Mag check on shutdown

[FenderManDan]

Do you perform a mag check before shutting down the engine(s). I have seen/heard people do mags off then quickly on/both. What does this do?

None of this have I seen in the POH or lycoming manual.

[cap41]

I always do it, im a rookie so take my opinion with a grain of salt. But, i thought it had something to do with confirming the ignition key off is working. last thing you want is the mag to be mistaking left on. if you were to move the prop even with the key out, if the mags are on/hot the propellor could kick/start. Yes the you pull the mixture full lean to starve engine of fuel. But i have seen first hand (no pun) a guy try to start his airplane, it turned over a little but the battery was dead. so mixture full rich, engine primed, key pulled out of ignition BUT mag was still live. when he turn the prop to a perfect horizontal position it kicked in. Unfortunately he now left handed.

[Beefitarian]

I do.

I believe I can hear the engine stop firing while the mags are grounded. I have never had a backfire/exhaust bang. I don't know how long guys leave the mags grounded to get that much fuel through the engine. Probably not long. I select it just long enough to hear the sound change seems like under a second.

I still don't trust the prop fully when I move it but I do like to move it to horizontal. When I park.

[ahramin]

Beef why would you move the prop to horizontal when you park?

[akoch]

because it is a whole lot more convenient to pull/push the airplane with the blades are horizontal? And the prop is not in the way of the wheel bar to steer the airplane in/out of the hangar?

[photofly]

Because it's less likely to get hit by the wingtip of an aircraft being carelessly manoeuvred into an adjacent space, and because that way less rain runs down into the prop hub to sit there and freeze.

[Beefitarian]

Even though guys should not be taxiing close enough for it to matter. It's to keep it out of the way for others maneuvering by.

there maybe OCD reasons too

[PointyEngine]

The idea of leaving the prop horizontal apparently started years ago when some machines were susceptible to water running down the blade and into the hub when the aircraft was sitting. Apparently it could seep into a seal or something and cause corrosion fairly quickly. How true it is? Not sure. Which machine? Wouldn't have a clue.

[Taco Joe]

Do you perform a mag check before shutting down the engine(s). I have seen/heard people do mags off then quickly on/both. What does this do?

None of this have I seen in the POH or lycoming manual.

As cap41 said, it's to make sure you don't have a hot mag with the ignition off. It also helps you determine if one gave out in flight because you'll notice the lack of an RPM drop as you cycle through each mag.

[iflyforpie]

I do it. I've seen them fail before where they will drop on L and R but keep running on OFF.

You won't do damage to any engine, provided that you do it at idle. You can also turn both mags off at run up RPM and provided you retard the throttle and wait until you can see individual blades, you can turn them back on without an apocalyptic fireball out the exhaust too... (this is also handy if you have a dead mag and still want to use the engine to taxi back to the hangar without restarting).

[triplese7en]

Yes, you should check the mags.

For a document about magnetos and how to check them, search Google for "caa magneto check basics" and click the first link.

As a side note, does anyone know how to copy the link to a search result on Google? I'm trying in Chrome and it's driving me up the wall...

[rob-air]

Most flight school checklist calls the mag check at least once every flight before shutdown. If i recall corectly, acording to lycom and conti it should be checked every 50 hrs.( dont quote me on the exact hour spread between mag checks, really not sure on that one)

I have seen an engine run on the off position, caught it at run up.

Another kind of related common thing, the keys will often come out at ANY mag position....so key out does not mean mags off.

I also put the blades horizontal....it looks nicer and like said before to avoid getting hit by a high wing.

[Colonel Sanders]

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?

