Yup, in a perfect world the engines are pulled lean till they die and the mag switches are grounded. And in a perfect world, computers that don't move and have air conditioning units that keep them nice and cool don't ever need worked on.

And that's how I made a living for 14 years, fixing non moving mechanical parts. Saw an engine come to life at school once, guy moved the prop from the back side, popped his hand pretty good.

But you're so smart, you know better, it's an old wives tail.

And I know some pretty smart old wives.

Safety will prevail with me...........but sounds like a great story for Mythbusters.

....you know, Tom....heh heh, after reading our friend bassfisher's post again....

....it looks like one of those "hook, line, and sinker" post's.

" Yes, the engine is leaned out and the mags turned to off. However, after working at a local airport throughout college, I developed a healthy respect for 100% scale engines. A piston engine can start with the mags "off". If a magneto has an internal short, the impulse coupler can produce a current to the spark plugs. The reason for two mags is to produce redundancy for the spark plugs. If one is not working, the other is there to pick up the load. It is not for safety on the ground. I have personally seen a plane start when someone leaned a little too hard on a prop. The stars where aligned, gravity was in place, and everything was perfect. Luckly for that person, they got out of the prop arc before they got hurt. The can happen especially on older engines that have lower compression in the cylinders. A good pilot or lineboy "proping" a plane is already ready for it to start no matter how long it has sat on the ground. After propping planes hundreds of times, my insurance policy was someone was always in the seat with the brakes applied. "Brakes On, Mags cold" was always the way to do it, but even with "cold" mags, I've had planes start on me as I pulled the prop through to prime the engine. If you ever have the notion to pull a prop through on a full size, always do it backwards so the impuse coupler will not engage. It's best not to even touch the prop on a full size unless you know what you are doing."

FBD.

I dunno flyboy I started in round motors, and seems to me a shorted mag just might cause an inadvertant start or more like a pop. I was always taught not turn a warm engine to dress the props. Just in case.
Now I've been in the business along time I've learned to never ever say it cant happen.
Regardless of all that. safety is a habit, if you get in the habit of reaching through the prop arc engine off, then one day you will, with the engine running. why? cause its a habit and the glow plug was gonna fall or something else.
I alway teach my students to reach over or around to put the glow driver on. Always approach from the rear to remove a glow driver.

Ok....I think it's time to clear up this cock and bull story about the magneto. First, let
me say that I have rebuilt personally and Professionally hundreds of magneto's in my
lifetime as a journeyman mechanic....don't try to B.S. me about a magneto.

(everything in quotes " is from Member bassfisher's post)

"Yes, the engine is leaned out"

Once the engine is leaned out and the engines dies from total lack of fuel, there in nothing left in
the intake manifolds and cylinders, so there will not be any firing of the cylinders, until the engine
is primed, and fuel is supplied to the cylinders. This is a simple fact.

"and the mags turned to off"

When the mags are turned "off" they are grounded, and will not produce a spark...period.

"However, after working at a local airport throughout college"

Doing what ?

"A piston engine can start with the mags "off".

No it cannot, because the Magneto swith also cuts off power to the starter solenoid, and there fore
the starter cannot be engaged.

"If a magneto has an internal short"

If the Magneto has an internal "short" is will not produce a spark ever becuase it is not
functional....and is need of repair.

"the impulse coupler can produce a current to the spark plugs".

This is the crux of this BS story. The "impulse coupler" does not produce any electrical
current, ever. Impulse coupler is a fancy word for a mechanical ignition advance mechanism,
of which I am very familiar. This mechanism uses bob wieghts to advance the timing after the
engine is started. It's function is to retard the timing when the engine is not running, so
when you crank the engine...the engine will not backfire, and break the starter gears.


"I have personally seen a plane start when someone leaned a little too hard on a prop. The stars
where aligned, gravity was in place, and everything was perfect. Luckly for that person, they got
out of the prop arc before they got hurt".

Oh, really ?

"The can happen especially on older engines that have lower compression in the cylinders".

