Inflight Mag Check

[rsuncloud]

I have been reading about testing the ignition system under stress.
Has anyone performed an inflight mag check?
What procedure did you use?
Who would be afraid to try without a flight instructor or fighter jock at their side?

[Al Sayre]

Bad idea.  Leave the nonstandard procedures to the mechanic

[TheSkyHornet]

Absolutely an appropriate thing to do when flying.

If the engine is running rough on both magnetos, follow your normal procedures (carb heat, if applicable, fuel, etc.). No harm in switching the key over to the right mag, then left mag, to determine if there's a magneto shorting. If you're running off of a rough magneto and a good magneto on "both," you may be drawing power away from the engine. If it's a magneto problem, you're going to want to identify it and plan to get on the ground as soon as possible.

[Panzerbjorn]

Why on earth would you test your magnetos in flight?  That's what a run-up is for.  What if you did lose a magneto in flight?  Now, by you 'testing' it, you've just potentially shut off your engine.  In my mind, testing your magnetos in flight to see if you have a magneto problem ranks right up there with turning your fuel selector valve to 'OFF' to see if you're experiencing a fuel line blockage problem.

If the engine is running rough, and you suspect a magneto, just get it on the ground.  Your engine is running, even if it's rough, so leave it running.  Don't do something that could turn a problem into an emergency just so you can try and diagnose a problem.  Get it on the ground if you suspect a real problem, THEN mess around with the mags or whatever else you think might be the problem.

[Live2Learn]

Why on earth would you test your magnetos in flight?  That's what a run-up is for.  What if you did lose a magneto in flight?  Now, by you 'testing' it, you've just potentially shut off your engine.  In my mind, testing your magnetos in flight to see if you have a magneto problem ranks right up there with turning your fuel selector valve to 'OFF' to see if you're experiencing a fuel line blockage problem.

If the engine is running rough, and you suspect a magneto, just get it on the ground.  Your engine is running, even if it's rough, so leave it running.  Don't do something that could turn a problem into an emergency just so you can try and diagnose a problem.  Get it on the ground if you suspect a real problem, THEN mess around with the mags or whatever else you think might be the problem.

Sometimes "just getting on the ground" isn't a really immediate option.  For example, while crossing an extended area of really rough terrain, or when over water at the margin of glide (we don't fly SEL over wide bodies beyond glide from land w/o proper gear, right?).   A mag check to isolate the bad mag is good trouble shooting.  As a 'normal' practice, I don't think a mag check in the air is a good idea since one click past the second mag is one click too many.  In flight mag checks are definitely an abnormal procedure done for a specific purpose.  I don't think an inflight mag failure is specifically addressed in the POH for the aircraft I've flown.  It really is surprising how much power we lose in our piston engine aircraft when one of our mags fails.

[TheSkyHornet]

Why on earth would you test your magnetos in flight?  That's what a run-up is for.  What if you did lose a magneto in flight?  Now, by you 'testing' it, you've just potentially shut off your engine.  In my mind, testing your magnetos in flight to see if you have a magneto problem ranks right up there with turning your fuel selector valve to 'OFF' to see if you're experiencing a fuel line blockage problem.

If the engine is running rough, and you suspect a magneto, just get it on the ground.  Your engine is running, even if it's rough, so leave it running.  Don't do something that could turn a problem into an emergency just so you can try and diagnose a problem.  Get it on the ground if you suspect a real problem, THEN mess around with the mags or whatever else you think might be the problem.

Sometimes "just getting on the ground" isn't a really immediate option.  For example, while crossing an extended area of really rough terrain, or when over water at the margin of glide (we don't fly SEL over wide bodies beyond glide from land w/o proper gear, right?).   A mag check to isolate the bad mag is good trouble shooting.  As a 'normal' practice, I don't think a mag check in the air is a good idea since one click past the second mag is one click too many.  In flight mag checks are definitely an abnormal procedure done for a specific purpose. I don't think an inflight mag failure is specifically addressed in the POH for the aircraft I've flown.  It really is surprising how much power we lose in our piston engine aircraft when one of our mags fails.

This.

A mag check in the air should only be performed as a method of trouble-shooting. It is not a normal procedure. It would be an abnormal procedure, not an emergency procedure. So, this is not something you would do when your engine quits, but if you did a run-up on the ground, leaned the mixture, used carb heat (if applicable), and you're still getting a rough-running engine, check the mags, WHILE making your way to a nearby airport.