76-07-12 BENDIX IGNITION SWITCHES: Amendment 39-2575 as amended by Amendment 39-3024. Applies to all aircraft employing magnetos and using Bendix ignition switches listed in the table below except switches identified by four digit date code (new) adjacent to the model number or a white dot (modified) on the support plate adjacent to the Bendix logo.
Rotary Action,
Bendix Switches Key or Lever Actuated
Switch Function Bendix (series) Part Numbers
Twist-to-Start 10-357XXX, 10-126XXX
Twist-to-Start/Push-to-Prime 10-357XXX, 10-126XXX
Push-to-Start 10-357XXX, 10-126XXX, 10-157XXX

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:

(a) Observing regular ground run-up procedures, allow the engine to reach operating temperatures and perform a normal magneto check.

(b) With the engine at normal idle, rotate the switch key or lever through the "OFF" detent to the extreme limit of its travel in the "OFF" direction.

(c) If the engine stops firing, this indicates an airworthy switch.

(d) If the engine continues to run with the switch in the extreme "OFF" direction indicating a malfunctioning switch, prior to the next flight accomplish Part III outlined in Bendix Service Bulletin No. 583, dated April 1976, for Repair and Replacement or use an alternate method approved by Chief, Engineering and Manufacturing Branch, Eastern Region.

2. The aircraft may be flown in accordance with FAR 21.197 to a place where these modifications can be accomplished.

3. The checks required by this AD may be performed by the pilot.

4. Upon submission by an operator with substantiating data, an FAA Maintenance Inspector subject to prior approval of the Chief, Engineering and Manufacturing Branch, FAA Eastern Region may adjust the compliance times specified in this AD if the request contains substantiating data to justify the increase for the operator:

(NOTE: If the engine continues to run when complying with paragraph 1 and repair or replacement cannot or will not be accomplished immediately, the magneto (primary circuit) should be grounded in accordance with Bendix Service Bulletin No. 583, dated April 1976.)

Amendment 39-2575 was effective April 14, 1976.

This amendment 39-3024 is effective August 30, 1977.

There is another (Canadian) AD on all aircraft requiring annual
inspection of the exhaust for cracks, to avoid carbon monoxide
entering the cabin via the shroud heat.

Using the same logic as testing the switch, I think that after
every flight, everyone should remove their engine cowls and
shroud and inspect their exhaust for cracks. After every flight,
instead of the annual inspection requirement in the AD.

Same logic as checking the switch every flight. And frankly,
far more important. Carbon monoxide poisoning will kill
you. If you don't play with the prop, a hot mag is unlikely
to kill you.

PS For IFP:

[Rookie50]

Our club had too many backfires on an "off " mag check. -- I've had them myself at idle power -- that they removed that from the shutdown list.

[Colonel Sanders]

New exhaust systems cost thousands of dollars. A
friend of mine just paid $3,000 to replace his.

If you don't mind writing a $3,000 cheque, go right
on ahead with that 100x unnecessary OFF check.

PS It's an afterfire, in the exhaust. A backfire is
up through the intake. Very different animals. An
afterfire, for example, is unlikely to fry your air filter.
It is however quite likely to blow the baffles loose
in your muffler and quite possibly plugging up the
exhaust.

PPS a hot mag is not quite the death sentence that
everyone here thinks it is. I know a guy who flew
with a hot mag for almost a year, until his next annual
inspection. His airplane ran fine - his p-lead was broken,
where it went through the firewall. Smart? No. But
an interesting data point. Given the choice, I would
rather have a hot mag than a cracked exhaust pumping
carbon monoxide into the cabin, but I guess my priorities
are screwed up.

Pop Quiz: why is it safer to have the right mag hot
instead of the left mag?

[iflyforpie]

I love that movie!

Like I said before, if you could do a 100 hour check and ADs at the flick of a switch, would you do it before every flight? I would.

If you are getting after firing from doing it, your carb or fuel system is not adjusted properly. Hundreds of aircraft I've run with engines from 65 HP to 600 HP never had any problems.

And yes, the danger from a live mag is not as bad as people think, but if you aren't going to be thorough, why do it at all?

[Colonel Sanders]

You must admit it is pretty funny that people
are stridently complying 100x as much as they
need to, for an AD that they don't even know
exists

Other memorable stuff from 1976:

[CamAero]

Pop Quiz: why is it safer to have the right mag hot
instead of the left mag?

