View Full Version : Magneto's - Live Propeller Question
Mamakim
21st Dec 2005, 02:55
Hey Gang,
Just reading through the theory books about mag's.
They say that when the mag's are off, there is still a chance of the propeller firing because of left over fuel/air mixture.
When the propeller is pulled down by hand, am I correct by saying that the spark produced is not by the magneto but by the impulse coupling contained within it?
Is that right?
Thanks
Arm out the window
21st Dec 2005, 03:25
The 'off' switch earths the primary coil of the maggie, so a faulty earth or switch connection will leave it live.
The impulse coupling is a spring arrangement that winds up and flicks around to produce a quick enough motion of the magnet to produce a spark when the engine is being cranked or the prop moved by hand, so the combination of the impulse coupling (allowing a spark to be produced at low turnover speed) and a faulty earth (leaving the maggie live even though the switch says it's off) can leave you with the dangerous live prop situation.
In answer to the question, it's still the magneto that produces the spark, but the impulse coupling is the bit that causes the magnet to move quickly enough to make a spark when the engine isn't running to drive it.
Mamakim
21st Dec 2005, 04:29
thats answered it
thanks AOW.
Woomera
21st Dec 2005, 07:10
"...there is still a chance of the propeller firing because of left over fuel/air mixture."
No, in a cold engine that could only happen in a combustion ignition engine - i.e. a diesel engine.
In a gasoline engine an external ignition source is required for ignition - i.e. the magneto (or a "hot spot" in a hot engine.)
Beware - you may be surprised at the number of aircraft engines with faulty off switchs leading to a "live" magneto - and potentially, very live prop!!!!
Counter-rotation
21st Dec 2005, 08:50
Can I just add two things to this discussion...
First, this is why you want to see a drop in RPM when you check your mags pre-flight. Absence of this drop means that the magneto you have just supposedly earthed, is still firing (or was never working in the first place - but in that case you'll get silence when you check the other one).
Some wise old types will encourage young pilots to check for a mag drop before shutting down also, for this reason, but it doesn't seem to be a common habit (bit like tuning up 121.5 prior to shut-down).
Second, the part where you were taught to treat every prop as a live one, is very important and may save your life. You will not appreciate this fully until you carefully turn one, and IT STARTS RUNNING. This has happened to me. Had I been less careful at the time, I would not be writing this now. Please be careful...
:ok:
CR.
No, in a cold engine that could only happen in a combustion ignition engine - i.e. a diesel engine.
Highly unlikely to happen in a diesel either, when cold (unless you have a diesel with no glow plugs).
Yeah, yeah. OK. You are correct. But you know what I was trying to say.............
:{
Woomera
AerocatS2A
21st Dec 2005, 21:30
Checking for a mag drop won't always show up a live mag. The mag may only be live when the ignition switch is in the "off" position, i.e., it may be a problem with that position of the ignition switch. The only sure way of checking that the mags will not be live is by switching them both off briefly at idle.
gspinz
22nd Dec 2005, 08:21
bang :p
gspinz
The Messiah
22nd Dec 2005, 09:50
Yes a quick flick to off before shutdown is the way to check for a live mag and furthermore, always treat a prop as live and if you must turn it, turn it opposite to its rotation, ie backwards, and it shouldn't fire even with a live mag.
After shutdown its a good habit to get into of placing the keys on top of the dash.
Counter-rotation
22nd Dec 2005, 11:06
Yep, very true. A dead cut is the only way to be 100% sure. Thanks for pointing that out.
A question for those who advocate turning a prop backwards... I have been told in the past that it can damage other things, like vac. pumps. This sounds like baloney to me, but I am certainly no authority. Can anyone confirm or deny?
CR.
I have been told in the past that it can damage other things, like vac. pumps.
Quite true. The hub slots of Airborne pumps, for example, are canted in the direction of rotation, turning the pump backwards can damage the graphite vanes. That is why they have different pumps for engines that rotate in different directions
if you must turn it, turn it opposite to its rotation, ie backwards
Not a good idea for the reason noted above, and also a very, very bad idea on any radial engine. If you rotate a radial engine backwards that has any oil in the cylinders, the oil gets pushed into the intake tract. If you then subsequently try and start the engine, the oil gets sucked back into the cylinder and joins any other oil there... instant hydraulic lock and a bent con-rod (if you are lucky).
For those that think these engines don't drain oil into the bottom cylinders that fast, I have seen it happen after only ten minutes from engine shutdown.
The Messiah
22nd Dec 2005, 12:27
Radial engines fair enough but I never saw a half a turn backwards by hand on an old Lycoming or Continental harm any vac pump ever but I have seen a person knocked to the ground unconcious lucky to be alive after turning it the normal way.
Slightly different topic but I was always amused to watch guys 'pull thru' a fixed shaft turbine eg Garrett, as fast as they could and about 20 turns, when in reality being geared approx. 18:1 you only needed to move the tip 1 blade width to guarantee the shaft would not cool in the same position it stopped.
