Live mag check before shutdown

[TLAW]

Read a message by a friend who is currently in Florida, who said she had picked up a fault with the ignition switch resulting in a live mag when in the off position. She did this by doing the old mags L-R-OFF-BOTH check at idle before shutting down with mixtures ICO. When she explained to her aircraft mechanic what she had done, he was horrified! He had never heard of this check and was certain it would cause engine damage.

Questions;

Is the mechanic right?

How many people reading this forum do the same, not do it, or do it slightly differently?

[aldee]

I'd be interested in the mechanics definition of "bad for the engine"
He's obviously concerned about the "off" position otherwise he'd be freaking about every runup:)
Probably get a bit of a pop out the exhaust as the unburnt fuel ignites if the ignitions "off" for too long.

With the r22 we were told if we go to off by mistake during the mag check to let it die and then restart due to no "flywheel effect" because of no prop, I assume due to risk of kickback if you turn the spark back on at a bad time in the cycle.

Definitely like to know there's no live mags on the tiger moth when we shut her down but once again its never all off:ok:

[PlankBlender]

On the R22 if you switch the mags off you need to check if the engine fan turned over its bearing because of the rapid deceleration..

[TLAW]

Sorry, I should clarify, this was on a PA-31.

[aldee]

There is a perception that the TIO 540's can be a bit "fragile" if mishandled and they are bloody expensive to repair so I can understand an engineers nervousness but still I wonder if there is any fact behind the phobia?

[kiwiblue]

Gawd another of these bloody old-wives tales with which aviation abounds surfaces yet again...

...he was horrified! He had never heard of this check and was certain it would cause engine damage.

First, I'd be wanting to know what rock the engineer has been living under, and for how bloody long??? Magneto dead-cut checks have been a part of aviation about as long as it took for the people investigating the first death caused by a live mag to figure it out. MOST engineers (in my experience) recommend executing a magneto dead-cut check every time you shut the engine down and again most will be very happy to demonstrate an appropriate technique if your training has not covered the how.
As a pilot my (personal) attitude is, any magneto that I have not (personally) proven dead will be treated as live.

First, as to being "bad for the engine"... what crap. A properly executed mag dead-cut is executed with the engine at the idle or just above, certainly never above 1000-1200 RPM (which is where most training organisations teach it be done). Personally, mine are done whilst taxiing (both in and out), with the throttles at the back of the quadrant. The switches are only in cut-off momentarily, then immediately back on. When I say 'momentarily' I really do mean momentarily! There is no need to pause with the switches off to be sure the mags are earthing properly -listen to your engine, if you hear the engine 'miss' with the mags off, all is good. If you don't you may have a faulty earth on a mag, a P-lead adrift or a switch problem. Aircraft that are equipped with a key-start rotary-type ignition/start switch may require a little more care and attention, particularly during the higher power mag-drop check during the run up. Should you inadvertently rotate the switch to 'off' with say 1800 RPM on, then back to on, you run the risk of damaging the exhaust system at a minimum. Should this happen, leave the switch off, allow the engine to shut-down completely then re-start normally. You will have done little more than raise your own blood-pressure and given yourself a good reminder to pay more attention next time.

If you check left and you get a drop and you check right and you get a drop you can be pretty darn sure that when you turn em both off, they will stop.

Nope. All you can be sure of is that both mags are earthing individually when selected to do so. What if there is a switch problem at the 'off' position that is leaving either one or both mags hot? You haven't identified and proven a safe condition at all. The only way to be sure your mags are both off when selected to do so is prove it with a mag dead-cut check, particularly again on the rotary key-start switch equipped aircraft.

Years ago, there used to be a poster that was usually prominently displayed at every training organisation/FBO you might visit... it stated "Treat Every Prop as Live" or words to that effect. Damn good advice IMO and still every bit as relevant now as then. NEVER put a hand on a prop you haven't proven dead. A glance in the window will let you know the switch positions, but how confident are you that the guy before you properly conducted a dead-cut check prior to shut-down??? Worth thinking about if you value your life.

[mince]

Ever thought to read the POH? The manufacturer knows what is good and bad for it's engines. Just do it their way and at least your arse is covered:ok:

[NutLoose]

At Idle on a piston aircraft it will be fine, its only above idle if you turn it off you should let it stop then restart otherwise you will get the mother of all bangs and really pissed off engineer.............

I have always done a live mag check after any maintenance and mag changes I have done, it is part of my procedures, all I do is cut the mags, look for the rpm to drop then turn them back on at IDLE, this is to ensure that neither mag is live.

If you did it on the cessna 182 (new One) there was a service bulletein to check certain exhaust models as you could split them a couple of years ago.

[chimbu warrior]

Kiwiblue

Excellent post

[MR. PROACH]

Agree with kiwi - I have always done this check, and recently flew with a youngish instructor that thought all you needed to do was go to L/R on the mags and then shut down. I explained, and always thought the only damage through the backfire (if ham fisted and left in the OFF posit for too long) can be to the exhaust.

I've found three broken P leads this way, so can't see the down side, and never heard a mechanic tell me otherwise?

