both...left...both...right...both...Why?

[dtuuri]

Exactly, it's still burning, but it still burns completely as long as the mixture was in the nose of the LEL-UEL range. The plugs don't know the difference.

A mechanic notices the difference when looking at two plugs from the same cylinder, one that's been firing and one that's not been, so I don't know what you're saying either. I'm betting, too, that an EPA check would show the gas mixture doesn't burn as completely with one mag turned off.

dtuuri

 

[Henning]

A mechanic notices the difference when looking at two plugs from the same cylinder, one that's been firing and one that's not been, so I don't know what you're saying either. I'm betting, too, that an EPA check would show the gas mixture doesn't burn as completely with one mag turned off.

dtuuri

Not to any point where it will cause a failed mag check.

 

[dtuuri]

Not to any point where it will cause a failed mag check.

You don't know that to be true all the time, but even if it's true most of the time--so what? It just takes a few seconds on Both to mitigate any possible risk. Seems like a good investment to me.

dtuuri

 

[Clark1961]

you two should get a room...who knows, maybe you'll like it...stranger things have happened in the world...

 

[Henning]

You don't know that to be true all the time, but even if it's true most of the time--so what? It just takes a few seconds on Both to mitigate any possible risk. Seems like a good investment to me.

dtuuri

Yes, I do know that all the time, because I know exactly what makes a plug degrade, and I know what that causes from a running aspect, and if you would have an issue you will not pass the test whichever way you choose to do it.

It makes ZERO, NADA, NONE, ZIP, difference going through the extra procedure. Insisting it be done outside all evidence to the contrary that it is unnecessary is just OCD.

 

[dtuuri]

Yes, I do know that all the time, because I know exactly what makes a plug degrade, and I know what that causes from a running aspect, and if you would have an issue you will not pass the test whichever way you choose to do it.

It makes ZERO, NADA, NONE, ZIP, difference going through the extra procedure. Insisting it be done outside all evidence to the contrary that it is unnecessary is just OCD.

If the mag check calls for a maximum drop of xxx rpm on either mag and the mags are known to be equally at that limit under the ambient conditions that prevail, the second mag will fail the test without going back to Both because the plugs are less efficient. In theory, anyway.

You are the one insisting on a change in operating practice, contrary to most manufacturers' instructions and historical practice, based on ZIP. OTOH, if you have some references pidgeon-holed, show 'em.

dtuuri

 

[Henning]

If the mag check calls for a maximum drop of xxx rpm on either mag and the mags are known to be equally at that limit under the ambient conditions that prevail, the second mag will fail the test without going back to Both because the plugs are less efficient. In theory, anyway.

You are the one insisting on a change in operating practice, contrary to most manufacturers' instructions and historical practice, based on ZIP. OTOH, if you have some references pidgeon-holed, show 'em.

dtuuri

Your theory is incorrect as there is no degraded state on the plug of any affect if the mags are ok. I don't insist anyone change their procedure, never have. I answered the question in the title. The reason the procedure is written that way is because the procedure was written when planes used toggle switches rather than rotary switches, and no one has spent the money and effort required to make a documentation change because it's not needed. You will notice that it is not called out that way in many POHs.

The only difference there is is that the switch will wear out faster, that's it, the only thing.

 

[ElPaso Pilot]

Interesting peeing match.

Ya know, it ain't that big a deal, and either method will work, but Continental and Lycoming each detail returning to "both" between L & R checks, and include plug clearing and/or fouling references.

http://www.tcmlink.com/pdf2/maintenancemanuals/x30041/x30041.pdf

CONTINENTAL® AIRCRAFT ENGINE OPERATOR’S MANUAL

5. Increase engine speed to 1700 RPM only long enough to perform
the following checks:

a. Check Magnetos: Move the ignition switch first to "R" position
and note engine RPM, then move switch back to "BOTH" position to
clear the other set of spark plugs. Then move the switch to "L"
position and note RPM. The difference between the two magnetos
operated individually should not differ more than 50 RPM with a
maximum drop for either magneto of 150 RPM. Observe engine for
excessive roughness during this check.​

http://www.lycoming.com/Portals/0/t...ns/SI 1132B (06-18-2010)/Magneto Drop-Off.pdf

Service Instruction No. 1132B

Switch from both magnetos to one and note the drop-off; return to BOTH until the engine
regains speed and switch to the other magneto and note the drop-off, then return to BOTH.
Drop-off must not exceed 175 RPM and must not exceed 50 RPM between magnetos.
Smooth operation of the engine but with a drop-off that exceeds the normal specification of
175 RPM is usually a sign of propeller load condition at a rich mixture.
...
For both fixed and rotor wing installations, do not operate on a single magneto for too long a period;
a few seconds is usually sufficient to check drop-off and to minimize plug fouling.

 

[Henning]

Interesting peeing match.

Ya know, it ain't that big a deal, and either method will work, but Continental and Lycoming each detail returning to "both" between L & R checks, and include plug clearing and/or fouling references.

http://www.tcmlink.com/pdf2/maintenancemanuals/x30041/x30041.pdf


http://www.lycoming.com/Portals/0/t...ns/SI 1132B (06-18-2010)/Magneto Drop-Off.pdf

Both publications were written at a time when mags were primarily (as many planes still are) on toggle switches. There it makes sense to state to return to both as the engine will shut down if you don't.

