Mag checks

[newtrainer]

Hey everyone - On a lesson the other day and my instructor set rpm at 1200 and checked both right and left mags seperatly?

What is the reason for this?

Thanks

[Lister Noble]

I think you should ask him as he should have explained why!
Lister:hmm:

[newtrainer]

This is true! However when trying to take it all in things like this slip me by!

Thanks

[MikeJ]

newtrainer - DUMP YOUR INSTRUCTOR!!!

If it was not clearly explained that he was testing the two independent ignition systems, DUMP YOUR INSTRUCTOR.

MikeJ

[FlyingForFun]

Hey, steady on.....

In the very early lessons, there are a whole load of things which my students will see, and even do. There's the pre-flight, startup, shutdown, power checks, taxying, refuelling procedures, r/t, booking out, signing out..... and that's before we even get on to the contents of the actual lesson itself!

I try to get my students involved in as many aspects of the flying as early as possible, and teach them as much as they can take in. But each of the early lessons has a very specific aim, which should be explained to the student at the start of the pre-flight briefing. To explain every single thing which happens before, during and after the flight would not only be extremely confusing for the student, but would also detract from the main purpose of the exercise. So I will show the student what I'm doing, or tell him how to do it if it's something I want him to get into the habit of doing, but I don't explain every single thing to my students from the very beginning.

On the other hand, if someone asks me a question, I will do my very best to answer it. And as students progress through the course, they learn more and more about what's going on.

So, newtrainer, do not rush to dump your instructor! Instead, ask him the question, and if he does not know how to answer, then you can consider dumping him!

But, since no one else has answered the question:

Each aircraft has a slightly different checklist of things to be done, and there would typically be up to three times that the magnetos are checked.

The most important of these is done at high power, to check that each of the two ignition systems will function correctly on its own. Since you said that the check was done at low power (1200 rpm), this is not what you saw.

There are two times that the check could be done at low power; either just after starting the engine, or just before stopping the engine. Just after starting the engine, the magnetos are checked for a similar reason as the check at high power - to make sure everything is working correctly. The high power check is the main one which is most likely to highlight a problem, but by checking them before taxying you can save yourself the trouble of starting the high power checks if there is a problem. I've always been a little unsure of the importance of these checks, since you aren't really checking anything you're not going to check more thoroughly in a few minutes time, but they are present in many checklists.

Alternatively, you may have seen the check just before stopping the engine. Here, the reason for the check is two-fold; first of all, if there is a problem, it enables you to get it fixed before the next person goes flying (which saves the next person from having to cancel the flight because he didn't know there was a problem). And secondly (and most importantly), because of the way the magnetos work, a broken wire could actually cause the magneto to be on even after you switch it off, which will result in a possibility of the engine firing if someone turns the prop. If the aircraft is going to be manoevred around by hand once it is parked, it is clearly vital that you know if there is a problem which might result in this being dangerous.

Hope that helps, but as others have said, speak to your instructor who will know your aircraft type and checklist inside out.

FFF
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[GusHoneybun]

Good Reply FFF,

The high power mag check is to check that each independent ignition system is running OK. A mag drop of more than a few hundred RPM will indicate that one of the spark plugs connected to that magento is not firing correctly, and the engine will appear to be shaking itself loose from the mounts.

The low power magneto check is to check that there are no short circuits which will leave a magneto live even though the ignition switch is off. I believe this is equally important to check at both the beginning and end of flight. I always check after start as it satisfies myself that the magnetos can be isolated if any subsequent problem occurs.

[Lister Noble]

newtrainer,
I am also a student,nearing the end of my course.
Do you have the appropriate checklist for the aircraft you are traning in?
It is a series of lists for all the actions for each part of the flight from walk around checks,start up through to shutting down at end of the flight.
You really do need this as there is so much to see and do,at the school where I am learning it is almost the first thing we buy,they are around £5.
This may help a lot,also you can read it at home and then ask even more questions!
Good luck in your training.
Lister:)

[newtrainer]

Hey guys - Great stuff, cheers for clearing that up!

I shall invest in a checklist of my own and sit in a small dark room with a strong light above my head.... :ok:

[Them thar hills]

GusHB
You mean OPEN circuits !
The mag switches rely on earthing the mag primary to kill the spark. But you knew that !
TTH

[SlipSlider]

A mag drop of more than a few hundred RPM will indicate that one of the spark plugs connected to that magento is not firing correctly, and the engine will appear to be shaking itself loose from the mounts.

Can just one plug out really make that amount of difference?

Guess who this afternoon had ZERO mag drop on left mag circuit at 1800 rpm and a GINORMOUS drop on right, with banging and misfiring ..... plugs come out tomorrow!

Slip

[Shaggy Sheep Driver]

Zero mag drop indicates that the mag switch isn't grounding out the appropriate mag. Even with everything perfect, switching off one ignition system should produce a small mag drop, since the fuel burn in those relatively large aircraft engine cylinders will not be as complete with one mag grounded as it is with both plugs up and firing.

So mag drop checks have two fail modes - excessive drop, and no drop.

[foxmoth]

Zero mag drop can also indicate that the engine is effectively only running on that side mag! This sounds like the case - imagine one mag not working at all, turning off or grounding that mag (i.e. selecting just the good one) will have no effect and you will get no mag drop as the drop is caused by reduction in engine efficiency from using just one set of plugs which in this case is happening already.:hmm: