Mag Drop

[Skates97]

So there is plenty of mention about the EGT readings rising during the mag check. I'm just new to this and watch the RPM drop as per the POH when doing my mag checks on run-up. The POH does't say anything about the EGT rise during the mag check, so what values should I be seeing in an O-360? And is it worth noting it at all since I only have an EGT coming off one cylinder, not a full monitor giving me readings from all four?

[MB65E]

Hi Skates, each engine will be different but maybe 20-45° hotter?? With an engine monitor you will see a trend that happens during the mag check. You should see something move on the bars/numbers during your run up. The one that doesn't change will likely be your suspect cylinder. 

Also, cold CHT followed by high EGT could mean a fuel injector is clogged or possibly a severely fouled plug. There are a few trends that I don't understand but most make sense. The EGTs rise because the fuel is still moving thru the cylinder but that plug that is being "tested" during the mag check goes cold and doesn't ignite the fuel. This causes that same fuel to be burned later in the exhaust. Thus elevating the exhaust temp slightly. I'm not sure you would see it on a single needle- non instant type probe.

I'll drift slightly with hopes for an explanation by someone that knows more on the subject-On the injected engines there are debates on how much fuel is being pushed thru injector lines during idle, cruise, or Take off. 

Many say the fuel lines on the upper deck only supply fuel to the engine during idle. The take off fuel comes mixed thru the intake tubes. Well...why the heck so I only notice a clogged injector during full throttle take off runs??

Big fan of post flight run-ups, but I've always done preflight mag checks. I've turned back to the hangar several times due to small ignition problems. 

Hope that helps,

-Matt

[Guest]

8 hours ago, glafaille said:

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.

 

Modern engine monitors record everything while the engine is running.  If there were an accident I'd think the FAA or a good lawyer could use the data to easily prove that a proper run up didn't take place.  

Clarence

[PTK]

16 hours ago, 201er said:

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

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.
 

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.

To be fair the op is not talking about fuel status. This is about ground run up.

As for "...your chance to check that the engine is running properly and calmly look at all gauges..." the most useful inormation is obtained if this is done prior to takeoff and depends on how you handle the engine just prior to take off. If you taxi onto the runway and rush to full power you have no chance to observe gauges and how engine is handling it. 

If, on the other hand, you slowly advance the throttle to about 50% power and pause to allow the engine the time it needs to adjust and respond you have the time to check gauges. If egt's coming up reasonably together and gauges good then slowly go to wot and roll continuing to scan gauges and egt's. The best time to check the engine is when you're asking it to produce full available power. And that's prior to takeoff.

 

[Marauder]

So there is plenty of mention about the EGT readings rising during the mag check. I'm just new to this and watch the RPM drop as per the POH when doing my mag checks on run-up. The POH does't say anything about the EGT rise during the mag check, so what values should I be seeing in an O-360? And is it worth noting it at all since I only have an EGT coming off one cylinder, not a full monitor giving me readings from all four?

 Thought I would chime in on your question as well. The actual values of the EGT during a runup are not as important as what they are doing. The reason they are not important is the placement of each EGT probe on the exhaust stack will vary the actual temp. 

Here is an example of a healthy runup. The red arrow points to the runup. All the EGTs are rising uniformly as the engine moved from one mag then to the other. I turned on the fuel flow so you can see the rise in fuel as I did my runup.

 

 

This next set of pictures are of a unhealthy runup. I thought I was experiencing a fouled plug but the plane was running intermittently rough. This set of pictures was all on the ground. I tried to "burn off" what I thought was a fouled plug. Fortunately, it didn't work and I cancelled the flight. I should also note that this data and a number of previous flights were looked at by SavvyAnalysis and confirmed prior flights had shown no impending failure. None...

 

This picture shows a red arrow pointing to a yellow EGT line (#2 cylinder) that is above the rest when running on one mag. This will show up on the cockpit display of the engine analyzer as an increased EGT above the rest. This is what you normally would see if a plug is fouled. Burn it off and the EGT returns to normal.

