Mag Check Failure

[RLCarter]

7 minutes ago, Vance Harral said:

A local flying club with a pair of 172s where I am an affiliated instructor requires the towbar be left attached …………… It is an unbelievably stupid practice in what is otherwise a great flying club.

I agree, mine was the 3rd aircraft we had moved/flown and just smooth forgot about it, like I said I was extremely lucky

[A64Pilot]

4 hours ago, PT20J said:

I don’t think the little mags on Lycoming and Continentals will generate enough voltage to fire without a pretty good spin UNLESS the impulse coupling is cocked and the mag is hot. In that case a small prop movement will fire the mag. The BT-13 has a R-985 with big mags without impulse couplings and I’ve heard of people hand propping R-985s back when I was flying Beavers.

Nonetheless, I do a check to see that the ignition switch kills the mags before shutdown, I put the key on the glareshield where I can see it from outside, and I turn the prop backwards 1/8 turn to connect the tow bar.

Speaking of tow bars, I was in the shop checking on the progress of my GFC 500 installation and saw a nice A36 with all three blade tips mangled. Started with tow bar attached. Mechanic said it was the second one in a month — the other was a Mooney.

Skip

Even the 985’s big brother the 1340 is not hard to hand prop, it’s due to the inertia once you get all that mass turning, I’ve done it a couple of times just to see how hard it was. Remember a 1340 will idle happily at less than 500 RPM too.

I have tried to hand prop an IO-520 with a dead battery unsuccessfully. mostly because without a battery you can’t prime an IO, but even if you could I think that three blade prop would be way more dangerous.

In an aircraft with keys, I ALWAYS put them on the glare shield and make a habit to look prior to touching the prop.

My 46 C-140 has two toggle switches for the mags, and I admit I have forgotten and left them hot as the C-85 you can’t kill with mixture, you have to turn the fuel off and just wait

Hot mags can hurt you, and there really is no good reason why we don’t use the switch to shut down, I don’t how why or where the prohibition of not shutting down with ignition started from.

So, it’s not going to hurt if you do occasionally. When I check the mag switch, I let the engine die as I have had backfires from turning ignition back on, and backfires can damage mufflers.

Your mileage may vary

[jaylw314]

6 hours ago, ltdriser said:

I’m not sure where I missed this during training but I’ve never been taught to check mags after flying, only during initial run up. I will say though, after all this I will be checking at the end of my runs. Who knows how long I’ve been pushing the plane back with a hot mag. 

My understanding is a normal hot mag check is to turn the switch to off to make sure the motor starts dying.  Before it completely dies, turn it back to both and run it a few seconds longer before shutting it off by killing the mixture.

The AD specifically is to check for a switch that has a poor grounding connection in the OFF position, so that moving it in the OFF position can actually break the connection and lead to an intermittent hot mag.  The AD specifically calls out moving the switch to the extreme OFF position (and I jiggle it around just for good measure).  I think you're supposed to actually let the engine stop to ensure there are no shenanigans going on in the switch.  Since there's some fuel in the cylinders, I usually start up the motor again and run it for a few seconds before shutting it down normally.

[Ragsf15e]

8 hours ago, EricJ said:

My insurance guy told me once that tow bars are a big source of prop strike claims.

 

I saw a relatively new SR22T that did that.  Luckily it was parked in front of a Cirrus Service Center.  The guy had been leaving to fly to his weekend home at night and forgot.  I saw the airplane with the engine removed the following week and asked the DOM how long it would be down.  He said it didn’t really matter as the owner already bought a new one to replace it and was just trying to unload this one.

[RLCarter]

6 hours ago, A64Pilot said:

Hot mags can hurt you, and there really is no good reason why we don’t use the switch to shut down, I don’t how why or where the prohibition of not shutting down with ignition started from.

I was told that going to idle cutoff gets rid of any fuel charge reducing the chances of harm should you have a Hot mag, I cycle the key fairly quick at idle and have never had a backfire

[PT20J]

It's not really a backfire which is a fire up through the induction system. It's more properly called an afterfire which is a fire in the exhaust. Backfires are usually caused by excessively lean mixture which burn so slowly that there is still burning gas in the cylinder when the intake valve opens igniting the mixture in the intake manifold. An afterfire usually occurs due to overpriming or shutting the ignition off when the engine is making power. In either case unburned fuel accumulates in the exhaust system and is subsequently ignited buy hot exhaust gasses when the engine fires. At closed throttle idle, momentarily tuning off the ignition only long enough to note that the engine begins to die and then turning it back on should not cause an afterfire because not enough fuel accumulates in the exhaust. If you accidentally switch the mag off during a high power mag check, it's safest to pull the mixture to ICO and allow the engine to stop and restart it.

