Magneto Checks
 

When I was learning to fly, I was taught to check the mag. drop after starting the engine. Later on, a different instructor told me that this initial mag check was not to check the mag drop, but to confirm that turning the key actually turned the mags off. The theory being that the engine was not up to temperature and checking the drop at idle was not a valid check of the magneto performance. A full mag check should be carried out at 2000rpm and the revs. should be allowed to stabilise on each mag. prior to moving to the next switch position, rather than a quick, 'Both-Left, Both; Both, Left, Right,Left,Both'.

While doing the mag. checks I have to confess to inadvertantly turning the key too far the wrong way and causing a Dead Cut. Now, my Cessna check list had 'Check for dead cut' as a valid check, which my instructor then scored out. I feel that stopping the engine momentarily, is a Bad Idea, as it must put a shock into the engine as well as releasing a small charge of unburnt fuel into the exhaust system. This could then ignite causing damage to the manifold or a backfire.

I don't feel that engine handling is taught as well as it should be.

Certainly I was taught (by our maintenance man) to give it a time on left, then back to both, then to right, then to both, stopping at each for while. That has more than once revealed an oiled-up plug where I suspect we wouldn't have noticed otherwise.

Turning both mags OFF is likely to wake up the neighbours with the BANG when you turn back on. It would be an absolute no-no for a car, but probably isn't quite as disastrous for an aircraft. Still wouldn't do it.

Turning off the mags totally is so daft that it's unfunny. All sorts of things can get blown to pieces like that.

Mag checks should be done to allow the engine to stabilise at the RPM rather than just a quick drop for the reason Keef has given.
If there is one thing that drives me nuts, it is having people rushing through their checks, the Carb heat being a particular favourite.

The mag check just after start is only to check that you can actually switch the mags off. It isn't a meaningful check of RPM drop since the rev's will be low and the engine isn't at all warm.

The RPM that you check the mags at is different depending on the type you are flying, eg. C152 1700rpm, PA28 2000rpm, HR 200 1800rpm etc..... The POH will have the right numbers in it for the machine you are in.

But if discovering a mag problem or oiled up plug immediately after start up saves you taxying all the way to the hold before you find the problem checking at 2000 rpm then there has been a positive benefit for everyone on the airfield.

If stopping the engine on the mags is so bad for it, what about some of the very old a/c such as the Tiger Moth that doesn't have a mixture control and is always stopped by switching off the mags.

I appreciate that this has to be done after a 30 second "idle" but is there a damage risk here?

Waiting for the a/c to run out of fuel is the only other option to stop something of this vintage!!

Monocock

It's not the switching off that does the damage, it's switching back on while the engine is still turning. The engine will re-start and the unburnt fuel mixture which has been pumped into the exhaust manifold through the cylinders during the period the mags were off will ignite and exlode. This can cause serious damage.

The Chippy is shut down by switching off the mags, but it is done at idle power setting, then the throttle smartly opened to full once the mags are 'off'. The fully open throttle at very low rpm means there is little vacuum in the carb venturi so litttle or no fuel is drawn into the engine (which could cause run-on if there are hot carbon deposits in the cylinders, and in any case would cause oil to be washed of cylinder walls, increasing wear at next startup).

SSD

Owenlars, when the mags checked at start up, then you cannot really tell if you have one fouled up plug. The pre takeoff power check is the the best place to tell if you have any problems such as a fouled plug, since the engine should be coming up to temperature and the great gobs of fuel you chuck in at start up should have disappeared by then.

A fouled plug isn't a problem really. the vast majority of times it can be burnt off by increasing the rpm and leaning the mixture for a few seconds. Get your FI to show you if they haven't already.

A SSD says, switching an engine off by the mags isn't going to harm it, it's the unpredictable nature of what happens when you turn them back on that's the issue. If you have a motorbike. try running the engine up and then hitting the kill switch momentarily, see what happen next! Either a flame out the exhausts with an almighty bang, or a kick back through the carbs leading to singed privates. Now try that in £100,000 a/c!

When gaining my licence, it had always been explained to us both by the various FI's and our powerplants groundschool instructor that the initial 'dead cut' check of the mags after starting was just that - a check to see if the individual mags were 'dead' or not. If the key (or switch) was turned to select either left or right mag and the engine died, we took our hand off it immediately and allowed the engine to stop before calling for an engineer. The check is always carried out at low RPM as this is the situation which would result in the least damage to any components in the event the mag was inadvertantly turned back to both and the engine restarting.

The other mag check at higher RPM is to check that the RPM drop from both mags to single mag operation is within limits, and that the engine would continue to function in the event of losing a mag in flight. As has been said, during this check the RPM should be allowed to stabilise for a few seconds on each mag to check for fouling of the plugs. There should be little or no risk of a dead cut at this stage which could damage the engine as that check has already been carried out.