An older engine would be even less likely to fire from a resting position, simply because
the comperssion will leak down after it has been stopped for a minute. A cylinder with no
compression will not fire at rest ever.


"I've had planes start on me as I pulled the prop through to prime the engine".

Really ? Why don't you try the prime function of the carburator and the elcetric srarter on the
engine next time. What are you "propping", WWI biplanes ?

"If you ever have the notion to pull a prop through on a full size, always do it backwards so
the impuse coupler will not engage".

This is another load of bull. The impulse coupler (advance unit) is keyed to the magneto shaft,
and is engaged all the time. It cannot be disengaged ever.

"It's best not to even touch the prop on a full size unless you know what you are doing."

That's why the mechanic told you not to lean on the prop....just keepin' the record straight.

Here's a factoid for those keeping score at home....a magneto has to be spun over rather briskly
for it to produce any spark. First the magnets have to spin around the field coils fast enough to
produce enough current to then supply the secondary windings enough current to produce a
spark of 20,000 volts or so to the apark plugs.

Trust me, this does not happen by "leaning against a propeller".

FBD.

Excuse me commenting from "over the pond" but I remember what I was taught long ago.

I learnt from my father in the 1940s "Never point a weapon at anyone unless you mean them harm."

I firmly believe that a propellor deserves similar caution.




old git - - - - - - - aka John L.

I agree....

.....never point a propeller at anyone, whether it's loaded or not.

Capt. John...I don't know how I missed this? (sorry ) You got it...and Phil also did a pretty good job of explaining what I meant as well...(thanks Phil )

Flyboy Dave are right..

We has same system of the impulse coupler in the magneto mounted at the marine engine, when the engine are handcranked to starting the engine, it is to prevent backfiring and damaging arm/hands since impulse coupler will rotate magneto fast and ignite after TDC, it make easier handstart the marine engine.

Jens Eirik

....thank you, Jens....[sm=thumbup.gif]

FBD.

Those proceedures are to prevent the engine from accidently starting. However, it does not mean that all people follow them at all times. Magnetoe swith's have been known to fail and people forget to turn them off, People even turn off the magneto instead of leaning the engine dry. People have been known to try to start an engine and prime it, only to change their mind and leave the prime in the engine. If an engine is at or near top dead center it doesn't take much to turn it just enough for the impulse drive on the magneto to trip, spin and strike an arc. Yes, at least some impulse coupling's do this, some magnetoes are not turning fast enough to create a spark, perhaps some are confusing this with the shower of sparks method! My point is that it has happened, not that it is common.

Just because an engine has leaked down does not mean there is not enough fuel air mixture to turn the engine over. Even a leaked down combustion chamber is enough to cause the engine to turn over past the next firing over, and thus start.

Anyone who has spent any time working on planes and expecially hand proping older planes or even modern ones with dead batteries, then you know that just when you get the cylinder past top dead center you hear a click with a metallic sound, sorta like "tic, poing". This is the implulse coupler going off. When the engine is not turning the advance weights are retracted and a ratchet engages a spring, the spring winds up till it meets a point just past top dead center where it is released and momentarily spins the magneto fast enought to create a spark. After that the magneto snaps back to its original point and then fires at its normal advanced rate. But the point is that the engine does not have to be turning fast, because the impulse coupling spins the magneto.

If it rotates the magneto fast then it doesn't have to be turned over fast at all. In other words it is doing more than retarding the ignition. That is why it makes it easier to handstart.

I had it happen to me when I was flipping a Fox .61 Eagle 4. Now this was with the engine in my right hand and flipping the prop with my left, lucky there was no fuel line attached
Why don't they start so easy when we want them to
Pete

In the impulse coupler are there a spring who are twisted first via timing gear in the engine before the impulse coupler are activated after TDC then the spring are rotating the anchor (rotor with coil inbuilded in the magneto) to make more energy in coil to give spark in the sparkplug while the engine are hand started. After the engine are started, the impulse coupler are deactived by centrifugal force (weights inside the impulse coupler) and the ignition timing are advanced.