This is something I was taught when I was doing my flight training. I actually had this discussion with a coworker after reading this, who is a CFI and A&P. He, too, said it's totally fine, if not a "good idea" to do in a case such as this.

[Cliff_Chambliss]

I did that exactly one time - never again.   I was asked (tasked?) by the FBO owner to go pick up one of the rental aircraft that had been left (abandoned?) by a renter pilot approximately 100nm away.  The renter pilot said that after his last refueling stop and about 30 minutes into the flight the engine started running  real rough and shaking so he landed at the first airport he saw (a very smart move).  The next morning the owner contacted the FBO at the airfield and asked for a check of the aircraft.  A short while later they called and said except for a bit of water in the fuel they could find nothing wrong, and on runup both mags checked good, so off we went on a recovery mission.
  I did a really in depth preflight and runup and tested the throttle through its full range.  everything sounded good and felt good so we launched planning to fly formation back to homedrome.  About 15 minutes into the flight the engine started acting like it wanted to jump out of the airplane.  missing, backfires, lots of vibration.   Trying different power settings and mixture setting would result in brief improvement but only for moments.
  It was then I decided to "Test the mags"  One click right, no change.  AH HA, thought I, no change so it's the bad mag, lets go two clicks right and then total silence.  I mean that engine stopped stone cold dead.  now 2500 feet over the hills and forests of North Carolina is no place for a Cessna C-172 pretending to be a glider.  Back one click left - no change, back to both and I got some power back and I left everything alone until we got back.
aftermath:  A full inspection of the engine later revealed a "perfect storm" of several issues:   The failed mag was not approved for the aircraft.  Someone made a mistake and installed the wrong part.  However, the mag was perfect.
Other mag way out of time and way past overhaul.
Ignition switch:  This was the real demon:  Highly worn and suffered a heat breakdown which completely shutdown the good mag and would not always make contact when switching between mags.   Lesson learned:  Sometimes a mag issue may not be a mag issue at all and may be an ignition switch issue.  I was lucky that day because playing test pilot could have turned ugly.

[TheSkyHornet]

At the end of the day, you're responsible for the safety of the flight.

Make a decision. If it seemed like a reasonable choice and worked out well, good. If it seemed like a reasonable choice and didn't go so well, suck, doesn't it? If it was a terrible choice and you knew better, own up to it and be prepared for the consequences.

There's no "perfect" way to troubleshoot when you don't know what the problem is, especially if you haven't experienced it before. If you knew what the issue was before takeoff, you wouldn't have taken off.

[Live2Learn]

At the end of the day, you're responsible for the safety of the flight.

Make a decision. If it seemed like a reasonable choice and worked out well, good. If it seemed like a reasonable choice and didn't go so well, suck, doesn't it? If it was a terrible choice and you knew better, own up to it and be prepared for the consequences.

There's no "perfect" way to troubleshoot when you don't know what the problem is, especially if you haven't experienced it before. If you knew what the issue was before takeoff, you wouldn't have taken off.

Bingo.  But, let it be an 'informed' decision if at all possible.  If it doesn't help... return to the previous settings! 

IMHO, abnormal procedures are just that.  They're to address the unusual event that may (or may NOT) rise to an emergency -- and therefore ain't likely to be covered in the emergency checklists and procedures.

The first 'abnormal procedure' begins with:  If I don't know why the plane isn't behaving and previous bad behavior hasn't been thoroughly trouble shot... don't launch.

[Panzerbjorn]

Sometimes "just getting on the ground" isn't a really immediate option.  For example, while crossing an extended area of really rough terrain, or when over water at the margin of glide (we don't fly SEL over wide bodies beyond glide from land w/o proper gear, right?).  A mag check to isolate the bad mag is good trouble shooting.  As a 'normal' practice, I don't think a mag check in the air is a good idea since one click past the second mag is one click too many.  In flight mag checks are definitely an abnormal procedure done for a specific purpose.  I don't think an inflight mag failure is specifically addressed in the POH for the aircraft I've flown.  It really is surprising how much power we lose in our piston engine aircraft when one of our mags fails.

Okay, so in that kind of scenario, your instinct tells you that you should do something that could potentially shut your engine off?  That really is something that can lead to..."Wow, I was right!  It was the mag!" in the deafening silence as your engine quits just like how Cliff narrated in his story (Thank you, Cliff, for illustrating my point.)

