Mag checks

[shumway76]

I noticed that in C172 (not sure about other types) POH normal checks there is no mention of "dead mag checks" in the after start and "live mag checks" in the before shutdown checks. The only mag check mentioned in the POH is during run up.

But club SOP does mention dead & live mag checks. Are these checks not mandatory, but up to operator / organization, that's why it's not in the POH?

[RTN11]

But club SOP does mention dead & live mag checks. Are these checks not mandatory, but up to operator / organization, that's why it's not in the POH?

The checks are effectively serving different purposes.

The "dead cut" check just after start is checking that the engine doesn't stop on one mag, showing that a mag has completely failed. Similarly if there is no drop, it would mean one mag is not being isolated by the switch which would also need further investigation. If the engine stopped during this initial check it would be a complete no go, and save you having to taxi all the way out to do power checks on one mag, then be stuck on the apron with a knackered engine. During this check you are really just checking "drop, no stop". You could easily skip this check, but then after taxiing out find you have a problem which you could've spotted much earlier.

The "drop check" during power checks is checking that each mag is functioning fully, an excessive drop would indicate a poorly performing mag, rough running could be a fowled spark plug, something which could be cleared using the mixture. This is certainly a check you should perform before each flight, and you are really looking for how much the RPM drops on each mag.

You should also do a dead cut check before shut down to make sure both mags are being isolated by the switch, otherwise you could unknowingly leave an aircraft with a live mag.

[Big Pistons Forever]

The danger with any of the mag "dead checks" is switching off and then not immediately switching them back on. Even a slight delay will result in a backfire and probable damage to the muffler and/or other parts of the exhaust system.

Personally I think that the after start "dead check" is not worth thr trouble as it will only detect a totally dead mag which is a relatively rare failure. I have only had one mag fail (and it was in flight) in 35 yrs of flying, so the benefits of the after start check seems extremely limited to me. In the case of my mag failure it would not have mattered as I detected the failure on the shutdown check.

As for the shut down check, I just set 1000 RPM and cycle the mags Both/Left/Right/Both. You will be able to see the mags cutting out and the chance that the mags will cut out normally, but be live with the mag switch I the off position is extremely remote.

One point that IMO, is insufficiently emphasized at flight schools is to check the mag switch key lock. The key should not be removable at any position except off. Therefore when I cycle the mag switch during my shutdown check, I give the key a gentle tug at each position to make sure thr key does not come out. I have twice seen worn mag switches that allowed the key to be removed when the key was selected to the right mag (the one next to off on most switches) :eek:. This IMO is the most likely cause of a hot prop and is not tested for in the traditional shut down "live mag" check.

[piperboy84]

PBF
As for the shut down check, I just set 1000 RPM and cycle the mags Both/Left/Right/Both. You will be able to see the mags cutting out and the chance that the mags will cut out normally, but be live with the mag switch I the off position is extremely remote

Sorry can you clarify, are you saying doing a quick OFF then back to BOTH quickly is not really of value due to the remote chance of this OFF function is not working properly as opposed to the risk of not doing it quick enough and causing a backfire with potential hardware damage

[Big Pistons Forever]

PBF


Sorry can you clarify, are you saying doing a quick OFF then back to BOTH quickly is not really of value due to the remote chance of this OFF function is not working properly as opposed to the risk of not doing it quick enough and causing a backfire with potential hardware damage

No I never switch the mags off with the engine running. Cycling the mags Both/Left/Both/Right/Both, and observing the RPM drop and engine sound change as each mag individual cuts out will prove the mags are properly grounded.

The chance that the mags would cut out normally when the switch is in the left or right position but then not cut out in the off position and thus leave the prop hot is IMO very small.

A more tangible danger IMO is slipping the key out when the mag is in the "right" position but you thinking it is in the off position, something I have personally seen was possible to do in two old worn out switches. The logical time to give the key a tug to make sure it does not come out except in the off position is when you do a Both/Left/Both/Right/Both check at shut down.
The "live mag" check where you turn the switch to off and listen for the engine to die does not check for this fault and as I have said in my earlier post will damage the engine if you are slow turning the mags back on.

[Big Pistons Forever]

Anything with a Stromberg or Bing carb requires it.

I guess I wasn't specific enough. I never turn off the mags on a running engine which has a conventional carburator equiped with the idle cut off feature........ which is Oh, I would guess about 97 % of the GA aircraft out there.

I post information which is relevant to the majority of the aircraft out because it would seem to me to be of the most use.

You will always be able to find the exception to just about any of the advice I post but it would seem to me that you are implying that my advice is not correct because it does not apply to your aircraft with its obsolete and now very rare carburetor set up.

I would suggest that a better take away is to note the importance of understanding aircraft systems particularly when you are dealing with rare and/or unusual and/or old aircraft but that there are many best practices which apply to all the common Cessna/Piper/Grumman/Socata aircraft fitted with 4 or 6 cylinder non turbocharged Lycoming and Continental engines.

Also I corrected my post. I of course meant to type switch to "off" and listen for the engine to die not as I had originally posted switch to "both" and listen for the engine to die.

[Big Pistons Forever]

I have to smile a little when a Stromberg carb is described as rare and obsolete though. How many are on airworthy aircraft today? I'd guess maybe 3-5000.

Out of a world wide piston GA fleet of around 180,000, yes I would agree 3 to 5000 sounds plausible.

[Crash one]

My Continental C90 was fitted with a Stromberg which was changed for a Marvel Schebler. However I still kill the engine with the mag switches (no key) & leave the mixture rich. Is this a bad thing? & why not stop any engine with the mags? Why the use of the ICO?