 

[ElPaso Pilot]

Both publications were written at a time when mags were primarily (as many planes still are) on toggle switches. There it makes sense to state to return to both as the engine will shut down if you don't.

No. The publications are 2010 and 2011 revisions, and refer to rotary switches in both text and wiring diagrams.

The quote is "then move switch back to "BOTH" position to
clear the other set of spark plugs", not to prevent engine shut down.

 

[dtuuri]

No. The publications are 2010 and 2011 revisions, and refer to rotary switches in both text and wiring diagrams.

The quote is "then move switch back to "BOTH" position to
clear the other set of spark plugs", not to prevent engine shut down.

Interesting diagram. Isn't the switch shown in the Off position? Doesn't that leave the P-leads ungrounded, i.e., "hot"?

dtuuri

 

[ElPaso Pilot]

Interesting diagram. Isn't the switch shown in the Off position? Doesn't that leave the P-leads ungrounded, i.e., "hot"?
dtuuri

The diagram appears to be a slightly effed up conceptual representation.

Electrically the arrow is in the BOTH position, and rotating clockwise from there is R, L, and OFF. The start circuit is not present.

 

[dtuuri]

The diagram appears to be a slightly effed up conceptual representation.

Electrically the arrow is in the BOTH position, and rotating clockwise from there is R, L, and OFF. The start circuit is not present.

Good on you! I could have stared at that for a month and never seen it.

dtuuri

 

[Henning]

The diagram appears to be a slightly effed up conceptual representation.

Electrically the arrow is in the BOTH position, and rotating clockwise from there is R, L, and OFF. The start circuit is not present.

Not all rotary switches have the start circuit in them and use a separate push button switch. Nobody at Lyc or Cont cared enough about the issue to get the drawing right.

 

[asicer]

Interesting diagram. Isn't the switch shown in the Off position? Doesn't that leave the P-leads ungrounded, i.e., "hot"?

Not if you're looking at the backside of the switch.

 

[asicer]

The diagram appears to be a slightly effed up conceptual representation.

Electrically the arrow is in the BOTH position, and rotating clockwise from there is R, L, and OFF. The start circuit is not present.

If you are trying to connect electrical leads to the backside of a switch, it helps if the diagram matches what are looking at. If that is the case here, then rotating clockwise would be OFF-L-R-BOTH.

 

[dtuuri]

If you are trying to connect electrical leads to the backside of a switch, it helps if the diagram matches what are looking at. If that is the case here, then rotating clockwise would be OFF-L-R-BOTH.

Brilliant! So then, why isn't the right mag on the left?

dtuuri

 

[ElPaso Pilot]

If you are trying to connect electrical leads to the backside of a switch, it helps if the diagram matches what are looking at. If that is the case here, then rotating clockwise would be OFF-L-R-BOTH.

Thought about that too, but that's not what the diagram shows. The labels on the mags would still tag it as OFF-R-L-BOTH. Hence my conceptual note.

 

[asicer]

Thought about that too, but that's not what the diagram shows. The labels on the mags would still tag it as OFF-R-L-BOTH. Hence my conceptual note.

Straight up grounds both mags, therefore that is the "OFF" position. Straight down opens both mags, hence "ON". One click from the "OFF" position grounds the mag labeled "RIGHT", so that's the "LEFT" position. Thus, the sequence is OFF-L-R-BOTH.

 

[bluerooster]

I had a rather unique experience during mag check once.
Airplane was '56 172. Mag check was precisely 50 rpm drop on both mags. (I did the both-r-both-l-both thing) Cool! but as I took off it seemed a tad sluggish. I flew around a while, landed, and as per habit, just before shutdown, I switched the mags to "off", and the engine kept running. Hmmm, P lead must be loose. I ge ahold of the A&P, who found 2 things wrong, niether of which were a broken/loose P Lead, But a faulty switch, and a bad mag.

 

[denverpilot]

Not all rotary switches have the start circuit in them and use a separate push button switch. Nobody at Lyc or Cont cared enough about the issue to get the drawing right.

Would you, knowing that pilots will do whatever they want while still arguing about it, anyway?

 

[Dan Thomas]

, I switched the mags to "off", and the engine kept running. Hmmm, P lead must be loose. I ge ahold of the A&P, who found 2 things wrong, niether of which were a broken/loose P Lead, But a faulty switch, and a bad mag.

The faulty switch was probably the Bendix against which an AD apllies.

There are thousands of ignition switches out there that are in violation of ADs, I'm sure. I find them all the time. Doing an AD search on the FAA database catches the model-specific ADs but doesn't find a whole mess of appliance ADs on stuff that can be found in many different makes and models. Ignition switches, wheels and brakes, vacuum pumps, magnetos, many instruments (alitmeters in particular), seat belts, radios, engines, propellers, lots of stuff. I use a long list of stuff to check I made up when going through an airplane for the first time.

Dan