no RPM drop on mag check

[Dave Butler]

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]

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.
>

[Dave Butler]

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

[Art]

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

[Ron Rosenfeld]

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)

[Ron Natalie]

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.

[Ron Natalie]

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).

[John Stevens]

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.

[G.R. Patterson III]

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.

[Don Hammer]

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

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[The Weiss Family]

I remember reading another post in which the owner said that his ignition
switch was mis-wired. L & R were shorted together, so all three settings
resulted in both mags being energized.

Don't know if this is possible, but it's something to consider.

Adam

[Doug]

Whoever said this is not an issue is WRONG. Although I might fly the
plane with this problem (depends), I would get it fixed as soon as
possible. No mag drop on runup is UNACCEPTABLE. You need to know both
mags are working. You could have timing issues, bad mags, bad ignition
switch. The suggestion about the EGT guage is good. EGT will go up
when the engine is running on one mag.

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.

[WARREN1157]

>No mag drop on runup is UNACCEPTABLE. You need to know both

You can wait until it gets dark. Pull the airplane by a mercury vapor street
light. Try to set the RPM at 1800. If the prop appears to stop at 1800 RPM your
tach is right on the money.

Run the engine until the prop appears to stop like if a timing light was
shining on it.Do your mag check. The prop should appear to turn to the left
because the RPM dropped. If it does this you should be able to tell if the mags
are independent of each other.

This takes some precision throttle movements to get the initial 1800 RPM
setting, but in the end the whole process is self explanatory.

This is also a tach check. The prop should show some motion change at 600 -
1200 - 1800 - 2400 RPM. Three blade props might or might not work like this,
logic says they will but at a different RPM.

Hope this helps.

Just My 2¢ Worth

[Dave Butler]

WARREN1157 wrote:
>>No mag drop on runup is UNACCEPTABLE. You need to know both
>
>
> You can wait until it gets dark. Pull the airplane by a mercury vapor street
> light. Try to set the RPM at 1800. If the prop appears to stop at 1800 RPM your
> tach is right on the money.
>
> Run the engine until the prop appears to stop like if a timing light was
> shining on it.Do your mag check. The prop should appear to turn to the left
> because the RPM dropped. If it does this you should be able to tell if the mags
> are independent of each other.
>
> This takes some precision throttle movements to get the initial 1800 RPM
> setting, but in the end the whole process is self explanatory.
>
> This is also a tach check. The prop should show some motion change at 600 -
> 1200 - 1800 - 2400 RPM. Three blade props might or might not work like this,
> logic says they will but at a different RPM.

Thanks, Warren and everyone! To summarize: There could be a problem with the
switch or the p-leads, but since the mags were just timed, it seems likely the
timing is off. Thanks for the mercury vapor light tach check suggestion, Warren.
That should be a more sensitive test than looking for small movement on the
analog tach. The consensus seems to be this is a symptom that should not be ignored.

(assuming the timing is wrong): the danger is (1) reduced power output? (2)
detonation? (3) inability to test magnetos at runup. Is that the kernel of it?

Thanks again to everyone. Dave

[Rich Badaracco]

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
>

[G.R. Patterson III]

Dave Butler wrote:
>
> (assuming the timing is wrong): the danger is (1) reduced power output? (2)
> detonation? (3) inability to test magnetos at runup. Is that the kernel of it?

Reduced power, increased EGT, and increased risk of burning exhaust valves. Inability
to test mags comes way last. Risk of detonation is actually decreased (probably
eliminated).

By the way -- if my IA gave me the reply that he "would expect very minimal RPM drop"
after timing them, I would take it elsewhere to get the timing checked.

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

[John Galban]

"G.R. Patterson III" > wrote in message >...
>
> By the way -- if my IA gave me the reply that he "would expect very minimal RPM drop"
> after timing them, I would take it elsewhere to get the timing checked.
>

Excellent point. Most of the smaller ContiLycoSaurus engines
should show a minimum 50-75 rpm drop. This should be noticable just
by the sound. If the mag drop is not perceptable, that's a bad thing
and the IA should know that.

John Galban=====>N4BQ (PA28-180)

[[email protected]]

Bigger bore cylinders (like an IO-360) should show even more mag drop
than small cylinders (like an O-300). It takes slightly more time for
the flame front to get across the cylinder head. Something is wrong
here.

[Dave Butler]

wrote:
> Bigger bore cylinders (like an IO-360) should show even more mag drop
> than small cylinders (like an O-300). It takes slightly more time for
> the flame front to get across the cylinder head. Something is wrong
> here.

Thanks everyone. Despite gorgeous weather here, I haven't had a chance to try it
again lately.

Partly based on the feedback from this group, I went back to my partner who
manages the maintenance and asked him to investigate it a little more. He says
he tried it and got a 25 RMP drop on each mag. I haven't been able to check it
out again myself, but I'm wondering why he sees a 25 RPM drop and I saw nothing.
Anyway, that's where it stands for now. I'll let y'all know if I learn anything
more. Thanks for all the information and advice.

Dave

[Ray Andraka]

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