 

 

What was troubling was when I saw the high EGT on one mag, I thought it was a fouled plug (I don't get many because I lean on the ground). When a plug fouls I normally don't see much in the way of it running rough. I just see the elevated EGT. It was different this time because it was periodically running rough. Well, turns out the reason it was running rough was because the other plug in the cylinder was periodically dying as well. When both plugs weren't firing, the EGT for the cylinder will actually drop below the rest and the plane will run rough on 3 cylinders.

 

 

When I pulled both plugs on the #2 cylinder, I measured the resistance of each plug. They should be below ~ 1,000 ohms when they are healthy. Mike Busch I think says they should be pitched if they go above 5,000. One of the plugs was a bit over 37,000 ohms, the other read over 3,700,000 ohms -- in other words dead.

 

This is why I fly with an engine analyzer and always have. It was the first "upgrade" I did on my plane way back in the early 1990s. Hope this helps clarify your EGT question.

 

 

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[Bartman]

I always do a quick run up before departure. Never found a fouled plug of problem with a P lead. 

However, I did lose one mag on my A3B6D IO360 about 5 minutes after departure once and evidence was there, but didn't understand how to interpret the data. After setting cruise power I noticed all EGTs were high, and the CHTs were all a little lower, but I didn't understand why. After landing and and shutdown she had a couple of misfires, but would not start again. 

I now understand the reason for the cool CHTs, elevated EGTs , misfires on attempted restart, and why it should not restart. 

An expensive dual-mag exchange got me going in a couple of days

[jlunseth]

4 hours ago, Hyett6420 said:

No no and double no. Sorry. But there is ONLY ONE WAY you can make sure both your Mags have not changed their status since you last took off and that is with a mag check.  Just doing it at the beginning of the day shows you that when you did that mag check they worked.  It does not show you whether after three hours of spinning at thousands of rpm or what ever the magnetos have not failed in some way prior to your next flight.  

How many of you do a parking / shutdown mag check as per the POH?  I always do. It lets me know the prop is not live before I go to touch it to put baby away. 

I personally am dumbfounded by this thread.  I never for a million years beloved there were people who "advocated" not doing pre flight mag and other checks on the most important piece of hardware on their aircraft. 

Andrew

I respectfully disagree.  First, let me make clear what I disagree about.  I do not disagree that run-ups should be done and tanks should be sumped.  I do both.  What I disagree with is your taking issue with the "first flight of the day" practice.  So let me comment more thoroughly on what I do.

We all (I hope) go out on occasion and do consecutive landings, I hope to a full stop, and with a re-run of the "Before Takeoff" section of the checklist (which includes fuel check, flaps, mixture setting and some other things, but not run-up).  I have done this many times, I guess the most I did in one "sitting" was when I got my commercial.  There is a requirement to have done 10 takeoffs and landings at night at a towered airport with the tower in operation.  I realized that although I had done a ton of them at my home field, which is towered, I had never logged whether the tower was open or not (it closes at 10 pm and the airspace becomes "E").  So I went out one night and did 10 landings and takeoffs to a full stop, plus a couple extra for good measure.  It didn't take very long.  Then I made sure to log that the landings and takeoffs were while the tower was open.

In fact my logs are full of entries like that, with multiple practice landings in one "sitting" for various reasons - practicing power off 180's, practicing perfomance landings, practicing high crosswind landings with the speed up, or just plain practicing.  

No, I don't do any runups after the first one.

We also had a program in Minnesota called the Fly Minnesota program, where you could get a leather jacket if you visited 135 of Minnesota's 138 public airports.  That meant multiple landings in a single day, I think my record was 20, with a shutdown at each long enough to run into the terminal, find the stamp for the airport, stamp your book, and maybe use the restroom.  It was great landing practice, there were some very short (for a Mooney) fields, and some grass strips.

The only difference between category 1 and category 2 was that with the category 2 landings (the Fly Minnesota stuff), the engine was shut down, whereas with successive practice landings it is not.  With the category 2 landings the engine never cooled, and in fact most starts were hot starts.

