Mag Drop

[Greg_D]

I rarely do run-ups on the ground.  One of the occasions that calls for one is when the airplane has sat for a while and/or work has been done in the engine compartment.  That occasion is what caused me to do one yesterday.  When switching to the right mag, I get no RPM drop and no EGT rise.  Switching to the left mag causes a noticeable mag drop (125 rpm) and EGT rise along with some engine roughness.  Any thoughts on troubleshooting and where I should l look 1st?

[DVA]

Hi Greg_D!

One of the reasons why you want to do a run up on the ground may be to find exactly what you just discovered. Probably a faulty P-lead; you are not grounding the mag, so it is likely hot all time. This is a dangerous condition for your safety on the ground, and a great reason to always do a mag check before you fly.

I’m not trying to be terse or anything like that, but why do you not do ground mag checks routinely?

[epsalant]

Hi Greg_D!
One of the reasons why you want to do a run up on the ground may be to find exactly what you just discovered. Probably a faulty P-lead; you are not grounding the mag, so it is likely hot all time. This is a dangerous condition for your safety on the ground, and a great reason to always do a mag check before you fly.
I’m not trying to be terse or anything like that, but why do you not do ground mag checks routinely?

I agree with you. Even more importantly how could you possibly know both mags are working if you don't do mag checks on the ground?

A few years ago I had engine run very rough on left mag during mag check. I thought it might be fouled spark plug. I increased throttle to burn off deposits. Things got better. Flew 30 NM, , got lunch. Mag check, engine shut off when mags set to left mag. No-go condition. Friend picked me up. Mechanic sent mags for overhaul. Without mag check I would have been flying on one mag.

Mag check confirms two things:. Both mags working when set on "both" and that neither will be on when turned to "off"

MAG CHECK IS CRITICAL PART OF EVERY PRE-TAKEOFF CHECK LIST.

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[Jozu]

I rarely do run-ups on the ground.  One of the occasions that calls for one is when the airplane has sat for a while and/or work has been done in the engine compartment.  That occasion is what caused me to do one yesterday.  When switching to the right mag, I get no RPM drop and no EGT rise.  Switching to the left mag causes a noticeable mag drop (125 rpm) and EGT rise along with some engine roughness.  Any thoughts on troubleshooting and where I should l look 1st?

Agreed. Do a run up. Every time you start the engine. But to answer your question, you most likely have a fouled plug. Lean the mixture and run at higher rpms then recheck the mags. Usually will clear in about a minute.


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[KSMooniac]

Many folks do an in-flight LOP mag check because it is a much better indicator of mag/wire/plug health vs a ground check at low power. A ground check is prudent after a maintenance action, though, as Greg appears to have done in this case.

This case is puzzling to me...seems to have a dead mag per that monitor indication but the engine kept running.

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[kortopates]

I would suspect the ignition switch isn't grounding the left mag when in the right position. You can confirm it with an ohm meter after disconnecting the p-lead. If so the bendix switch can be dissembled and cleaned up and you'll probably be good for a long time. 

[Andy95W]

^^^^^^^^^^^^^. What Paul said.

[pinerunner]

I agree that the in-flight mag check can give you better information. But a mag check on the ground is the only kind that will save me from taking off with only one mag working. That actually happened to me when I was a kid. Had recently soloed in a Piper Cub and was doing a bunch of take offs and landings. On final take off engine sounded different and I l cleared the trees by just ten feet instead of thirty feet (guesstimate; I was mostly looking ahead after all). I didn't think I needed to keep checking mags every time. After landing I did a mag check and one was dead. So that convinced me to always check mags before taking off. 

[MooneyBob]

Maybe I am missing something, but I have one really stupid question. What is the exact reason for not performing run up on the ground? I probably called sick when they talked about it in the ground school class. 

[Greg_D]

I guess I should have explained a little better.  I don't fly my plane using much of the conventional "wisdom" that was taught when I learned to fly about 35 years ago.  Yeah, I lean during the climb (target EGT), don't pull the power back to 25 squared after takeoff, etc, etc.  Things that would probably make most 300 hour CFIs cringe.

