Mag check on shutdown

[Big Pistons Forever]

One fact that almost never gets covered at FTU's is the fact that ignition switches are designed so that the key can only get removed if the switch is in the off position.

At the end of the flight when you go to turn off the Mags you should give the key a little tug at the Both, Right, Left positions as you rotate the key to the off position. If the key comes out before you get to the off position, the aircraft is unserviceable and maintenance should be alerted immediately

When I do the intial cockpit check as part of my walk around I always check the position of the ignition switch. I have never seen a switch that did not have the Mags actually off when the switch was was in the off position. I have however seen the mag switch in the left position with no key in the switch when I first got in the airplane.

I believe it had been like that for quite awhile and pilots had been simultaneously turning and pulling on the key when they got to the "Mags Off" part of the shut down check. The danger of doing this is if the key lock is worn the key can come out before the switch gets to the off position.

Just because the key is not in the switch doesn't mean the mags are not turned On !

[photofly]

You don't think that by tugging the key in the Both, Left and Right positions every flight you could possibly cause that self-same wear that allows the key to be removed in those positions?

[Big Pistons Forever]

You don't think that by tugging the key in the Both, Left and Right positions every flight you could possibly cause that self-same wear that allows the key to be removed in those positions?

Is that a question or a statement ?

[photofly]

It's a question. Is it a concern?

[Big Pistons Forever]

Well as a general statement you should always use the minimum force when moving any switch/control/surface/door/access panel etc etc

So with respect to the mag switch I do not see how a gentle pull is going to wear it out. IMO the benefits of finding out that the switch has an unsafe condition out ways the possible extra wear. In any case since almost nobody does this action I don't think I am going to cause a sudden increase in the profits of Bendix Corp

[photofly]

I'll sell my shares then :-/

[AirFrame]

Those prone-to-failure key switches were part of the reason I removed the one in my AC and replaced it with two stout, reliable, toggle switches. More visible from outside the cockpit, too.

[esp803]

I usually shut down with the mags so that when I horribly botch my docking I can quickly restart and pretend it was intentional... this technique has proven difficult on the garret engines...

E

[Colonel Sanders]

replaced it with two stout, reliable, toggle switches

Yes, on a homebuilt, replacing that complicated, expensive
bendix switch with two toggle switches and a $5 pushbutton
switch for the starter makes a lot of sense.

That's exactly what the Citabria/Decathlon has as stock
configuration:



Just remember, with this sort of manual configuration,
that you select LEFT mag only for start - right mag off
until the engine is running, to avoid kickbacks. Stock
Lycoming is impluse coupling on left mag only.

The maule (with TCM IO-360) has as stock configuration,
separate magneto and starter pushbutton switches. Because
both magnetos have impulse couplings on this Continental,
normal start is both magnetos on. Starts easier when
hot, too, than a IO- Lyc, probably because all spark plugs
are firing when cranking, instead of just half of them.

And let's face it, the bottom plugs fired by the impulse
coupled mag probably don't do much when you're
cranking, so you're really only trying to start on half
the cylinders (top spark plugs) with a Lyc.

Note to newbies: high tension leads from magnetos
are configured so that each magneto drives the TOP
spark plugs on one side of the engine, and the BOTTOM
spark plugs on the other side of the engine.

Otherwise, the magneto drop would be horrendous.

[Alberta_Canada]

OK CS, newbie question time based on your last note. With one mag sending spark to the top of one bank, and the bottom plugs of the other, does that make it a "waste spark" system like older cars where it fires 2 cylinders at the same time? (one cyl would be at the top of compression, and the other cyl on exhaust....being a wasted spark). The idea being that the first cylinder has spark jump from the electrode to the ground strap of the plug, then through the engine block to the 2nd spark plugs ground strap, across the gap to the electrode and then back through that wire for the complete circuit??.....or is it a different setup?

Also, to add, I thought I read somewhere that you don't mix bottom plugs with top plugs. Is it because of this setup where some of the plugs have spark that jump from the electrode to the ground strap, and others have a spark that jumps from the ground strap to the electrode so there has to be a slight design difference to accommodate this? Kind of like what various automotive manufacturers had on some of their 90s cars where you couldn't mix up the factory LH spark plugs with the RH spark plugs (different part numbers) unless you went with the replacement plugs which were all the same.

Maybe I'm off base with all of this??

