wiring harness or the switch and would be like
having only one mag system.
Art Sundeen

On Wed, 27 Oct 2004 09:46:07 -0400, Dave Butler
wrote:

"I discussed mag drop problem with the IA: he complied with the mag AD during
the annual stating that he also timed the mag and set the points and would
expect very minimal RPM drop. He also tested the P lead and found it normal. The
only confirmation is turning the key off and the engine should stop, I guess
that was done. Looks like a non issue."

"Minimal" drop is different than "tachometer needle doesn't move at
all".

There are other failure modes that can cause this problem.

If the mags are properly timed, and if the ignition switch is shorting the
P lead, RPM should drop. You can have a faulty switch that is failing to
short the P lead(s).

Do you have an EGT gauge for each cylinder? If you do, then in single mag
operation, all EGT's should rise.

Don't hand turn the prop until this is corrected.


Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

Art wrote:
Dave,
You might try measuring the mag drop using a device like the Proptach
Digital Optical Tachometer.
www.proptach.com
Mechanical Tachometers do not have sufficient resolution to measure a small
RPM drop.
The Proptach has a resolution of 1 RPM and an update rate of 1 second and no
hook up required.
There should always be a drop on each mag if the engine stops in the OFF
position of the switch.
Another possible failure mode could be a short between the Right and Left P
leads but not to ground.
That failure could be in the wiring harness or the switch and would be like
having only one mag system.
Art Sundeen

There should be a visible (with a regular tach) drop when on one mag.
If not, there is a either a problem with the P-lead (the mag is not getting
grounded) or the mags are mistimed.

Never believe that the behavior of the mags in the L or R position tell
you anything about what will happen when you put the switch in OFF.
Believe me, my ignition switch (before it was replaced) could be set to
what appears to be off (you could even remove the key) and it not ground
the mags (but the normal L-R positions give you mag drops).

Of course, then there's always the old Rod Machodo click-click-click-BOOM!
mag check his students do. If the engine dies for whatever reason when you
turn the mag switch, close the throttle before you turn it back.

Dave Butler wrote:

Our Mooney just came out of annual and now the tachometer needle doesn't
move at all during the magneto check as you switch through R, L, BOTH.
The engine does stop running with the switch in OFF.

Not OK.

It can be a bad switch or the mags mistimed (since we ruled out the
P-lead is ok).

Dave Butler wrote in message ...
Our Mooney just came out of annual and now the tachometer needle doesn't move at
all during the magneto check as you switch through R, L, BOTH. The engine does
stop running with the switch in OFF.

I asked my my partner who manages the maintenance relationship to look into it
and he contacted the IA who did the annual. The answer I got was:

"I discussed mag drop problem with the IA: he complied with the mag AD during
the annual stating that he also timed the mag and set the points and would
expect very minimal RPM drop. He also tested the P lead and found it normal. The
only confirmation is turning the key off and the engine should stop, I guess
that was done. Looks like a non issue."

This is a Mooney M20J with the Lycoming IO-360-A3B6D with the "dual magneto",
two magnetos in a single housing.

I will check the POH to see what it says about RPM limits during the magneto
test. I'm not where I can access the POH right now.

My question is: is it OK to not have any RPM drop at all when switching from
BOTH to R or L? I'm vaguely uncomfortable with it, but I can't come up with any
reason it's a-bad-thing.

Thanks.

Seems to me (a rank novice, almost) that the RPM drop is the
indication of actually switching off one of the mags to allow you to
test the other in isolation. If you get no drop, are you really
testing the single mag? If you are not, how do you know you have a
fully redundant ignition system?

That would be my concern. Understand, I am a student with ~12 hours in
C-172s and _no_ experience with a Mooney (other than envy).

John S.

John Stevens wrote:

Seems to me (a rank novice, almost) that the RPM drop is the
indication of actually switching off one of the mags to allow you to
test the other in isolation.

It usually is, but if the timing on both mags is retarded a little (about 10 degrees
does it on my O-320), the engine will run at the same speed on either or both mags.
This could be the problem, or, as others have said, the switch could be failing to
ground the mags in the "L" and "R" positions.

George Patterson
If a man gets into a fight 3,000 miles away from home, he *had* to have
been looking for it.

I concur with several others - the timing's off.

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