As Scott said, I do a mag check in cruise just before I begin my descent.  When LOP, that puts the greatest stress on the entire ignition system.  If there is a problem, it will show up there first.  I'm the only one who flies my airplane.  If I hangar it, and it doesn't get touched before I fly it again, it makes little sense to me to perform a complete ground run-up that creates hot spots on the engine and sucks junk into my prop before departing on the very next flight.  I do check the ignition switch before shutting down after every flight.

The problem isn't a broken P lead or fouled plugs.  I'll need to get a meter and check the ignition switch.  This in an Ovation by the way, so it's one of those high dollar switches, not the simple bendix device found in the older planes.

Edited February 4, 2017 by Greg_D

[PTK]

I don't always do a runup either. But I do check the ignition as I taxi using the EDM. Engine is running very lean as I lean aggressively on the ground. Increase the sensitivity on the edm by putting in normalize mode and cycle the mags watching to be sure all egt's do rise and none drop. This gives a very good indication of ignition health and properly firing plugs and if a mag is inop. I'll do a standard runup if I have time, again using the edm and watching egt's. I gave up rpm drop a long time ago. Also I do inflight lean mag check almost every flight before starting decent.

[201er]

1 hour ago, Greg_D said:

A: I guess I should have explained a little better.  I don't fly my plane using much of the conventional "wisdom" that was taught when I learned to fly about 35 years ago.  Yeah, I lean during the climb (target EGT), don't pull the power back to 25 squared after takeoff, etc, etc.  Things that would probably make most 300 hour CFIs cringe.

B: As Scott said, I do a mag check in cruise just before I begin my descent.  When LOP, that puts the greatest stress on the entire ignition system.  If there is a problem, it will show up there first.

May I point out that statement A has nothing to do with statement B? One does not conflict with the other (like 25 squared vs full bore does). There is no wisdom in not checking the mags before flight. There's no harm in doing a ground mag check and I can't imagine much benefit of skipping it. What do you save? 8 seconds?

Just because an in-air LOP mag check may be more diagnostic, it won't save you from taking off on a single mag in the first place! Why wouldn't you want to be made aware of the failure prior to leaving the ground rather than when you're already in the air and can't do a thing about it?

There are plenty of things we CANT effectively check until we're in the air and are taking a chance with. But this doesn't have to be one of them.

Edited February 4, 2017 by 201er

[Guest]

1 hour ago, Greg_D said:

I guess I should have explained a little better.  I don't fly my plane using much of the conventional "wisdom" that was taught when I learned to fly about 35 years ago.  Yeah, I lean during the climb (target EGT), don't pull the power back to 25 squared after takeoff, etc, etc.  Things that would probably make most 300 hour CFIs cringe.

As Scott said, I do a mag check in cruise just before I begin my descent.  When LOP, that puts the greatest stress on the entire ignition system.  If there is a problem, it will show up there first.  I'm the only one who flies my airplane.  If I hangar it, and it doesn't get touched before I fly it again, it makes little sense to me to perform a complete ground run-up that creates hot spots on the engine and sucks junk into my prop before departing on the very next flight.  I do check the ignition switch before shutting down after every flight.

The problem isn't a broken P lead or fouled plugs.  I'll need to get a meter and check the ignition switch.  This in an Ovation by the way, so it's one of those high dollar switches, not the simple bendix device found in the older planes.

The switch in your Ovation is a standard ignition switch fitted with a Medco style key.  The replacement from Mooney is quite expensive.  As other have posted it can be disassembled and cleaned.

Clarence

[Greg_D]

33 minutes ago, 201er said:

May I point out that statement A has nothing to do with statement B? One does not conflict with the other (like 25 squared vs full bore does). There is no wisdom in not checking the mags before flight. There's no harm in doing a ground mag check and I can't imagine much benefit of skipping it. What do you save? 8 seconds?

Just because an in-air LOP mag check may be more diagnostic, it won't save you from taking off on a single mag in the first place! Why wouldn't you want to be made aware of the failure prior to leaving the ground rather than when you're already in the air and can't do a thing about it?

There are plenty of things we CANT effectively check until we're in the air and are taking a chance with. But this doesn't have to be one of them.