[iflyforpie]

No, it's not a waste spark system. The spark plugs fire only once for the four strokes, not twice like in a modern car.

Also, not all mags drive a split of top and bottom plugs. The O-300 for example has one mag driving the top and one driving the bottom. The reason being is because they have a two degree stagger between timing so the flame fronts will reach the piston at the same time--so switching would have the opposite effect.

For spark plug erosion, typically we rotate the plugs from top to bottom and to the next cylinder in the firing order. The reason being is that the bottom plugs get dirtier than the tops and advancing them will change the polarity of the spark for even wear

The problem with doing it that way is you can change which magneto the plug is on--which is easy because of the various configurations (different firing orders, different HT lead setups). When magnetos are assembled, inspected, or rebuilt, there is no requirement for the rotor to be a certain polarity. So going from #1 top to #3 bottom on a Lycoming 4 will put the plug on a different mag but it could be the same polarity, resulting in uneven wear.

[ruddersup?]

How about mag checks in the air? I do it all the time, especially over unfriendly geography. It's great to know what could be happening.
Don't be afraid, but be prepared to throttle back to idle if there is a dead mag and then go back to both mags before applying power.
It's really handy when breaking in new engines/cylinders. Might as well land and clean the plugs before both mags start to run rough.
Same thing with engines that have been sitting for several years. Plug fowling can happen real fast.
Almost all pilots I've come in contact with are afraid to check the mags in flight. Is this not taught in flight schools?

[Colonel Sanders]

Actually, I know of an aircraft that had it's muffler
blown off in flight, by a pilot messing with the mags.

It structurally damaged the skin of the aircraft, and
it's been down for months now, because no one in
Canada knows how to fix it.

Likely the engine has been damaged by internal
corrosion during that time, too.

[rob-air]

There are really two things you are checking:

1) p-leads to the magneto, and
2) switch contacts

They actually have nothing to do with each other.

If you can select L, and see an RPM drop, then
go back to both, and then select R, and see an
RPM drop, then you know that your p-leads and
magnetos are grounding out perfectly via the
switch contacts for L and R, and that they are not
grounded out on BOTH.

However the nightmare scenario is that the switch
might be faulty in the OFF position. If you do test
the OFF position with the engine running, please do
it at absolute minimum RPM for a very brief moment.

The higher the RPM, and the longer you leave it in
the OFF position, the more unburned combustible
fuel-air mixture is going to be in the exhaust, which
is going to light off when you turn the mags back on.

I am sure that we have all witnessed student pilots
doing the click-click-BOOM method of magneto testing
during a warmup. No one ever listens, but if you ever
accidentally select OFF during a runup and kill the engine,
just let it die, and restart it after the gas evaporates out
of the exhaust.

Now onto AD's. Lycoming and Continental really don't
have much to do with accessories such as magnetos
and airframe items such as ignition switches.

The governing AD is from 1976 (reallly) before most
people here were born. It's still in effect, but its in the
misc section so no one ever complies with it, or even
knows it exists:

http://wwwapps3.tc.gc.ca/Saf-Sec-Sur/2/ ... -07-12.htm

Note that it specifies checking the switch every 100
hours. Checking it 100x as frequently as that (every
flight) is a little over the top, but if it makes everyone
feel like part of the team, and if it makes the FTU checklist
longer, hey, why not?

Thanks for the clarification, I kinda knew, but now I know.

[ruddersup?]

CS, you mean you do not check your mags in flight? You are afraid of blowing off the muffler? Have you ever had a mag failure in flight?
I was going to say have you ever lost a mag in flight but that's open for the proverbial answer, lol. How would you know you've had a failure? If you have had a mag failure in flight what would you do? How would you check it? What would be next flight destination?
If the engine runs rough at altitude and you find it clears up with the mixture - what are you going to do? Just leave it and dismiss it's not a mag?
I know the potential of a backfire but I've had a pilot taxi into my hangar door with perfectly good brakes. I even know of someone who landed with the gear up with perfectly functioning landing gear. Don't scare people away from really understanding what's going on with their engine. It's a great check when you have a rough mag on the ground and leaning it out clears it up, so why not check it in flight to feel that yes it was just a fowled plug and I'm good to continue.
Slow night here just making conversation. Oh, I've been doing it for over 50 years and never a backfire, but sure have discovered countless mag and plug problems to pass on to the mechanic, oops that's me too.

