no RPM drop on mag check

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

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.

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

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

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 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.

"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)

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.

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

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."
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