ICO versus Mag Switch

[modelman]

In reading the recent thread on propellors,I have never really understood (shameful really as a PPL holder) why the ICO is used to shutdown engine as opposed to the mag switch.If the mag switch was used and the motor kept running,then you would know straightaway that you had a duff switch.
Anyone please explain

MM

[S-Works]

Turning the mags off leaves unburnt fuel in the cylinders which will strip the oil. Running it ICO runs it out of fuel and leaves some lubrication as one reason.

You do a dead cut before you shut down so you know if the mags are working properly.....

[modelman]

You do a dead cut before you shut down so you know if the mags are working properly.....


Hello Bose
I do a check for dead cut by switching from 'both' to 'l' and 'r' in turn but the engine still runs ok as you are still running on one and mag drop can sometimes be quite hard to discern (as opposed to a dead cut).Have never taken the key around to the off position.Should I be doing this a part of shutdown?

MM

[Dave Gittins]

Leaning the mixture to starve the engine into submission also ensures there is not a combustible mixture left in one or more cylinders so that IF the mag switch remains live and somebody swings on the prop, there is little risk of a kick back or the engine starting.

[S-Works]

modelman, you check both mags on the deadcut. You do not turn both off and back on again as that is very bad for your engine.

When you check each mag in turn you get a slight rpm drop that indicates the mag is off. Two mags two drops in turn and you have done the dead cut check. Surely you were taught this?

[Rod1]

“why the ICO is used to shutdown engine as opposed to the mag switch.”

This is used mostly in older designs of engine from the USA. Russian engines have always been stopped using the ignition system, as are Rotax, VW (most versions) and most modern engines using FADEC etc. Remember back in the down of motoring car engines were fitted with mixture controls and used magnetos, which were quite popular in the 1920’s.

Rod1

[DFC]

The "dead cut" should not be confused with a mag check which is what Bose describes.

It is called a dead cut because you do indeed select the OFF position briefly. As Bose says this is not good for the engine if you do it at anything above idle and go back to L,R or BOTH causing a backfire etc

If you fly an aircraft with individual switches for the left and right mags then turning each to off and back to on in turn as you do on a mag check tells you that with both switches in the off position, both mags are grounded.

However, on the rotary switch, selecting left and right may give you a drop but this only checks the L and R positions you have not checked the OFF position with this check.

Far better to check the OFF position before the first flight of the day -

Start the engine and allow it time to warm a little. Check the L and R positions give you a mag drop (they are working). Then with the throttle closed, select the OFF position. Leave the switch off until the engine stops. You can then re-start the engine and be happy that the switch is mostly working as it should (you have not checked the start position).

Using ICO to shutdown as has been said earlier leavs the plugs dry etc etc.

However, it is no guarantee that the engine will not fire if you abuse the prop. How many have hopped into the old C172 and started the engine with the mixture ICO. It will fire and promptly stop but enough to hurt your parts that are in the prop arc.

Regards,

DFC

[modelman]

I appreciate all the contibutions and yes,I was taught the 'dead cut' check at the end of the flight.I think DFC is onto it:should be called mag check not dead cut because turning the key to 'l' or 'r' only really tells you if the mags are working (or otherwise),not whether the switch grounds them both when in the 'off' position.
As discussed in the other thread,best to treat the prop as live and likely to bite even if you give it a funny look.

Thanks again
MM

[Russell Gulch]

Turning the mags off leaves unburnt fuel in the cylinders Not if you advance the throttle to "full" as the engine winds down (For engines without accelerator pump).

[S-Works]

Not if you advance the throttle to "full" as the engine winds down (For engines without accelerator pump).

I think we will have to agree to disagree on that one. Without a spark (having turned the mags off) you are not going to burn the fuel.

[Russell Gulch]

No-one's talkin about burning the fuel.

With the butterfly fully open, the fuel wont get sucked in to the cylinders (as much) because the venturi will be at atmospheric pressure, particularly during the last few gasps as the prop winds down.

[S-Works]

Like I said we will have to agree to disagree. My engineer read the post and laughed. I have enough faith in him to keep me safe.

[gasax]

To be fair the 'approved' method of stopping a Gipsy is to idle to cool the engine then earth the mags, whilst smartly advancing the throttle to fully open.