[Colonel Sanders]

Have you ever had a mag failure in flight?

Oh, yes. I am sure you will recall the AD about 25 years
ago on the yellow Bendix coils. I had one fail on an IFR flight
on a Continental IO-360 and never even noticed it - constant
speed prop picked up the slack, I guess. I noticed it the next
time I started and did a mag check. Both coils were replaced,
of course.

Much more recently, I had a red coil fail in a Slick magneto
on a Pitts S-2C. Replaced it with a new magneto.

A friend of mine - awesome airshow pilot - carries around spare
points, condenser, coil and buzz box so he can do magneto
repairs on the ramp when they fail.

An old "B" engineer friend of mine - not sure if you know what
that means - really does not like pilots to play with the mags
in flight.

[pelmet]

I had a mag problem in flight a couple of years ago. The rubber shock mounted instrument panel was vibrating more than normal as the primary indication. The mag was still working when I checked its operation but very rough when operating alone. After landing during another runup it was like the previous runup.....operated just fine. Was only malfunctioning at high power.

The only complete failure I can remember was on an Aztec many years back. There was an occasional strange vibration for a few seconds once in a while during cruise. A ground check showed the one mag to be dead.

[glorifieddriver]

I did not read all the posts, but a personal recommendation of mine is to simply repeat the "Mag Check" before shut down, ground the left, then the right, if they individually turn off and you can register the RPM drop, then you are just as safe as doing the old school way of going to the OFF position for a few moments. This is a safe and reliable check in my opinion.

[cgzro]

Was only malfunctioning at high power.

Had that same problem last summer so rather than overhaul we took mag apart and replaced cheap bits and retried after each. Turned out not surprisingky to be the capacitor. High RPM, higer voltage so it jumped the capacitor gap to ground.

Theoretically you could diagnose and replace the capacitor without removing the mag.

[ahramin]

I did not read all the posts, but a personal recommendation of mine is to simply repeat the "Mag Check" before shut down, ground the left, then the right, if they individually turn off and you can register the RPM drop, then you are just as safe as doing the old school way of going to the OFF position for a few moments. This is a safe and reliable check in my opinion.

If you think about this, or read the thread before posting, you'd realize that your version of the check is completely useless.

[Big Pistons Forever]

I did not read all the posts, but a personal recommendation of mine is to simply repeat the "Mag Check" before shut down, ground the left, then the right, if they individually turn off and you can register the RPM drop, then you are just as safe as doing the old school way of going to the OFF position for a few moments. This is a safe and reliable check in my opinion.

If you think about this, or read the thread before posting, you'd realize that your version of the check is completely useless.

Ahrim

If with the engine at idle you switch the mag to left there will still be a noticeable drop when the mag stops producing power as it goes to ground. Do the same thing by selecting the right mag will also produce a noticeable drop when that mag goes to ground. You have now demonstrated that both mags can be grounded. While it is possible for a mag switch to work in the left right and both position but not ground out both mags in the off position the possibility of this exact failure, the only thing that this procedure does not check, is very small.

This is the procedure I teach because I have observed too many inexperienced pilots screwing up the "mags momentarily to off" method. Any significant delay in turning the switch on will result in a backfire which will invariable destroy the muffler.

So please explain to me why this procedure is useless, as I don't get where you are coming from

[Colonel Sanders]

It's not useless. He has proven that:

- both magnetos are functioning
- both magnetos have functional p-leads

As BPF says, the only thing he didn't check
is the internal contact of the Bendix switch
in the OFF position, which the ancient AD
says to check once a year.

Do you pull your exhaust shrouds off after
every flight, too? They're another annual
AD, which IMHO is a heckuva lot more
important.

If you're going to check all annual AD's every
flight, you should really do that one, too.

[ahramin]

The reason it's useless is because you have already checked all that during the runup. The only thing you haven't checked is the off position of the ignition switch, which as had been posted ad nauseum, is an annual AD.

[photofly]

There's clearly some value in detecting an ungrounded mag sooner rather than later.

So to politely disagree, it's not useless to test at the end of a flight. if a p-lead drops off, it's more likely to do so in flight than on the ground. In which case waiting until the next runup to test the grounding leaves the un-grounded mag undetected for much longer.

[pelmet]

Actually, I know of an aircraft that had it's muffler
blown off in flight, by a pilot messing with the mags.

Messing with the mags? What exactly happened?