Say again

Quite right but you can tell if you've got a knackered magneto (I guess most of us check magnetos just before we switch off so we can do something about it before we go flying next, it's the same principle). I, for one, have also identified fouled plug(s) and cleared them(it) by leaning on the way to the fuel pumps. The point is that if you can identify something before you taxi anywhere it's got to be better than taxying to the hold and finding something during your power checks. And of course the power checks (which we all do don't we) are the acid test to show stuff up that you cannot see at idle.

Kolibear

Just reading your second paragraph. I may have misunderstood you but dead cut is not referring to turning the both mags off, instead you are checking briefly that both mags are at least working.

I think engine handling is best learnt in your own share - cos then you get it right to keep down on the maintenence bills!

One more point - don't forget to check each mag before shut down - and watch the RPM to make sure it drops a little (50rpm or so), this shows that the mag will not be live when you shut down the engine, always a bonus if your about to pull it byt he prop!

Bodie

The purpose of the "dead cut" magneto check after start is two fold in the sense that when selecting an individual magneto there should a) be some drop but b) the engine does not stop.

a) confirms that you do not have a live mag

b) confirms that you do not have a completely dead mag!

In the case of a) - ie in the event of ZERO drop - one might then troubleshoot further by closing the throttle completely and turn the magneto switch to OFF to see what happens. If a mag is live then the engine would continue to operate in which case maintenance action would be required since it would not be possible to switch off the mags completely in the event of an emergency which requires a shutdown in the air and also the engine is live should anyone be imprudent enough to pull the propellor through whilst on the ground.

In the case of b) if you have a completely dead mag there is no point in going any further unless you are prepared to risk your neck flying around on one mag!

All these points should have been covered in basic flying training but as an instructor it is not infrequent to discover that some pilots are a little unsure as to WHY these checks are conducted as they are.

Finally, the magneto system is NOT fail safe in the sense that when you switch a mag off you are MAKING a circuit (ie grounding the mag to the engine etc.) This is yet another reason for treating a propellor as live at all times. I recently watched with horror as a private owner pulled the prop through whilst the aircraft was in the hangar!!

Hope this helps

SAS

My POH requires a all-off mag check too, saying it must be done below 1000rpm to prevent exhaust system damage! I've never done it, preferring the traditional half-off, half-off check.

There is a possible failure mode within the mag switch which would be detected by an all-off check and which would not be detected by the traditional check, but it's extremely unlikely.

IO545

It's been bugging me as to why my checklist says to select Mags OFF then Both checking that it would have stopped. Seems to me that checking a drop with LEFT and RIGHT checks the same thing.

How would that failure occur?

Brodie,

I think that you are correct & I have misunderstood the term 'dead cut' - you learn something new everyday here - thanks!

Slow-Rider

Don't ask me..... perhaps if I spent some time examining the internals of the mag switch I might be able to come up with a highly theoretical scenario of a breakage in a particular precise manner which might do that.

Personally I don't believe it, and it's not worth the risk because if you get it wrong the resulting combustion inside the exhaust system sounds dreadfully dangerous.

IO, I assume that because your machine is injected, then the procedure may be different, but I still wouldn't recommend switching the mags off totally and can't really see how it would be totally different anyway.

Mind you, I still hate the idea of magnetos compared to a proper electronic ignition system!

The cynical part of me wonders if the engine manufacturers want us to dead cut, so that their aftermarket parts business gets a boost! How much is an new 540?:{ :uhoh:

If you want to be cynical, pick up a handbook on engine management from the Lycoming disti stand at the next GA exhibition. It basically says: we think you are all idiots so we want you to run engines well rich of peak because that way you can't damage them. It also says things like "our engines are built to the highest standards so there is no point in doing anything to make them vibrate less".

It is apparent that a lot of the instructions/procedures in GA have at best no justification; at worst they are dictated by lawyers.

To be fair, if you look at the standard of instruction when it comes to engine management in GA, Lycoming are stuck between a rock and a hard place... they do sort of say that if you have proper engine instrumentation then you can run LOP but they distance themselves from it as far as they can.

Deakin should be compulsory reading for everybody who has 3 levers, and all of them should have a multi cylinder engine monitor :O

This is what I've always understood the purpose of the dead-cut check to be. However, if you have badly fouled plugs then this can cause the engine to stop as well. I discovered this one when preparing for a night flying sortie. When I did the post-start dead-cut check the engine stopped when I selcted one set of mags. I switched everything off, went back to the clubhouse, and reported the aircraft U/S with a dead mag. One of the instructors went out to check (why do they never trust us?) the aircraft. He confirmed that the engine did stop on one of the mags, but after running at 2000rpm with the mixture leaned out for a few minutes, the problem had gone, showing that it must have been due to fouled plugs.

Brooklands

Why don't we trust you to make decisions that would cost us lots of money? I think you answered the question yourself!!;)

Can anyone give me an idea how long you should lean
the mixture to 'clean' the plugs. Is it just a case of leaning until the revs drop, then full rich again. Or should the engine be run for a couple of minutes leaned.

Many thanks,

TO