Jens Eirik

Which means that there is a spark as soon as the impulse coupling is triggered, even if the prop is barely moving. It is not just for retarding the ignition, for that you only need to rotate the mag, or rotate the breaker plate inside the mag.

Hey Flyboy Dave, don't take this personally i'm thinking you might be soemone i know. Does your real last name mean Hedgehog? If this makes no sence your not who i think you are. If it does send me a line, it's been a long time.

No, it doesn't. I'll PM you.

Even though the coupler will rotate the magneto slightly to improved the spark....

....the prop has to be pulled through in a fast, positive hand propping motion to
generate a spark, or be turning with the electric starter to produce a spark.

The safety feature of this built in factor is so the magneto will not fire if it is
rotated slightly by accident, and also the safety factor of not firing in an advanced
timing mode....therefore protecting the starter and flywheel on the engine from
damage resulting from a kick back.

The engine will still not fire if you lean on the prop, or move it slowly by hand....

....this is balogna.

FBD.

I had this happen on a Jett 90. Shut it off and was off mayb 30 seconds and started right back up again.

You must be mixing apples and oranges, as this sounds like it might be the shower of sparks (not familiar with that system). Impulse coupling mags click off suddenly and can do so with little movement. Look at about 2/3 throught this document.


http://www.allstar.fiu.edu/aerojava/...ht.htm#impulse

However, some old engines DID have magnetos that did not ground properly, which is why we were taught to consider EVERY airplane as having a "hot ignition" system. The old Cubs and Champs at the small field I learned to fly at did not always have the switches off, or proper maintenance, so you had to have a LOT of respect for the engines when you went out to help the owner get his engine propped. Several of the planes I learned to fly in had no starters...T-Craft BC-12D, PA-11, 7AC...several of us learning to fly there got a LOT of lessons and practice on hand-propping those airplanes.

With models, it's a different story. The methanol in our fuels will cause the glow plug element to get hot. Combine this with enough compression or an already-warm/hot engine, and you can get one to fire without the glowplug battery attached. I know, I've done it!

I would like to make a comment if I may. The odds of any one of us having been in every possible situation involving every possible configuration is at the very least, unlikely. There are factors which can cause unexpected ignition. We all know this. This is why we all practice due diligence while operating our aircraft be it full scale or smaller. I'm no stranger to risk and trust me when I tell you that it is possible for an engine to fire unexpectedly. I was asked to dispel the roumor that an engine can fire on it's own. I can say that a model airplane engine is far less likely to spontaniously ignight when after-run oil is used.

Eric M.

FBD I have to disagree so I guess I get no thumb up.
1. Dave you assume that a mechanical kill switch is 100% reliable (never break) to ground mag.
2. Most mags don't produce more spark when turned faster. Take in point a Ford Model T will produce 70,000 volts ,low amps 1908 technology with a simple pull up 1/4 turn of crank. I've started huge antique farm tractors up to 600 CID with a 1/4 crank pull ( 6 to 9 o'clock position)
The permenant magnet passing by field windings at a slow speed is fast enough as long as the magnet doses not crawl thur cycle pulse.
There are 200LB catfish at the bottom of that dam so says the divers. HAH

Sure....if you give the crankshaft a positive good pull with the crank handle, it will produce
a spark....it's designed that way. Same thing with the prop....give it a good hand
prop force, and it will produce a spark....It's designed that way.

However, turn either machine slowly....turn he crank shaft tooth by tooth with a prybar on the
flywheel and see what happens. I give you a clue....

....nothing will happen. It will not produce a spark.

Lean on the crank handle of the Model A, or lean against the propeller of an airplane and
see what happens....nothing will happen....sorry.

FBD.

In theory, I could catch a 200 lb catfish on 2 lb test line, huh ?

If you airplane engine won't shut off with the magneto switch....guess what ?
It's defective....get a new one. Duh.