I don't stop and do runups for each of those.  I have also flown with a good many highly qualified Mooney instructors, and they do not either.

There is a cost-benefit to a run-up.  Overtime, it grinds the prop.  If you have never owned a prop long enough to have to replace it, you think this is inconsequential.  But when it comes to prop overhaul time and you need to replace the prop with new because the leading edge is too ground up from picking up sand and pebbles during run-ups, then you understand.  In fact, I had a couple of very good, very experienced Mooney qualified instructors school me on that early on in my flying career - that run-ups should be as short as possible and not done when not necessary - and I did not believe them until I got to replace my first prop all chewed up.

If you let the aircraft sit and cool, that is when things change in the engine, and the mag that worked in the morning may decide not to participate in the afternoon.  Do a flight in the a.m. and then one in the p.m., then I will do a second run-up, same day.  But otherwise no.  Doing a quick stop in the terminal or successive full stop practice landings is no different as far as the engine is concerned than doing successive IFR practice approaches with a low approach and go around.  We don't do in-flight checks then either because it is not necessary.

BTW I also do a P lead check every shut down, and yes, I have found a broken P lead.  I immediately told the FBO where my plane is stored, we put signage on the plane so no line guy would move the prop by hand, and got it into maintenance pronto.  It is a flick of the ignition switch and worth doing every single time you stop.

[carusoam]

Richard,

A couple of notes regarding variations.  

1) Most Mooneys prior to the 90's didn't have engine monitors.  

2) Many Mooney's got them installed later in life so their install has some non-factory variation.

3) The significant level of education of the pilot community has taken a while to catch on.

4) knowing that the EGT rise occurs on all cylinders comes from advanced knowledge of how the fuel burns inside the cylinder, and some more outside the cylinder when running on one plug...

5) fortunately we still have the freedom to chose to run-up or not.

6) The Savvy folks have been really good at helping people understand some of the issues found in the data of their engine monitor.

7) Some MSers are still getting used to the idea of posting engine data while asking engine questions.

8) Some engine data for the run-up is slow to display what is going on.  Some engine monitors only take a data point every six seconds.  This is an eternity during the run-up...

9) To minimize the ground run-up issues it takes a good amount of mental planning to hit all the target rpm and switch and knob positions.  It takes effort to get the perfect JPI graph! 

10) Some engine monitors really include more data than ever. Rpm, oil pressure and oil temp too...

Thoughts for the new owner,

-a-

Edited February 4, 2017 by carusoam

[ArtVandelay]

I do a mag check just to warm up the engine, so skipping it saves no time. On a fuel stop I skip it, but always check the JPI for anomalies as I power up.

[Greg_D]

 

I understand the thinking behind those who insist  that a full blown run-up must be completed before each takeoff roll.  If you don't have an engine monitor, do and don't know how to use it, or If it makes you feel good, then go ahead and do it.  As far as that procedure being a good diagnostic test, I'll have to disagree.  At least when it is conducted the way most pilots are taught.  A full rich run-up at 1700 or 2000 RPMs, as called for in most POHs will only catch the most obvious errors.  A bad ignition switch, fouled or dead plug, or broken P-lead being good examples.  A better test is a LOP ground test of the mags, but that isn't covered in the POH.

I have had two magneto failures in the time I've been flying.  Both were preceded with full run-ups, just the way I was taught when I was a student pilot.  Neither of those run-ups detected either mag failure.  The first time it happened to me, I didn't have an engine monitor.  The engine seemed to be running rough when I was on the crosswind leg.  After turning downwind and at pattern altitude, I switched mags and found one of them running extremely rough.  I called the tower and did a precautionary landing.  An inspection of the left mag revealed some missing gear teeth, even thought the mag had just undergone a 500 hour inspection a few months earlier.  The second mag failure happened many years later, but this time I was flying with an engine monitor.  A quick glance at the monitor after I had applied full power was all that was needed to see there was a problem.  And I had just done the whole run-up routine per the POH.  I aborted the takeoff run and taxied back to the run-up area thinking maybe I had a fouled plug and I would burn it off.  The full rich POH run-up procedure didn't show anything wrong.  I even did it again, several times, just to make sure.  Thinking all was OK, I attempted another takeoff.  As soon as the engine got to full power on the new attempt, the problem came back and I aborted again.  This time, the diagnosis turned out to be a bad capacitor in the magneto.  It worked just fine at 2000 RPMs during the run-up though.