Well you won't see it fail there and then, but you will probably reduce it's life. The manufacturer does warn against turning the pump backwards... so... but then it really isn't a big deal.
Regarding Garretts, good point. However, when I did my J31 conversion many years ago at BAe Prestwick, we were told by the BAe engine guys that it was good practice to turn it several times - not to rotate the shaft, but to force some cooling air through the engine to lesson the effects of shaft bow.
Always seemed to me that parking it into wind would work just as well... :p
Woomera
22nd Dec 2005, 23:27
Backwards is Bad (http://www.avweb.com/news/maint/182905-1.html)
Woomera
The Messiah
23rd Dec 2005, 00:32
Woomeras article is entirely about backwards installation not turning it backwards by hand. For propellor safety I have never seen it cause an early vac pump failure.
There is a very good picture of the internals of a vacuum pump at http://www.sacskyranch.com/rapco.htm
The pump is designed so that the when the pump is rotated in the correct direction, the sliding surfaces and the curvature at the end of the vane work together to provide optimium strength. When you rotate it backwards, you stress the vanes differently and the result can be (very slight) damage to the corners and "back" of the vane. This damage leads to accelerated wear, as only a tiny piece of loose carbon in the pump will cause further damage as it gets jammed under vanes and in grooves. The pump may not fail immediately, but it will fail prematurely. Also, making two carbon faces slide against each other under stress, when they don't normally do so, can lead to galling of the surfaces which in turn stresses the vanes.
The following is reprinted from that page, I have added emphasis:
7. Reverse Rotation
As mentioned earlier, Airborne pumps come in clockwise (CW) and counterclockwise (CC) flavors, designed to rotate in one and only one direction. The profile of the elliptical rotor bore is "note" symmetrical in the Airborne pump; also, the rotor slots are cut at an angle. (These design features may improve performance, but some experts feel they make the carbon rotor more susceptible to damage.) Attention to label instructions can eliminate incorrect rotation of pumps in normal use, but avoiding occasional engine "kickback" on startup (or shutdown) is not such an easy matter - yet if vane/slot clearances have opened up, one kickback may be all that's needed to jam a rotor and trash a pump. Pamco Industries' Thomas Zompolas (designer of a new standby vacuum system for Mooney's) feels this is a major reason why Airborne pumps used as standbys tent to last longer than the same pumps installed on engines. "That's why you often hear guys say 'but it was working fine when I shut down'" Zompolas maintains.
A kickback is not that violent, and hand-turning backwards can cause similar damage.
I mean, turn props backwards if you want, but nobody who knows anything about these pumps (like the manufacturer, and service stations) will agree with you about the wisdom of doing so.
It is interesting that your own government agrees, if you have a look at AAC 1/98, there is a virtual word-for-word copy of the text I quoted above (and the article it comes from), see http://www.casa.gov.au/airworth/AAC/PART-1/1-098.HTM
Lefthanded_Rock_Thrower
23rd Dec 2005, 03:45
I am uncertain the you are actually talking about a Magneto in this instance, some aircraft such a C310/402 do not have an "impluse coupling", they have a "starter vibrator".
The starter vibrator is an electrical method of producing the spark required to start an engine.
In the C310 it is a black box located above the pilots knees, in the 402 it is under the panel to the left ( under your elbow ).
The Impulse coupling works as described above.
The starter vibrator feeds an electrical ( oscillating voltage ) through the LH magneto coils, then out the retard points to the cyclinders.
With a starter vibrator, the signal is around a 1,000 RPM in frequency.
Don't try this at home: if you are flying along a 2400 rpm and press the starter button on a starter vibrator equipped aircraft the engine RPM will immediately drop to 1,000 RPM.
the wizard of auz
23rd Dec 2005, 15:00
No, in a cold engine that could only happen in a combustion ignition engine - i.e. a diesel engine.
Just being pedantic, but I would assume you were refering to Compression ignition. ;)
yup, thats what I meant. :p
Just being padantic, but I would assume you were refering to Compression ignition.
Just being pedantic, but I would assume you meant to say pedantic ... ;)
AerocatS2A
23rd Dec 2005, 16:32
bang
gspinz
Not if you do it at idle.
Capt. On Heat
23rd Dec 2005, 18:22
Depends how brief your brief check is.
DeBurcs
24th Dec 2005, 11:34
While we're on the topic...
What exactly is a "shower of sparks"...??
I know what a shower of sh!t is, more or less, but somehow I went right through GA without ever working out what a shower of sparks is. I always wondered though....
It's something to do with ignition.
gspinz
25th Dec 2005, 02:07
basically the 'shower of sparks' is a induction vibrator similar to that of a booster coil and is energised with the starter circuit.
after the eletricity passes through the induction vibrator it becomes a pulsating current. the current then passes into the primary coil of the left maggy. here the coil induces a higher voltage in the secondary winding and as a result a pulsating high voltage spark that is sent to the retard points and then to the spark plug.
so in the end... all the shower of sparks does is to produce many hot (retarted) sparks instead of the single spark that is generated by an impulse coupling system.
i hope the above made some sense :)