A certainly has to do with B in this case.  Both are methods that were taught for many years.  The wisdom in not doing a fully blown run-up and mag check is as was stated before.  I checked the mags before descending on the previous flight.  Nobody has touched the airplane since shut down.  Everything with the ignition system should be as I left it after shutdown.  On the other hand, doing a run-up at near full power creates hot spots on the engine's cylinders because there is very little air flowing through the cowl during a static run-up.  Static run-ups tend to pull rocks and other debris into the prop.  It's not about saving time, it's more about being nice to the engine and prop.

Does doing a full run-up before takeoff ensure that a mag won't fail after the takeoff roll has started?  Of course it doesn't.  

[201er]

12 minutes ago, Greg_D said:

A certainly has to do with B in this case.  Both are methods that were taught for many years.  The wisdom in not doing a fully blown run-up and mag check is as was stated before.  I checked the mags before descending on the previous flight.  Nobody has touched the airplane since shut down.  Everything with the ignition system should be as I left it after shutdown.   

So would you not check your fuel tanks before flying just because you know there was gas in it last time?
 

12 minutes ago, Greg_D said:

 On the other hand, doing a run-up at near full power creates hot spots on the engine's cylinders because there is very little air flowing through the cowl during a static run-up.  Static run-ups tend to pull rocks and other debris into the prop.  It's not about saving time, it's more about being nice to the engine and prop.

I appreciate that it's not just for time saving. You may be right about those issues. But on the flipside, a run-up (not just mag check) is your chance to check that the engine is running properly and calmly look at all gauges. On the takeoff roll, you can miss something. This is a chance for a calm and deliberate assessment of the engine.

I know another darn good reason for the run up. Many a plane has fired up, taxied to the runway, took off, and then crashed because of water in the fuel. Taking a little extra time and running at higher power gives you a chance to pull some more fuel through the system, calmly assess the engine's operation BEFORE you are in the air, and it's a chance to get it warmed up some more before going full power as well.
 

12 minutes ago, Greg_D said:

Does doing a full run-up before takeoff ensure that a mag won't fail after the takeoff roll has started?  Of course it doesn't.  

This is definitely true. And I've experienced it. I did a run up but lost a mag within 1 minute of takeoff. However, having done a mag check on the ground prior to takeoff, I at least came out of it knowing that I wasn't a fool for taking off with a potentially knowable failure. I agree that it isn't likely, but I do think there still exists the potential that since the last in air mag check, a wire could have chafed or fallen off (especially upon touchdown), a mag could have gotten jerked and messed timing, a plug/wire failed, or the internals of the mag just happened to corrode/wear enough that during the current engine start they finally came to fail.

There are so many things we can't check or know during preflight as it is. I would at least like to do my part in checking the ones that are within my capability to reasonably check.

 

I've converted over to the full power climb, target EGT, LOP cruise, engine analyzer, no more paper charts, gear up asap crowd. But I just don't buy the arguments against a run up or mag check on the ground. It does not seem like a way to enhance safety or at least improve efficiency without any potential harm to safety.

Edited February 4, 2017 by 201er

[Hank]

I always do a full runup. Just like the preflight makes sure the airframe is ready to go, the runup verifies that the propellor and engine are ready to go. On those really cold winter mornings, it can take 2-3 seconds before the propellor moves the first time I pull it back, I'd really rather not have that happen in the air. . . 

[Marauder]

Interesting perspective on not doing a ground runup. As someone who did have a dual spark failure on one cylinder caught on a runup, I'm glad I didn't discover that as I was climbing into a 400 foot ceiling.

If you look at my engine data for the prior dozen flights, there were no indications of the pending failure either in flight or on the ground.



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[glafaille]

I know a guy that never sumps his tanks because he never puts water in them.  He figures if there is water from condensation, that it will reveal itself during the taxi.  I've known other folks that perform their own annuals, buy hardware from Home Depot, run avgas without an STC and fly IFR without a license or clearance from ATC.

One of the reasons to do things "by the book" is because by doing so you have an easily defended position when called into court after the accident.  Operating outside of approved procedures puts you in the position of explaining why you know more about the aircraft than the manufacturer.

If your POH says that a mag check should be done before take off, I encourage you to do one.