Impulse coupling on the L/H mag only.

[FenderManDan]

Thank you for a lot of good info. I forgot ot mention that i have the slick mags and not bendix. Does that make any difference?

[Colonel Sanders]

er, no - it's the ignition switch that's made by Bendix,
and to which the 1976 AD applies. Note the part numbers
of new ignition switches:

http://www.aircraftspruce.com/catalog/e ... kkey=43807

The 1976 AD still applies, to the brand new parts produced
37 years after the AD was issued.

[Colonel Sanders]

Impulse coupling on the L/H mag only

Correct. Many aircraft (eg Lycoming engines) only
have impulse coupling on the left mag (with some
notable TCM exceptions, which have impluse couplings
on both mags).

Without an impulse coupling, a hot mag really isn't
as much of a problem. If it fires at all - extremely
dubious, if you move the prop by hand - it's going
to fire very weakly at least 20 degrees BTDC, and
kick back - not start.

Not all magnetos use impulse couplings. With a buzz
box (again, generally on the left mag only, with a
retard breaker) it is extremely unlikely that you will
ever generate a spark, moving the prop by hand,
even with both mags hot, if the buzz box has no
power.

For fun, try hand-bombing an AEIO-540 with a buzz
box, carefully primed, master on, mags on.

[pelmet]

In terms of which mag is the impulse mag, it may be correct in general but...there are always the rare exceptions. I fly one engine type where it is the right mag. Can CS confirm for me that almost all modern engines have it on the left side.

On another note, according to a respected author on operating older aircraft engines about if you accidentally select off during a mag check....

"Something to keep in mind for the rest of your career, “when” you inadvertently move the magneto
switch to OFF, when you’d only intended to move it to LEFT, resist the temptation to immediately
return the switch to the proper position. I deliberately used “when”, not “if”, because I don’t know of
anyone who hasn’t made this mistake at least once! If you fall victim to the almost irresistible impulse
to attempt to cover up your mistake you’ll only succeed in drawing unwanted attention to the mistake, I
guarantee it! I‘d guess that in every instance I’ve seen it’s resulted in a healthy backfire! What
you need to do is “don’t flinch”; just say to yourself that this is another case of “big deal – so what”!
Leave the switch off, get your hand well away from it and put it to use in retarding the throttle to the
idle position. Allow the RPM to fall naturally to the idling speed of approximately five hundred RPM.
Then you can return the mag switch to BOTH and allow the engine to regain its footing at idle. Now
you can restore the power and return to the check you had originally set out to do. You’ll need to
practice this or at least strongly think about it in order to be able to resist the temptation to “flinch”
when it happens to you. And it will!"

However, this was written primarily for radials. I'm not sure if there is any difference for newer horizontally opposed engines. Any comments are welcome.

[iflyforpie]

Done it personally on the TSIO-520..... absolutely nothing to write home about.

[Colonel Sanders]

Try a GTSIO-520 sometime!

[PilotDAR]

A long time ago, a friend of mine, whom I quite trust, asked me to hand prop his flat battery Cessna 150. I was very comfortable hand propping, and happy to help him. It was an aircraft also I regularly flew, so I knew it to be in good mechanical condition, including the live mag check, which I had recently done…

As it was the first flight of the day, pulling it through a few times was appropriate. I called to him "Mags off" and he called back “Mags off”. I swung the prop once, and it started, and ran quite nicely. I looked through the turning propeller at my friend in the pilot’s seat, and gave him that look, which he knew was my question to him, “are the mags really off?”. He immediately held up the keys, which I saw through the still turning prop!

This particular 150 was equipped with one of the mag switches which was later AD’d for internal failures which lead to two live mags. What a dangerous co-incidence that the time of the switch’s first failure, was the first time I would hand prop it!

If I have to touch a prop, I touch it assuming that the next thing it will be, is running!