I saved those stories and intended to use them for questions when I attended the APS seminar in Ada, OK the following year.  I didn't even need to ask them.  Among many other things, the course covered how to read and diagnose engine monitors. We also saw two heavily instrumented engines in test cells that were operated under a variety of conditions, including mag checks.  The course also covered in detail why a full rich, ground mag check, at partial power was just about worthless for determining the health of an engine that was equipped with an good engine monitor and a pilot that knew how to read the monitor.  These last two points are key here.  The run-up procedures I was taught as a student and that are in most POHs were developed before engine monitors on small GA airplanes became popular.  To be sure, they are better than nothing.  But again, they will only detect the simplest of problems.  And every problem they detect can be detected with a good engine monitor.  This was all clearly demonstrated in the engine lab.

Back in the classroom, I shared my experience regarding the two mag failures previously mentioned here and asked for suggestions.  Their advice was simple for aircraft flown exclusively by a single pilot:  If you don't have an engine monitor, get one and learn how to use it.  Until then, complete the run-ups as taught, minus cycling the prop 3 times and turning into the wind.  If you do have an engine monitor and have learned how to use it, conduct a LOP mag check prior to descent.  If that checks good and if nothing changes on the airplane prior to the next flight, perform a quick mag grounding check on the taxi out and then use what you've learned here to check the engine monitor on the takeoff roll.

 

Incidentally, I'm sure nobody here has ever taken off with only one mag selected after dutifully performing their POH defined run-up:)

Edited February 5, 2017 by Greg_D

[Cyril Gibb]

I check mags and egt rise on leaning every engine start.  It only takes a few seconds.  I need to have rpm >1800 for my prop check, so I no longer do that.  I picked up a stone on the FIRST runup after a prop overhaul causing a ding that almost needed a new blade after dressing out.  The failure mode of a prop is full fine.  If that happens, I don't perceive a danger. I'll just land.

And yes, I do an oil pressure green, asi alive, mp ok and rpm 2700 on the takeoff roll.

[epsalant]

Is it possible to have an RPM drop (and EGT rise)testing left and right mags and still have a hot prop in the "OFF" position ?

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[Andy95W]

Yes, if the ignition switch were very screwed up.  The way to check this is to use the ignition switch to shut the engine down by going to the Off position.  The engine should stop, after which you can restart and shut it down with the mixture to idle cut off.

You could also momentarily go to Off then quickly back to Both without the engine completely dying.  The only problem with this is that you could get a back-fire in the muffler which could cause damage if done incorrectly.  There is actually an AD that requires this shut off check to be done on certain, older Bendix ignition switches.

Not sure which model of Mooney you own, but one of the pluses to the Shower-of-Sparks ignition on many of our airplanes is that it is fairly unlikely for the engine to fire with a hot mag, unlike an airplane that has impulse couplings.

[cnoe]

4 hours ago, epsalant said:

Is it possible to have an RPM drop (and EGT rise)testing left and right mags and still have a hot prop in the "OFF" position ?

Sent from my XT1254 using Tapatalk
 

With all due respect to Andy, for practical purposes I would answer "no".

If you get an RPM drop and corresponding mag drop on each switch position then it means that you've effectively turned "off" each mag at some point which would verify p-lead integrity. But...

I should agree with him though that it might be possible for the switch to fail such that one side could conceivably remain "on" when in the "both off" position. So in that case I'll agree with Andy that a "very screwed up" switch could still allow the engine to run in the "off" position.