 

[jlunseth]

2 hours ago, pinerunner said:

I agree that the in-flight mag check can give you better information. But a mag check on the ground is the only kind that will save me from taking off with only one mag working. That actually happened to me when I was a kid. Had recently soloed in a Piper Cub and was doing a bunch of take offs and landings. On final take off engine sounded different and I l cleared the trees by just ten feet instead of thirty feet (guesstimate; I was mostly looking ahead after all). I didn't think I needed to keep checking mags every time. After landing I did a mag check and one was dead. So that convinced me to always check mags before taking off. 

+1. I always do a mag check on the ground before the first takeoff of the day.  On several occasions the mags have failed the test.    On three or four occasions I have been able to cure it by running the engine way lean for a period of time, the problem begin some kind of contamination on one or more plugs.  On one occasion, a Piper Warrior rental, there was a carbon build-up shorting the electrodes of one of the plugs, I went back to the FBO, maintenance cleaned the plugs, and off I went, safely. I have found failed mags during the ground runup twic, and in one instance just the afternoon before an instructor and I have been flying around in IMC doing several approaches with no problem.

The moral of the story is that takeoff is no time to find out there is a spark problem. 

Sometimes I won't do run-ups on later flights, same day.  If I am doing repeated landings and takeoffs it just takes too much time.

[jlunseth]

1 minute ago, glafaille said:

I know a guy that never sumps his tanks because he never puts water in them.  

One of the reasons to do things "by the book" is because by doing so you have an easily defended position when called into court after the accident.  Operating outside of approved procedures puts you in the position of explaining why you know more about the aircraft than the manufacturer.

 

Leaks around Mooney gas caps are a known issue.  You don't have to put water in the tanks, or bad gas, to wind up with water.  The cap sits in a well, if it rains and the O-ring leaks, the result is water in the tank.  I sumped about a pint out of a rental J once, that's how I know.

I am a lawyer, and frankly I could care less about what might happen later on in court.  What I care more about is living long enough to get to court in the first place.

[cbarry]

You may have this problem if you're not getting a mag drop.

[Hank]

^^^^^^ ouch!!

[kortopates]

I use the LOP Mag test in the air as well but I never skip on the run-up. I've managed to foul a plug in the descent once when I got a slam dunk approach and I've managed to lose a magneto on startup. If I shutdown, I perform a run-up. Its a simple cost vs benefit - I am not that much in a hurry. But as has been mentioned, my runup is based on being in normalize mode looking at EGTs and if I look what i see wrt to EGT rises I don't much care about RPM drops. I've caught other non-ignition issues from the runup including a partially blocked injector in a company aircraft not long ago.  If I am departing from a dirt strip, then I'll do a rolling run-up, preferably while back taxiing again to check for EGT rises. A lot can happen between starting the descent and the next takeoff and as 201er points out there is a lot of checklist items that should be checked before departure to avoid creating an unnecessary emergency.

Its really another topic, but instructing I see too many pilots that don't know how to properly use their analyzer for the runup and are only focused on looking for the RPM drop rather than the EGT rises and don't take advantage of normalize mode to ensure they don't miss an issue. At least its very rare these days to meet a pilot that doesn't believe they need an engine monitor; even though most of survived fine without one years ago. But it really helps opens up our situational awareness of how the engine is doing which translates into safety - if we use it.

[LANCECASPER]

One of the main reasons General Aviation has such a bad accident rate compared to the airlines is that many pilots don't follow proven procedures and they take short-cuts on their checklists. Part of the certification of an airplane is that the POH be approved by the FAA. This has Normal and Emergency procedures which should be followed if you want the safety that was built into the certification. 

[carusoam]

Did I mention the full run-up shows that both mags are working and all 12 spark plugs are working today!

If I had a blocked injector, that would most likely show as well.

I also test/ observe MP and FF during the roll...

Safe to say, these tests prove that air, fuel, and spark are getting to the engine, and oil is getting out to the prop...Today, before flight.  

I have a brand new prop. I'm not a big fan of running it on the ground. I'm less of a fan of flying a plane that wouldn't pass the run-up tests.

 

Greg, I like that you shared what you are seeing.  I look forward to hearing what isn't working properly, the switch or the ground wire.

How many hours do you have on the ignition switch?

How long ago was any maintenance done on the engine?  Like the annual....

Best regards,

-a-