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

Yes Dave,
The website is laid out a little funny. There is actually more than one
suggested cause. One of which is improper timing which has been suggested in
a number of other replies. I'll post it here for your viewing pleasure.
No rpm drop when checking magneto in Lycoming or Continental aircraft
engine
Warning - see a mechanic immediately - if the propeller is moved even
slightly the engine may fire causing injury

A drop in rpm is expected when one magneto in a redundant ignition
system is shut off. Should the propeller be moved by hand (as during
pre-flight or maintenance) and a functional "hot" magneto exists, the engine
may fire and cause injury to personnel.
1. Open magneto primary (P) lead causing hot magneto.

2. Magneto timing advanced beyond the specified setting.

3. Open in the grounding circuit of the feed through capacitor (Bendix
S-1200 series magnetos see Bendix S.B. 624).

4. Defective ignition switch.

5. Open magneto capacitor. For Bendix magnetos see Teledyne Ignition
Systems Critical Service Bulletin CSB641 or latest edition.




"Dave Butler" wrote in message
...
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

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.

No, I think a short between P leads would kill the engine when put in either the
L
or the R position. Grounding the P lead kills that mag, an open circuit lets
the mag
run. If you had a short, then grounding either mag would ground both.

The problem is either a defective mag switch such that neither the L or R
positions
ground the respective mag, or the timing is way off.

Art wrote:


Another possible failure mode could be a short between the Right and Left P
leads but not to ground.



--
--Ray Andraka, P.E.
President, the Andraka Consulting Group, Inc.
401/884-7930 Fax 401/884-7950
email
http://www.andraka.com

"They that give up essential liberty to obtain a little
temporary safety deserve neither liberty nor safety."
-Benjamin Franklin, 1759

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)

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.