[Pilot DAR]

Stopping an engine with ICO assures that no fuel is left to burn as the engine slows to a stop. If there are glowing deposits in the cylinders, and you have shut down by turning the mags off, the continued flow of fuel into the combustion chambers can be ignited by the glowing deposits, and cause a backfire. Such backfire or run on can be unsafe, when you simply want the propeller to stop, and can be damaging to the exhaust, if that is where it occurs.

To a lesser degree, the now flooded engine can be more difficult to restart, if you want to do that right away.

I too have flown aircraft which had to be shut off with the mags, for lack of a mixture control, in which case you obviously have no choice.

[peterh337]

The danger with any of the mag "dead checks" is switching off and then not immediately switching them back on. Even a slight delay will result in a backfire and probable damage to the muffler and/or other parts of the exhaust system.

IMHO, it is more important to do the dead cut at a low RPM, than to do it very briefly.

My POH (IO540-C4) makes that fairly clear.

Another thing is high altitude mag checks ;) Especially with the mixture leaned ;) Very desirable to check out the ignition system. But needs to be done with care - if you get any issue you have to close the throttle before returning the key to the BOTH position otherwise you again risk exhaust damage, but more seriously than on the ground checks which are done at a very low power setting.

[Jabawocky]

Stopping an engine with ICO assures that no fuel is left to burn as the engine slows to a stop. If there are glowing deposits in the cylinders, and you have shut down by turning the mags off, the continued flow of fuel into the combustion chambers can be ignited by the glowing deposits, and cause a backfire.

Glowing deposits? And how did they get there??? Are you suggesting a good source of pre-ignition or the theoretical and hard to replicate highly carboned up due to poor mixture abuse situation? And I often shut big pistons down with the mags. Never been a problem. If the engine is in fine order, it should stop!

Otherwise, I would like to see hard data on these.

and cause a backfire. Such backfire or run on can be unsafe, when you simply want the propeller to stop, and can be damaging to the exhaust, if that is where it occurs.

A Backfire is when a flame front goes back through the carby, and not easily achieved. Not normally followed by any run on. A run on is usually from a faulty ICO. So which is it? Not likely to have both.

To a lesser degree, the now flooded engine can be more difficult to restart, if you want to do that right away.

Flooded? how so? you pumped the primer? Run the boost pump at WOT/Full Rich?

I too have flown aircraft which had to be shut off with the mags, for lack of a mixture control, in which case you obviously have no choice.

I have had .... i forget now, 700-800 hours on these types, LOTS of starts and stops, thousands, and not once have I had an instant restart problem, they start in 180 degrees, nor any backfire, after fire or anything. In fact hot or cold they were the easiest to start.

Less OWT's and more science please.:ok:

[Big Pistons Forever]

I have had .... i forget now, 700-800 hours on these types, LOTS of starts and stops, thousands, and not once have I had an instant restart problem, they start in 180 degrees, nor any backfire, after fire or anything. In fact hot or cold they were the easiest to start.

Less OWT's and more science please.:ok:

From the above you are implying that you had thousands of routine starts after shutting down the engine by turning off the Mags. Is that what you intended or am I misreading your post ?

Just to keep things interesting ;) I have a question for you

You are cruising along in your C 421 and one engine starts to develop a slight but noticeable high frequency vibration. What is the likely problem and what should you do ?

[Jabawocky]

Yes, engines often in a training environment being moved refuelled etc.

Synch the props ;) Get em balanced or find an RPM that is not conducive to a first/second order vibration.

Heck if is bad shut the engine down and divert. Better to diagnose from the ground. Unless you are sure you know what the issue is and how it can be dealt with.

Observe the trends in your engine monitor if only slight, it could be nothing more than an ignition issue easily seen by trends in the EGT values.

What were you looking for?

Unlike my question about a turbocharged engine with an unexplained loss of MP, which if unexplained could very easily be a fire in the making, this one could have many subtle causes. No doubt I have missed the one you are seeking :ouch:

[Big Pistons Forever]

Jabowacky

The mixture ICO function is ubiquitous in carburated aircraft engines for a reason. The manufacturers decided it is the best way to shut down the engine. All of the Lycoming and Continental engine manuals say to shut down the engine with the mixture as do every POH I have ever seen. I have yet to hear a compelling argument to do otherwise

As for the C 421 engine vibration question. Well this engine has a vibration dampener disk as part of the starter drive assembly. At cruise RPM it is spinning at over 10,000 RPM so if its bearing start to fail and it starts wobbling you will get a high frequency vibration. A total bearing failure will quickly result in catastrophic vibration which could result in the engine coming apart. Ignition or fuel issues will normally result in an intermittent and/or variable vibration, but the sudden onset of a steady high frequency vibration should result in an immediate landing at the nearest suitable airport. A vibration that is increasing in intensity should be cause to shut down the engine.

The point of my question is note the importance of a thorough understanding of the engine and its associated systems and that thinking all of us are Luddites who mindlessly follow OWT is to discount the experience of posters who have flight time measured in multiple thousands of hours and maybe have learned a thing or two in those hours.......

[Jabawocky]

The mixture ICO function is ubiquitous in carburated aircraft engines for a reason. The manufacturers decided it is the best way to shut down the engine. All of the Lycoming and Continental engine manuals say to shut down the engine with the mixture as do every POH I have ever seen. I have yet to hear a compelling argument to do otherwise

Could not agree more. :ok:

But don't for a minute expect me to believe the engine is flooded when stopped by mags only. :ugh: Read the post again, you may have missed that bit. :ok:

but the sudden onset of a steady high frequency vibration should result in an immediate landing at the nearest suitable airport. A vibration that is increasing in intensity should be cause to shut down the engine.

Absolutely, no argument from me.