Jump to content

LOP mag check


Bartman

Recommended Posts

Sorry if this is an elementary question, but I just want to make sure how to do, what to look for, and most importantly precautions.

I'm pretty sure I know what to look for, but what is the best altitude for my 201?  Should this be done at full throttle, or reduced throttle? What precautions to take?  My fear is that if there is a dead plug and rough engine, then when I go back to both mags could the unburned fuel damage the exhaust?  If I do find a problem should I throttle back prior to going back to both mags ?

Link to comment
Share on other sites

Doing a mag test doesn't dump raw fuel into the muffler... when done right...

turning both mags off does dump raw fuel into the muffler. The uncontrolled burn is hazardous to aged and worn hardware.

Popping sounds are like firecrackers in the muffler. Avoid any big kabooms... cracked mufflers and CO are a bad combination.

Similar to a mag test on the ground.  There is a lot to pay attention to including flying the plane...

Plan it out before flying and trying...

PP ideas only, not a mechanic or CFI...

Best regards,

-a-

Edited by carusoam
Link to comment
Share on other sites

The point is if you have a bad mag or plug, it will dump fuel into the muffler. It won't be a pleasant way to find out.


IMO this is a non-issue if you've simply got a bad plug as the small amount of fuel from one dead cylinder is quickly scavenged from the exhaust system. I can assure you that at least in a four-banger you'll be switching the ignition back to "both" quickly on instinct alone.

Now if your "mag" is dead there might be some greater potential for exhaust damage on restart (if the throttle is left open). But then I'm pretty sure I would recognize a dead mag long before turning the mag switch since all four of my EGT readings would be near 1,500 F.


Sent from my iPad using Tapatalk
  • Like 1
Link to comment
Share on other sites

1 hour ago, cnoe said:

Now if your "mag" is dead there might be some greater potential for exhaust damage on restart (if the throttle is left open). But then I'm pretty sure I would recognize a dead mag long before turning the mag switch since all four of my EGT readings would be near 1,500 F.

YOU would. But how about Mr-I-dont-do-ground-mag-checks? Isn't that just asking for trouble to find out you're missing a mag during an in-air mag check?

Link to comment
Share on other sites

39 minutes ago, 201er said:

YOU would. But how about Mr-I-dont-do-ground-mag-checks? Isn't that just asking for trouble to find out you're missing a mag during an in-air mag check?

You are misquoting what I said.  I stated that I rarely do full power run ups on the ground.  I do check the mags on the taxi out on every flight.  That's just as effective for checking a completely dead mag.

  • Like 1
Link to comment
Share on other sites

6 minutes ago, Greg_D said:

You are misquoting what I said.  I stated that I rarely do full power run ups on the ground.  I do check the mags on the taxi out on every flight.  That's just as effective for checking a completely dead mag.

Sorry I didn't mean to single you out, you just had the only written example that comes to mind, but I was just talking about that mentality in general. What do you mean by check during taxi?

Edited by 201er
Link to comment
Share on other sites

6 minutes ago, 201er said:

Sorry I didn't mean to single you out, you just had the only written example that comes to mind, but I was just talking about that mentality in general. What do you mean by check during taxi?

You can check ignition continuity while taxiing by selecting the each mag on the taxi out.  An EGT rise on all cylinders is what to look for.

If you go to the end of the thread you linked you will see the procedure I use and why/how I got there.  I didn't invent it on my own as some suggested.

Link to comment
Share on other sites

There was a time when multi-tasking was admired.  Run-up while taxiing helped keep stones out of the prop and air flowing across the cylinders while minimizing time spent on the ground....

Realistically, texting and driving has ended some of the more lousy multi-tasking ideas.  A full quality run-up requires set rpms and math to be done... taxiing isn't a really good time to for other things.

Some people are slightly less capable than others.  Watching a centerline and monitoring speed at the same time can be a difficult challenge.

Best regards,

-a-

Edited by carusoam
  • Like 1
Link to comment
Share on other sites

4 minutes ago, Greg_D said:

You can check ignition continuity while taxiing by selecting the each mag on the taxi out.  An EGT rise on all cylinders is what to look for.

If you go to the end of the thread you linked you will see the procedure I use and why/how I got there.  I didn't invent it on my own as some suggested.

How do you address the concern of gas getting into the exhaust during the LOP in-flight mag check if there is an undiscovered problem?

Link to comment
Share on other sites

The common undiscovered problem is a single wire or plug resistance challenge.  Having one problem per cylinder still burns most of the raw fuel.

Somebody posted a two plug problem in their engine recently.

A dead mag and a dual plug failure are pretty rare.  Eliminate the bad mag supplier and the bad plug supplier and the chance of putting raw fuel into the exhaust becomes drastically less.

i'm still doing the mag check while stationary on the ground...

Best regards,

-a-

 

Link to comment
Share on other sites

13 minutes ago, 201er said:

How do you address the concern of gas getting into the exhaust during the LOP in-flight mag check if there is an undiscovered problem?