Just be careful with a PA31 and don't attempt to handprop! :ok:

Be safe out there.

[27/09]

I agree with kiwiblue about doing the Left/Right/Both Off when using a key or rotary switch for exactly the reason stated, as different switching positions on the these styles of switches are used to test for the 3 situations and each key switch position needs to be tested.

However I don't see the need to switch both mags off (at the same time) when there is an individual toggle (on/off) switch used for each mag as is the case (if my memory serves me correctly) in something like the PA31, or even a Piper Cub.

[kiwiblue]

...I don't see the need to switch both mags off when there is an individual switch used for each mag as is the case (if my memory serves me correctly) in something like the PA31, or even a Piper Cub.

A valid point 27/09, and one with merit. However in 2 airframes with which I am familiar (C402 and BN2) -both equipped with individual L&R switches for the mags on both engines, I continue to check both mags individually as well as a complete dead-cut check with both switches selected off momentarily. In part, it's probably an instinctive/habitual action; one that puts a tick in that mental box that ensures I have a proven safe mag condition -I'm not relying on supposition or "this condition has been met so this must be so" logic. Partly it's based on experience of electrical components failing to behave as they are supposed to or behaving in a manner entirely unintended!

I have had conditions in the past where I have observed an apparently "normal" mag drop on selecting L&R individually, only to then have the engine run on as happy as a sandboy with both switches selected "off". In both cases a faulty P-lead was the culprit. Electrical components can be strange beasties, displaying all manner of false behaviours that make diagnosing the underlying fault frustratingly difficult. The upshot for me with mags is: if I ain't proven it, then it ain't so. My point is: it takes but a moment to prove that you have a safe mag condition, with absolutely no harmful potential to the engine when conducted properly -so why would you not?

In one experience of mine with the BN-2, our Deputy CP (a man with 10's of thousands of hours) was in the habit of putting the mixture and throttles full-forward after shut-down, with the intention of easing egress for the front-seat pax. Not a habit that many of the rest of us were comfortable with, but he was not going to change his ways, despite it being raised with him on several occasions to my knowledge.
On this particular day, I took over his aircraft around mid-day. There were 3 company Islanders parked abreast on the flight-line, with mine furthest out. I noticed (and corrected!) the position of those levers and checked switches Off during my pre-flight. Most of us were in the habit with the BN-2 of rotating the props to the vertical after shut-down, to reduce the likelihood of pax or ourselves getting a nasty smack across the forehead if walking forward of the engine, under the wing as we frequently did whilst working around them. This day the Stbd prop was horizontal when I got to the aircraft. I had my hand on it to swing it to the vertical but stopped -because I had not proven the mags dead. On the way out to the runway, I did my usual L/R/Dead-cut check... and guess what? A hot mag on the Stbd engine. Had I or anyone else swung that prop, most likely she would have fired. The consequences personally would have been fairly dramatic and virtually as dramatic and immediate for the company -the live engine with a full-rich mixture and wide-open throttle would have easily cleaned up all 3 aircraft. When I raised it again with the DCP, he confirmed having done a mag-drop check with no anomalies prior to shut-down, but not a dead-cut.

I reckon it's a pretty simple check to make with no consequences beyond proof absolute that things are as you expect them to be.

[MR. PROACH]

And sort of on the subject, in piston equip I was taught to (after mag checks) shut down the engines and then engage the starter. Better to know sooner, rather than later that you're stuck for a while if it doesn't engage:E

Unless you can hand prop of course........

[Jabawocky]

To shut down my engine.....we use both mags off at idle! It gets checked every time!:}

Of course if one failed......turn off the fuel tap!

J

[VH-XXX]

In once witnessed a student accidentally switch off both mags and immediately press the starter button before the engine has stopped rotating. Managed to sheer off the 6 x fly-wheel bolts.

[Jabawocky]

For every action there is an equal and opposite reaction.........plus one from the a/c owner as well:eek:

J:E

[kiwiblue]

To shut down my engine...

hmmmm... I'm not advocating a mag dead-cut as a technique for a normal shut down of your engine Jabawocky -that should always be by starving it for tucker -mixture to ICO or fuel off as necessary/appropriate to your engine. The dead-cut does however go a long way to ensuring your safety around the propellor when the engine is shut down.

[Jabawocky]

Kiwiblue,
agreed with all the other a/c I have flown its done with the mixture, however in the Rotax and Jab its done by the mags!

1 Radio/Intercom OFF
2 Ignition Switches OFF
3 Master Switch OFF


Keeps your mind active when changing a/c types!

J:ok:

[archangel7]

Could it be that Americans don't do this check at all?

[R J Kinloch]

Pulling a Cessna 206 through years ago.
To Pilot "Mags Off?" Reply "Yes" "Put your hands where I can see them"(Glare Shield) Pulled prop through, fired on 1 cylinder. Result 27 Stitches in my left hand and I was LUCKY!

[kiwiblue]

Keeps your mind active when changing a/c types!