I definitely agree that it should not be a 'normal' procedure under any circumstance.  But I have to admit that I can't think of a single scenario where it would make it a good idea in my mind to turn that key and see if my hunch that I've lost a mag actually happened.  What I WILL concede, however, is that with a windmilling prop, turning the key over to BOTH or the good mag should at least get the engine going again almost immediately.  But if I saw the pilot reach over and just decide to do an in-flight mag check with me in the right seat, there would be colorful metaphors aplenty.

[JeffDG]

On a risk-v-benefit basis:  What does determining that the problem is indeed a bad mag give you in flight?

OK, so you have a dead mag?  That's why you have 2.  You're going to be underpowered, but you already knew that before testing.  So, it really gives you no discernible benefit.

On the risk side, you have the scenario described above where it just might not come back, and then you've lost 100% of your power.

Risk of serious escalation of the problem vs. no discernible benefit...fairly easy call IMHO.

[Cliff_Chambliss]

Please let me add a couple items to my earlier posting.
* Before I even left our home field the maintainer at the off site location had examined the aircraft, checked the fuel and found some water which he drained out.  He an up the engine and checked the mags at various settings and said they sounded good.
* Before I took off in the "wounded" airplane I performed a through preflight including runup and mag check at various power settings and everything checked OK.
*  As soon as the engine began to misbehave we (me and the escort) diverted to the nearest airfield which was the one we left from.

My only mistake was in trying to play "test pilot" over very unfriendly terrain and not having a well thought out plan of action.

Defense:  I was still somewhat young, dumb, and invincible stationed at Ft Bragg, NC, and recently returned from Vietnam.

this was another item removed from the bag of luck and put into the bag of experience.

[JeffDG]

this was another item removed from the bag of luck and put into the bag of experience.

And, thanks to you sharing your experience, other pilots can add it to their bag of experience at no cost to their own personal luck bag!

[C/SrA Ravlin]

If you are experiencing a rough engine then the first thing you should do is turn on the carb heat... This will make the engine run rougher for a few minutes wether or not carb ice is present... If it is the engine will run extremely rough until the ice melts. The next thing would be to switch fuel tanks... Gas gauges are not the most reliable gauge in the cockpit. After that check your mixture. Try different mixtures and see if the engine runs better at a certain setting. After that try the magnetos... If one runs better or there is more than a 50-90rpm drop between the 2, then use the one with higher rpms and call for a priority landing if with atc or tell someone if you are on Unicom. Hope this helps.

[Panzerbjorn]

If you are experiencing a rough engine then the first thing you should do is turn on the carb heat... This will make the engine run rougher for a few minutes wether or not carb ice is present... If it is the engine will run extremely rough until the ice melts. The next thing would be to switch fuel tanks... Gas gauges are not the most reliable gauge in the cockpit. After that check your mixture. Try different mixtures and see if the engine runs better at a certain setting. After that try the magnetos... If one runs better or there is more than a 50-90rpm drop between the 2, then use the one with higher rpms and call for a priority landing if with atc or tell someone if you are on Unicom. Hope this helps.

The first suggestion is not really an option in fuel injected engines.

Adding carb heat does not "make the engine run rougher". You just simply have a slight loss of power and about a 50 RPM loss.  It's the carb ice that makes your engine run roughly, not the carb heat.

Cessnas normally operate drinking from both tanks at once.  You're only supposed to run on one if necessary and only in straight and level flight.  So switching the tank wouldn't really have all that big of an effect in a Cessna unless you have a fuel line blockage or water in your fuel.

Operating on a single mag and the engine is not showing signs of quitting on you is really more of a land as soon as practical situation than it is an emergency one and asking ATC for priority handling.  If you do that, you're more likely to be landing with a fire engine escort dispatched as a 'just in case' measure.  Also, more often than not, the difference in RPM drop has to do with somewhat fouled plugs, which can be remedied by leaning out the mixture, raising the CHT, causing the lead deposits to melt and clear the plug.  As far as your suggestion to "use the one with higher rpms...."  Please show me where that procedure is printed in ANY POH saying you should switch off a magneto in flight in accordance with your suggestion.