It's pretty easy to determine if you have a completely dead mag before doing the check by looking at the engine monitor.  On the off chance that you switch to a bad mag, pull the throttle back to idle before switching back to the good mag.

Link to comment
Share on other sites

34 minutes ago, Greg_D said:

It's pretty easy to determine if you have a completely dead mag before doing the check by looking at the engine monitor.  On the off chance that you switch to a bad mag, pull the throttle back to idle before switching back to the good mag.

Im no expert and might not prevent a backfire but I'd probably be more inclined to pull the mixture to ICO then back to both mags on, then introduce fuel back to the engine. 

  • Like 1
Link to comment
Share on other sites

1 hour ago, Greg_D said:

It's pretty easy to determine if you have a completely dead mag before doing the check by looking at the engine monitor.  On the off chance that you switch to a bad mag, pull the throttle back to idle before switching back to the good mag.

 

51 minutes ago, triple8s said:

Im no expert and might not prevent a backfire but I'd probably be more inclined to pull the mixture to ICO then back to both mags on, then introduce fuel back to the engine. 

in the case of a dead mag, I would pull the mixture to ICO to prevent a backfire and also pull the throttle back to prevent a prop overspeed.

  • Like 1
Link to comment
Share on other sites

The procedure described by @Greg_D is exactly what is taught at the APS class. I follow the same procedure on all long cross country flights. There shouldn't be anything scary about doing mag checks in flight, at altitude, even if you discover a dead mag and the engine consequently stops. There also shouldn't be any fear of running a tank dry at altitude. If you know and understand how the engine works, how the fuel flows, etc, it's a very simple and safe procedure. I do it at the top of descent. In the C that was anywhere between 8K and 13K feet. In the 252 it's usually somewhere between 12K and 24K ft.

I would think that the pilot population would have a higher than average concentration of people driven by reason and science rather than fear, emotion, and superstition. After all it's physics and math that allows us to fly in the first place. But unfortunately many of us learned to fly from very talented instructors, but then never got the detail instructions on engine management.

The APS class is enjoyable for many reasons, and certainly not the least because of all the myths, old wives tales, and just bad information that is exposed, dispelled and otherwise debunked. Especially for those of us who are driven by reason and science, it's nice to actually know and understand... which is something that most CFI's can't provide... out of no fault of their own... they didn't get it from their CFI.

  • Like 1
Link to comment
Share on other sites

8 hours ago, carusoam said:

There was a time when multi-tasking was admired.  Run-up while taxiing helped keep stones out of the prop and air flowing across the cylinders while minimizing time spent on the ground....

Realistically, texting and driving has ended some of the more lousy multi-tasking ideas.  A full quality run-up requires set rpms and math to be done... taxiing isn't a really good time to for other things.

Some people are slightly less capable than others.  Watching a centerline and monitoring speed at the same time can be a difficult challenge.

Best regards,

-a-

I would hope that anyone flying a plane would have the multitasking skills to turn a key and quickly observe a graphic engine monitor while on a straight taxiway.

And I'll suggest that a static run up while full rich isn't a "full quality" run up at all.  In fact, of the four methods a pilot can use to check ignition system integrity, it comes in dead last.  If you don't have a modern engine monitor, or have one and don't know how to use it, then I'd suggest a leaned mag check.  That method comes in 3rd place.  The best is a LOP mag check at high power.

I've had two mag failure in 35 years of flying.  Both were preceded by a full rich static run up per the POH.  I caught the second failure on the ground while on the takeoff roll with an engine monitor.  Suspecting a fouled plug, I taxied back and did another run up which didn't show any problems.  The subsequent takeoff roll was also aborted.  It turned out that the magneto coil was breaking down, but didn't show any problems until full power was applied.  The key point here is that the full blown run up is not a guarantee that your mags are 100%.  A quick check on the way out is almost as beneficial and when coupled with a modern engine monitor and the knowledge to use it, is a better diagnostic tool.

I understand the folks who don't agree with this method.  Much of the resistance is because of the effect of primacy, as this is the way most were taught during training.  But please keep in mind that most CFIs know very little about engine management.  And most POHs were written or adapted from those written before the advent of graphic engine monitors.

  • Like 2
Link to comment
Share on other sites

22 hours ago, 201er said:

Sorry I didn't mean to single you out, you just had the only written example that comes to mind, but I was just talking about that mentality in general. What do you mean by check during taxi?

Actually I've stated numerous times that my run up consists of continuity checks only. Mags and prop are cycled at taxi power. Oddly the last two ignition issues we've had were or would have been no shows in a standard run up. My partner departed and noticed in climb that #2 EGT was >1600. In flight mag check showed it was missing on one cylinder. He landed did what he thought was some trouble shooting. The ground mag check was fine so he assumed the plug was cleared and departed. Same problem. Turns out the plug was dropping out above 2200rpm.

I once had mag coil failure in which the coil would short to the mag case but only above 1800rpm.

I don't tell anyone not to do 1700rpm ground run ups; whatever floats your boat. I just don't see tremendous utility in them. I now do a mag continuity check at shutdown as well. I learned that here, but I don't recall who from.

  • Like 1
Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.