Doing Mag Checks...Properly

[cptn2016]

I was thinking about where to post this, opted for the training forum. As a student pilot I really did not have a clue about the difference between the type of mag checks, or what I was even looking for. It was a checklist item and I did it faithfully but with no real understanding. RPM drop <125, less than 50 RPM difference between the two mags, etc...

Now, I am in no way qualified to judge whether or not this article is 100% correct, so don't shoot the messenger. But at least now I have a much clearer idea about what I'm doing and what I'm looking for.

http://www.caa.govt.nz/Publications/Vec ... rApr08.pdf

Additional advice and discussion, as always, is appreciated.

[Big Pistons Forever]

Personally I do not teach either the live mag cut or the dead mag cut. If you are a little slow switching the mags back on the engine will backfire blowing the baffles out of the 800 to 1000 dollar muffler. The after start up dead check only shows a totally dead mag, which is not very common and should have been discovered on the last flight.

As for the shutdown check I teach my students to set 1000 RPM and cycle the mags Both/Left/Both/Right/Both and watch and listen for each mag to cut out (ie watch for the RPM drop and listen for the change in engine sounds. The mag is never turned off with the engine running.

In addition 2 facts that never seem to get taught at flight schools.

1) No mag drop means the mag is not connected to the switch and means that the mag cannot be checked rendering the aircraft unsevicable, and

2) The key is blocked from being removed from the switch at all postions except off. So on the shut down check the key should be given a gentle tug at the left and right mag positions and if the key comes out the switch is defective and the aricraft should be grounded until a new switch is installed

Finally and most important When you touch a prop treat it like the mags were live

[sidestick stirrer]

For the first time ever and thus with regret, I am going to have to disagree with BPF.
I always operated as he suggests until encountering an airplane that would indeed run on just one magneto when switched to Left or Right but continued to run when switched off.
Curiosity made me dismantle the old switch after it was replaced. The numerous little springs and thin metal, triangular-shaped cups that sweep across the contacts appeared to give ample opportunity for the switch to malfunction. In the case of this switch, one of the tiny springs had popped out and was floating around loose and looked like it had been burnt and subject to some arcing.
While BPF is correct that a live-mag check done improperly can indeed blow the muffler off the engine, this only happens if the check is done at a high-enough rpm or for a long-enough period that the muffler gets filled with a combustible mixture, ready to be ignited by the first exhaust valve to allow a burning charge out of a cylinder when the magnetos are again powered. Therefore, the check is done quickly, at idle rpm.
Much like a previous thread regarding checking the mixture control during the runup before takeoff, I used to just lean until I got either a rpm rise or drop, never checking that the engine would still shutdown when the mixture was pulled all the way out. I do now as I want to ensure that it really will die as it is the first action in the event of an inflight engine fire.
One follow-on post that suggested it was unnecessary if the engine had been shut down that way after the previous flight, should be viewed as logic that is the beginning of a slippery slope, culminating in never checking anything before flying.
Just saying...

[Instructor_Mike]

I agree with sidestick. If the end of flight live (off) check is done properly at low rpm etc, the risk of backfire is low and the benifit of knowing that that point in the switch works is important. my $0.02.

Always still treat as live though, even with MICO.

[Colonel Sanders]

Ah, the old ignition switch discussion.

Personally, I am in the camp of just checking for a mag
drop on L and R, rather then "off" because of the unburned
gas that's pumped into the exhaust, then lit off when you
turn the mags back on again. It must be done at the slowest
possible RPM to minimize the quantity of unburned fuel that
is pumped into your very expensive exhaust system.

Yes, the switch can fail internally so that a mag is still hot
when the switch is off, but this really isn't a very common
event.

Specifically, AD 76-07-12 addresses this particular issue:

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

Compliance required as indicated:

1. For switches subject to this AD, conduct the following checks within the next 100 hours' time in service and each 100 hours thereafter to detect possible switch malfunction:

So, the FAA (and TC) think it is adequate to perform the "off"
mag switch check, with the engine running, every 100 hours.
They have not seen a requirement to change this interval, in
the last 36 years.

Doing it every flight, IMHO, is a little excessive. One must wonder
if it is really worthwhile, from a cost/benefit standpoint.

There are another couple of AD's, for example, that are required
for the ubiquitous 172 - seat rail, and exhaust leaks to the cabin
heat. The interval for checking the seat rails & exhaust is pretty
much the same as checking the Bendix switch, but I don't see
too many people doing those AD inspections every flight, either.

PS Note that the 1976 Bendix Switch AD is still quite valid -
compare the part numbers in the AD, to new production parts:

http://www.aircraftspruce.com/catalog/e ... itches.php

[iflyforpie]

I too have encountered engines that have continued to run with the mags in the both off position. I've also got the occasional zap from a disconnected lead when an impulse coupling snapped through with the switch off.