[Flying Pig]

If you are experiencing a rough engine then the first thing you should do is turn on the carb heat... This will make the engine run rougher for a few minutes wether or not carb ice is present... If it is the engine will run extremely rough until the ice melts. The next thing would be to switch fuel tanks... Gas gauges are not the most reliable gauge in the cockpit. After that check your mixture. Try different mixtures and see if the engine runs better at a certain setting. After that try the magnetos... If one runs better or there is more than a 50-90rpm drop between the 2, then use the one with higher rpms and call for a priority landing if with atc or tell someone if you are on Unicom. Hope this helps.

Switch to one mag and run it at higher RPMs? What airplane are you flying that has this listed as a procedure?

[PHall]

If you are experiencing a rough engine then the first thing you should do is turn on the carb heat... This will make the engine run rougher for a few minutes wether or not carb ice is present... If it is the engine will run extremely rough until the ice melts. The next thing would be to switch fuel tanks... Gas gauges are not the most reliable gauge in the cockpit. After that check your mixture. Try different mixtures and see if the engine runs better at a certain setting. After that try the magnetos... If one runs better or there is more than a 50-90rpm drop between the 2, then use the one with higher rpms and call for a priority landing if with atc or tell someone if you are on Unicom. Hope this helps.

Switch to one mag and run it at higher RPMs? What airplane are you flying that has this listed as a procedure?

Microsoft Flight Simulator.

[Panzerbjorn]

If you are experiencing a rough engine then the first thing you should do is turn on the carb heat... This will make the engine run rougher for a few minutes wether or not carb ice is present... If it is the engine will run extremely rough until the ice melts. The next thing would be to switch fuel tanks... Gas gauges are not the most reliable gauge in the cockpit. After that check your mixture. Try different mixtures and see if the engine runs better at a certain setting. After that try the magnetos... If one runs better or there is more than a 50-90rpm drop between the 2, then use the one with higher rpms and call for a priority landing if with atc or tell someone if you are on Unicom. Hope this helps.

Switch to one mag and run it at higher RPMs? What airplane are you flying that has this listed as a procedure?

Microsoft Flight Simulator.

Considering some of the habits I saw some cadets come home with this summer, I can't help but speculate if he learned this at a Powered Flight Academy,

[PHall]

If you are experiencing a rough engine then the first thing you should do is turn on the carb heat... This will make the engine run rougher for a few minutes wether or not carb ice is present... If it is the engine will run extremely rough until the ice melts. The next thing would be to switch fuel tanks... Gas gauges are not the most reliable gauge in the cockpit. After that check your mixture. Try different mixtures and see if the engine runs better at a certain setting. After that try the magnetos... If one runs better or there is more than a 50-90rpm drop between the 2, then use the one with higher rpms and call for a priority landing if with atc or tell someone if you are on Unicom. Hope this helps.

Switch to one mag and run it at higher RPMs? What airplane are you flying that has this listed as a procedure?

Microsoft Flight Simulator.

Considering some of the habits I saw some cadets come home with this summer, I can't help but speculate if he learned this at a Powered Flight Academy,

So you're saying that your cadets were taught incorrectly by CFI's.  Interesting...

[Panzerbjorn]

If you are experiencing a rough engine then the first thing you should do is turn on the carb heat... This will make the engine run rougher for a few minutes wether or not carb ice is present... If it is the engine will run extremely rough until the ice melts. The next thing would be to switch fuel tanks... Gas gauges are not the most reliable gauge in the cockpit. After that check your mixture. Try different mixtures and see if the engine runs better at a certain setting. After that try the magnetos... If one runs better or there is more than a 50-90rpm drop between the 2, then use the one with higher rpms and call for a priority landing if with atc or tell someone if you are on Unicom. Hope this helps.

Switch to one mag and run it at higher RPMs? What airplane are you flying that has this listed as a procedure?

Microsoft Flight Simulator.

Considering some of the habits I saw some cadets come home with this summer, I can't help but speculate if he learned this at a Powered Flight Academy,

So you're saying that your cadets were taught incorrectly by CFI's.  Interesting...

When I had one who was taught that standard procedure to start the engine is to pump the throttle furiously as you crank it AND come home with the habit of making a 45 degree bank from base to final.....I MIGHT be suggesting that there are some issues with what's being taught, yes.  Not in all cases because I'm not going to generalize, but clearly there's at least one CFI there that could benefit from some remedial training.