Skip

[kortopates]

If you have a carbon blade dry vacuum pump. turning it backwards can lead to early failure.
 

old wives tale myth.


Sent from my iPhone using Tapatalk

[N201MKTurbo]

1 hour ago, kortopates said:

old wives tale myth.


Sent from my iPhone using Tapatalk

Some pumps have straight vanes, some have angled vanes. the ones with the angled vanes, that were near the end of life, turning backwards may make them fail. So not completely an OWT.

[A64Pilot]

48 minutes ago, N201MKTurbo said:

Some pumps have straight vanes, some have angled vanes. the ones with the angled vanes, that were near the end of life, turning backwards may make them fail. So not completely an OWT.

It’s the way the blades wear, the trailing edge will have a thin edge to them, if you disassemble one it’s obvious. Turn it backwards and this fine edge can break.

‘For whatever it’s worth the brushes in a turbines starter generator will show exactly the same thing and for that reason turning. starter generator backwards should be avoided.

Take the brushes out of any electric motor and you will see what I’m talking about , the leading edge will be dull. while the trailing edge will have a fine edge.

Motors that operate in both directions never develop this wear pattern of course.

[A64Pilot]

Pretty much every spark ignition engine in existence is turned off by ignition and not fuel starvation, except airplanes, and of course it does them no harm.

Sure it’s an after fire, but no one calls it that. Just like none of our turbos are intercooled. they are actually after cooled. but who calls it an aftercooler?

Dump a fuel / air mix into the exhaust, and then light it with hot exhaust gasses and some damage can occur, not often but why risk it?

[N201MKTurbo]

1 hour ago, A64Pilot said:

Pretty much every spark ignition engine in existence is turned off by ignition and not fuel starvation, except airplanes, and of course it does them no harm.

Sure it’s an after fire, but no one calls it that. Just like none of our turbos are intercooled. they are actually after cooled. but who calls it an aftercooler?

Dump a fuel / air mix into the exhaust, and then light it with hot exhaust gasses and some damage can occur, not often but why risk it?

Most spark ignition engines don’t have a giant killer propellor hooked to their crankshafts that the operators regular move by hand. And most spark ignition engines don’t have magnetos that will start them with no external power applied.

[A64Pilot]

40 minutes ago, N201MKTurbo said:

Most spark ignition engines don’t have a giant killer propellor hooked to their crankshafts that the operators regular move by hand. And most spark ignition engines don’t have magnetos that will start them with no external power applied.

Your taking it out of context, I’m not saying always turn off the motor by the ignition, just that doing so occasionally does no harm.

All old style Diesel engines have the same or similar concerns with self starting, and magneto ignitions are very common or used or be, all small motors like lawnmowers etc are magneto, pretty much any motor without a battery is a form of magneto, or self energizing ignition

Anyone should always treat that prop like they should a gun, that is, you always treat it as if it’s loaded, the ignition switch is the safety, and never, ever trust a safety.

So if you ever need to turn that prop, do it in such a manner that if it were to start or kick, nothing gets hurt.

[tmo]

On 9/6/2021 at 6:04 PM, PT20J said:

Started with tow bar attached.

I confess to doing that - luckily with my M20K and the collapsible towbar there is enough clearance for the prop to spin freely, and there were people around - their frantic waves clued me to shut down and ask what was going on. Close call, definitely.

One of our students took off in a C152 with the tow bar attached, and a FI on board. They landed safely.

[FlyingDude]

On 9/6/2021 at 6:39 AM, M20Doc said:

don’t they teach doing a live mag check prior to shut down

When I did my private in Italy in 2005, mag check at OFF at idle before shut down was required. We weren't told it was for an AD. Rather, the goal was to catch that fault and fix it so that the next guy doesn't catch it at engine run up and have to cancel flight... I don't know if it was the case only at our school/club or across entire Italy.

[Joshua Blackh4t]

I have had mag issues in ultralights that really ruined a day's flying plans. So its a habit to check ultralight mags on shutdown. That way I can order a new <$200 mag and fit it before next flight. 

Also, ultralight engines such as Rotax and Jabiru are turned off with the mags (or rather CDIs) and no problem there.

The idea of keeping the fuel out is kind of ironic considering you can restart the IO 360 without moving the mixture.

 

[carusoam]

It is a good technique….

Test mags

• Before the flight…
• during the flight…
• After the flight…

I have not had a mag failure yet…

Best regards,

-a-

[kortopates]

I have not had a mag failure yet…
Best regards,
-a-

You’re really lucky and probably overdue - hope it doesn’t happen on your next flight!

but pressurized mags fail much more frequently due to corrosion than the NA variety.


Sent from my iPhone using Tapatalk