Absolutely Jabawocky! Showing my lack of familiarity with 'recent' developments in the smaller end of the market :\ Ever had an issue with a prop firing when pulled through? Are there any particular procedures you observe due the fact there is still fuel present in the engine?

R J Kinloch: Luck indeed!!! :ok: What was the underlying cause? A hot mag? Fuel igniting on a hot-spot?

[Jabawocky]

Kiwiblue

This is turning into a good informative thread.

To answer your question, for the newer small end of town, there is no fear of a prop start on a Jabiru, it can't be started under 300RPM and unless you are superman, you can not achieve that by hand starting. This is a factory known fact not just my opinion. They are that way by design. They do not use the same kind of sprung magneto system as say a Lycoming.

As for Rotax, I do not believe they can be hand started in the 4 cycle engines, but that is not fact, just an opinion. The 2 stroke versions I think can be hand started. Maybe a Rotax guru can advise here.

As for policy around props, even the known to be unstartable by hand......Treat them as live. This is a habbit for all machines, but I must say its easy to become complacent.

J:ok:

[gassed budgie]

Should you inadvertently rotate the switch to 'off' with say 1800 RPM on, then back to on, you run the risk of damaging the exhaust system at a minimum. Should this happen, leave the switch off, allow the engine to shut-down completely then re-start normally. You will have done little more than raise your own blood-pressure and given yourself a good reminder to pay more attention next time.

Well done Kiwi. I haven't heard anyone else suggest this in a long, long time. If you happen to inadvertantly select magnetos 'off' when checking the mags, you run the real risk of stripping the teeth off the main gear drive in the magneto when the engine all of a sudden ramps up again. I've seen it done.

http://img169.imageshack.us/img169/8809/brokendistributoruj4.jpg

This is what the end result looks like. You ain't going anywhere anytime soon. I might point out that Bendix mags are more likely to strip the teeth off the gear than Slick magnetos.
As Kiwi has suggested, sight tight, relax, pull the mixture and then crank it again. No harm done.

[TLAW]

Gassed Budgie,
Yikes! :eek: Very informative photo.
I admit I once accidently selected 'OFF' during a run-up, realised what I had done, then quickly selected BOTH again. :O Won't do that again!
The instructor drolly asked if that was my normal procedure for checking mag drop.

[ItsOkMagsOff]

Keys on dash where i can seem them please...

[bushy]

Those gears are made of nylon and are definitely a weak point, and a good reason to use diesels which do not need mags. (if you could ever afford to buy one) The first engine failure I ever experienced was caused by a magneto gear failure.
Bendix mags need regular inspections and the nylon gears have a limited life. I once bought an aeroplane from an airline captain, who had had to overhaul the engine when an oil hose burst and dumped all his oil overboard.(I wonder why)
Within a few months we had a forced landing because the nylon gears in the mags were old and brittle, and one lost some teeth out in the middle of nowhere. We also discovered some necessary propellor work he had not told us about.
I think slick mags are better because you replace them instead of overhauling them, so you always have fairly new mags.
Magnetos need lots of attention, and they are critical.

[Sunfish]

I suspect that the reason the gears are nylon is so that they WILL break if the Magneto jams up. That way at least you should still have one functioning Magneto.

[Brian Abraham]

you should still have one functioning Magneto

Not necessarily so as some engines have the mags piggy backed on the same drive shaft. Whether they are driven by a nylon gear or not you would need to know your specific engine. Just the same, have had engine problems because a metal shaft stripped its gear teeth.

[kiwiblue]

A bit of thread-drift, but such is life :}

Should you ever find yourself in the unfortunate position of having to apply the Armstrong starter to your aircraft, a couple of mag-related things that may be useful...
know which (if any) of your mags is the impulse mag -usually the left, but be sure. More likely to get a nice clean start off that one.
know what type of mags you have installed... a shower-of-sparks mag will require the starter switch be engaged during the Armstrong -or there will be no shower-of-sparks... No spark = no fire :E By contrast, an impulse mag won't give a toss where the starter switch is.Anyone have others they can add???

[Lefthanded_Rock_Thrower]

If you are trying to hand start a Lycoming after a failed attempt with the starter motor, the starter shaft will not disengage ( cog wind back into the starter housing ) until the engine has operated above 500 odd RPM ( after hand starting ).

You can not hand start a Shower of sparks ( starter vibrator ) type engine without both battery power and the starter button being pressed.

Continential are quite suseptible to cracked cylinders, with oil consumption above half a litre per hour or manifold pressure above 15 inches at 1000 RPM ( at idle, warm ) that would warrant further investigation, may also be running a little rich at idle.

Flying a PT6 with the condition lever in "Low Idle" does not save fuel, you are an idiot for thinking so.

Multi engine aircraft with engine driven hydraulic systems, when operating on one engine, will generally take longer to cycle the landing gear.

High oil level will generally cause the oil temp to be high inflight.

Engines do not operate very well with air in the tanks.

If you abort a start in a turbine, let it sit for a couple of minutes to allow the oil levels within the engine to equalise ( PT6 ).

A warning light in flight, actually definately means the light globe is working.