It's an AD in my J and many others to turn the switch off briefly prior to shutdown to confirm its condition.

[Guest]

You're referring to AD 76-07-12.  It says to turn the ignition switch to the "extreme" off position to see if the engine stops.

http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAD.nsf/0/C14960A415D956BD86256E520053A53E?OpenDocument&Highlight=76-07-12

Clarence

[DonMuncy]

A P-lead check is part of my shut-down check list. Just before pulling the mixture to cutoff, I momentarily switch the ignition key to off, and back on. It the engine does not try to die, the P-lead is defective. 

[bonal]

As one without an analyzer I do a mag check before I take the runway every time it's what I do and will always do. I have a question about shutting down the engine. What is the reason for killing the motor with mixture cutoff rather than ignition. I always do the idle cutoff but was just curious the reason why.

[Tommy]

I follow Mike Busch's advice.

First flight of the day - full run up and paid close attention to the normalized EGTs

Subsequent flight - no mag test required. 

End of flight - Mag check.

Is this a sound advice? It seems to strike a good balance between safety and caring for your prop / engine? 

Insurance / legality-wise, is this a defensible action?

[DonMuncy]

Killing the engine with (lack of) mixture is a safety measure. If you kill it by shutting off the fuel, it is unlikely to have residual fuel available to start the engine if someone accidentally turns the prop, and the P-leads are defective.

[bonal]

Thanks Don, thinking about it I remember from my early ground school but not till you answered my question think I'll pull out my books and see what else I have forgotten 

[Skates97]

5 minutes ago, DonMuncy said:

Killing the engine with (lack of) mixture is a safety measure. If you kill it by shutting off the fuel, it is unlikely to have residual fuel available to start the engine if someone accidentally turns the prop, and the P-leads are defective.

This makes sense, but from what you had said earlier I had a follow-up question (or two).

I also always shut down per the POH using the mixture cut off. Some of you are doing what you're calling a P-Lead check prior to shut down. I'm not trying to argue, I'm just a new pilot trying to learn as much as I can. From the sounds of it, the P-Lead check, momentarily shutting the ignition switch off prior to cutting the mixture, shows that the P-Leads are not defective if the engine tries to die. Wouldn't that then mean that if you did try to kill the engine with the ignition switch that if the P-Lead was defective (allowing a possible accidental hand prop and start from residual fuel) the engine would actually not die? Likewise, if the engine does die from turning off the switch then wouldn't that show the P-Leads are not defective and therefore the engine wouldn't restart if there is residual fuel and someone turns the prop?

[DonMuncy]

It is "belt and suspenders". If the P-leads are good, you should be safe even with fuel available. If you shut it off with the mixture cut-off, you should be safe even with bad P-leads. But with both, you have redundancy.

Yes, when you turn off the ignition switch momentarily, you will know instantly if the engine starts to die. If it doesn't, you shut if off with the mixture and have the P-lead repaired before flight. (I would probably do the P-lead check a second time, thinking perhaps my ears were deceiving me.)

[Skates97]

Thanks, I appreciate the info. I think I will add that to my shut-down checklist.

[cnoe]

Thanks, I appreciate the info. I think I will add that to my shut-down checklist.

The AD Clarence and I refer to is only required every 100 hours but it's not a bad idea to do the check regularly. Just be sure to do it quickly so you don't load the exhaust with combustible gas and cause more damage than good. You'll know right away if the engine is shutting down.


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[kortopates]

7 hours ago, cnoe said:

I should agree with him though that it might be possible for the switch to fail such that one side could conceivably remain "on" when in the "both off" position. So in that case I'll agree with Andy that a "very screwed up" switch could still allow the engine to run in the "off" position.

That is the most common way our ignition switch will fail. Just some corrosion on a single contact for the Left or Right key position will cause the switch to prevent grounding the p-lead while all other positions function correctly - kinda minimally screwed up :). I have yet to see one that was screwed up in multiple positions at once.