These engines are prone to running on and the cooling plus cold air (actually mixture) will usually stop the engine. However in hot conditions the engine does sometimes run on lumpily for up to 5-10 seconds - the mixture being ignited by hot deposits in the combustion chamber.

Really the key to this is earthing the mags. My O-200 has a Stromberg carb which does not have an ICO so it is only possible to stop it using the mag earth - so I handle my propeller with complete contempt!!!

[Studefather]

So it's 'horses for courses' then

- you shut down an American 4 or 6 with the ICO (because you can)
- you shut down a Gipsy with mag switches (because there is no leaning possible at idle since either no mixture lever - Tiger - or mixture linked to throttle - Chipmunk)

except
- you shut down Chipmunk Gipsy with the fuel cut-off toggle (mod incorporated in-Service in later released examples, presumably for a very good reason e.g. minimise raw fuel washing the bores down)

[Merritt]

I just wanted to add to this thread following some flying with a different instructor on Sunday..

My PA28 check list gets me to go through the 'master switch on' / electrics on routine BEFORE calling 'Clear Prop' and turning the engine over. My instructor on Sunday tells me he has seen two instances where powering on the master switches has resulted in the starter being engaged. This would obviously catch you out if you weren't expecting it.. even more so if there was someone at the front of the A/C.

Now - the ONLY way I can see how this could happen is if the starter / solenoid somehow became engaged just BEFORE the engine was shut down on the previous flight. I say this because im pretty sure the starter / solenoid would not survive being activated for the duration of a normal flight...

Does anyone else perform this check before powering on the Masters? Has anyone else heard of this rare occurance happening??


Cheers

Steve

[JUST-local]

Yes, I've seen this happen on a PA28 also, faulty solenoid, could have been nasty if it were happen during flight those starters are pretty heavy duty.

Certainly gave everyone who saw it a shock! The engineer was horrified.

New solenoid fitted, good as new.

[Merritt]

Scary... I thought it sounded extremely unlikely but obviously not!


Steve

[SNS3Guppy]

It is called a dead cut because you do indeed select the OFF position briefly. As Bose says this is not good for the engine if you do it at anything above idle and go back to L,R or BOTH causing a backfire etc


Not backfire. After fire. A backfire goes through the induction and carburetor. Afterfire goes out the exhaust. If you shut off the mags and turn them on again while the engine is turning, unburned mixture exits the cylinder into the exhaut, where it is subject to be ignited when the engine starts sparking again; the bang is in the exhaust. In engines at low speed with poor induction flow, a backfire is possible; in this case you're increasing the potential for not only damaging the induction, but causing a carburetor fire. A backfire occurs when the valve timing isn't perfect or when valve overlap occurs.

A better practice is a post flight runup, which should be done anyway, along with your idle mixture check. Lycoming and continental both recommend this, though few instructors know about it and fewer teach it. Again, a product of a heritage of inexperience.

These engines are prone to running on and the cooling plus cold air (actually mixture) will usually stop the engine. However in hot conditions the engine does sometimes run on lumpily for up to 5-10 seconds - the mixture being ignited by hot deposits in the combustion chamber.


An engine without an idle cutoff feature requires either fuel, air, or spark to be removed. When grounding the magnetos, spark is removed. By opening the throttle, the mixture is leaned to the point of cutoff, with the spark gone. It can lope and idle for an extended period on the idle mixture on the idle jet. By opening the throttle the mixture is leaned rapidly and any dieseling tendency to run on without ignition will be extinguished. If a magneto has failed to ground, then the engine will continue to lope, and may bang on you when you advance the throttle quickly.

I say this because im pretty sure the starter / solenoid would not survive being activated for the duration of a normal flight...


Actually the starter drive may remain engaged. Most systems are of the bendix design which should disengage when not under a load and at a high enough speed...but not necessarily the case. If the starter does remain engaged, what it becomes a generator at a high enough speed, and can experience damage or damage other components. The nature of the specific system determins this. Many aircraft use a single starter-generator which begins operation as a starter, but after cutoff and stable engine speed has been achieved, serves as a generator.

The starter solenoid is only used to deliver power to the starter; it's a remote switch. If the starter is engaged mechanically, the position or functiion of the solenoid is irrelevant. If you're talking about the starter bendix drive, which functions somewhat like a solenoid, but is not a switch (it's a centrepital clutch, in most cases), that's a different matter.