There is no danger of having an afterfire with going to both off provided that the engine is at a dead idle. A65 to R1340 I've done it without so much as a pop.

If you accidentally switch both mags off at a high RPM (runup or inflight doing a mag check) simply retard the throttle and allow the engine to clear itself before switching them back on. Again, a non-event.

[Colonel Sanders]

I too have encountered engines that have continued to run with the mags in the both off position

You've also certainly had to change Cessna seat rails IAW
the AD, and I'm sure you've found many cracked exhausts,
too.

But do you check for those after every flight as well? If
you really do, I will lay off you, because at least you are
consistent in performing every applicable annual/100hr
AD after every flight.

I just don't understand why this obscure AD from 1976 is
singled out for so much attention by so many people.

Worn ignition switches talk to you. The keys bind when
you try to move them. Heck, I've seen plenty of worn
switches that you could take the key out when it was
running on BOTH. It's not exactly rocket science to
figure out that gee, after 10,000 hours and 30 years,
maybe that mag switch is getting a little worn and
should be replaced.

If you accidentally switch both mags off at a high RPM
(runup or inflight doing a mag check) simply retard the throttle

Sounds good, but you know as well as I do that when
the engine suddenly goes quiet, the adrenaline rush
hits, hands are a blur in the cockpit and pilots instinctively
in 0.01 seconds undo what they just did, and turn the
mags back on, severely hammering the very expensive
exhaust.

I call this the "click, click, BOOM" method of testing the mags.

I was do some type famil training recently, and I told the
pilot - his hand were constantly a blur in the cockpit,
frightening me - that the most important thing, when
the engine quit, was to not rip the fuel selector handle
off when the engine got quiet and the adrenaline got
pumping.

A very experienced pilot (65 yrs in aviation so far) once
taught me:

"The first thing you do when an engine quits, is light a cigar"

[iflyforpie]

But do you check for those after every flight as well? If
you really do, I will lay off you, because at least you are
consistent in performing every applicable annual/100hr
AD after every flight.

Heat muffs are every 150 hours.

Seat rails I check by making sure my seat is locked.

A on-off check of a mag switch can be done faster than a L R check. I'll do an L R check right after a 100 hour or mag change to make sure each mag and bank of plugs are working before continuing the runup.

If I could do a 100 hour inspection in 30 seconds, I would do one before every fight.

Never do anything fast in an airplane.

[sidestick stirrer]

If I may be permitted to slightly derail this thread, can anyone explain the backwards arrangement of the switch's positions?

[Colonel Sanders]

You a leftie? Start is clockwise, which is natural
for a right-handed person ...



Personally the above looks too much like a car for me. I
prefer separate toggle switches for the mags, and a momentary
switch for the starter.

This arrangement is found on the Citabria, Decathlon and many
of the round engine aircraft that I fly:

[sidestick stirrer]

Nope: not a Soutpaw.
I was referring to the "R" being to the left of the "L"...

[CpnCrunch]

And most people have carbon monoxide detectors in the cockpit, which performs the same function as checking the muffler for cracks during every flight. Perhaps that crack or hole will very gradually appear over 100 flight hours and will be spotted at the next annual inspection, but I wouldn't really want to bet my life on it.

[avyonx]

We had to write an SB for a few Flight Schools in the States operating DA20 C1's to remove the ACS Switch with the turn to start style (shown in the post by the Colonel). Students were doing MAG checks but going to far and engaging the starter. it was to install the push to start style switch. One more thing to be careful when performing that before shutdown live Mag check. It destroys the starter very quickly!

[Grantmac]

Nope: not a Soutpaw.
I was referring to the "R" being to the left of the "L"...

The "L" position is always next to "Both" because if only one mag has an impulse coupling its almost always the left one. The engine should only be started on the impulse coupled mag to prevent kickback.
Fun note: the "start" position should automatically have only the left mag hot, then when the key is released it automatically goes back to both.

-Grant

[Colonel Sanders]

the "start" position should automatically have only the left mag hot, then when the key is released it automatically goes back to both.

True - most of the time. But not all aircraft just have one impulse-coupled
magneto - I have an aircraft (TCM engine) with both magnetos impulse-coupled,
so grounding out the right magneto during cranking would be foolish!

Always know your airplane, like Truman Sparks does:

[Grantmac]

Its all in how the switch is hooked-up, something which has been known to be done wrong in the past. I've heard of once which was cranking on both when it should be just the left. I can imagine the reverse is also true, although it wouldn't eat starters in the same manner.

-Grant