You ask about checks, but don't specify which ones. If you mean ensuring the swtiches are off prior to turning on the battery master, then yes, you should always do this. If you're asking if it's possible for systems to power up simply by turning on the battery master switch, then the answer is also yes. I've seen this happen on a number of airplanes due to various malfunctions. In that case, a stuck starter solenoid would not cause the engine starter to rotate, unless the starter switch was either shorted closed, or in the on position.

With the butterfly fully open, the fuel wont get sucked in to the cylinders (as much) because the venturi will be at atmospheric pressure, particularly during the last few gasps as the prop winds down.


Not exactly, but close. By opening the throttle plate, the draw from the low idle jet is removed. If spark isn't removed prior to this happening, the engine will accelerate and continue to operate off the primary jet.

However, on the rotary switch, selecting left and right may give you a drop but this only checks the L and R positions you have not checked the OFF position with this check.


If you have a three position switch and you've checked on the left and right positions, what you've already done is verified continuity and grounding of the P leads the magnetos ground out. You've already verified this, without checking anything else. By checking the off position, you're checking the switch, and nothing else.

Running it ICO runs it out of fuel and leaves some lubrication as one reason.


Mixture killing the engine doesn't provide any lubrication, and provides no lubrication increase over an engine killed by removing spark.

Turning the mags off leaves unburnt fuel in the cylinders which will strip the oil.


Oil continues to be redistributed as the piston moves up and down, and controlled by the oil control and scraper rings.

[IO540]

The general idea of using ICO to stop the engine is that it removes fuel from the vicinity of the engine and makes it much less likely that (should the ignition be left ON) the engine will start if the prop is manually rotated.

The dead-cut on the ignition switch checks for the (very unlikely) ignition switch failure mode where cutting each of the two ignition circuits works but cutting both doesn't. This test risks damage to the exhaust system. My POH says "1000rpm max" but I never do it because I can't see how the switch can fail in such a way. For the very simple mechanism, it would be a really bizarre internal mechanical malfunction.

[LowNSlow]

Shutting down a Cirrus Minor is accomplished in a similar manner to the Gipsy.

1. Shut the fuel tap and run the engine at around 1,000-1,100 rpm to even out the cylinder temperatures.

2. Check the mag switches (two toggle switches) and leave it running until it starts to run unevenly.

3. Once the lumpy running starts the mags are switched off as the throttle is moved smartly to fully open.

4. Close the throttle.

My assumption is that this method allows the cylinders to cool evenly, the mags to be checked and the majority of fuel/fuel vapour to be cleared from the cylinders.

[Speedbird48]

Hi Guys,

A lot of inexperience has been taught here, none if which is really wrong, but also obviously not understood by some.

The Gipsy, Cirrus engines were all stopped by switching off the mag' switches and advancing the throttle. The opening of the throttle was to lean the mixture beyond the running stage. The Tiger Moth/Magister etc. did have a mixture control when built, 'studefather', but a lot have had the lever removed. The lever only selected Rich or Weak mixture, never a cut off position. The Chipmunk did, in later years, get a cut off toggle that cut the fuel.

When stopping one of these engines should either of the mags' be "hot' the engine will continue to run and you switch off the fuel and wait 3-4 minutes for the carburetor bowl to drain. Then call the engineer to fix the problem as the engine is still capable of firing.

One thing that has been missed here is the long standing Airworthiness Directive for the Bendix magneto switch that the majority of GA aircraft have fitted. The AD calls for the engineer/mechanic to do a "Dead Mag check" at inspection time, by switching the magnetos to OFF briefly, with the throttle at idle to make sure that the switch does indeed ground the magnetos. This is what you are checking at each "Dead Mag" check.

There are plenty of manuals and information out there that call for the mag' switch to be set to off momentarity, with the throttle at idle at every shut down. This is nothing new. The Left Right routine is OK as far as it goes but it does not check the actual switch, which is why it is called for.

On my own aircraft, which has a dead mag' check at every shut down before the mixture is pulled to cut-off, I had it start 2 days later when the starter was activated for a check. It only fired three times, but the guy standing nearby changed his shorts!!

They are always live!! Remember that, and you will retire with most of your essential components intact.

Speedbird48