G

gasman

Well Known Member
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Why not shut down with the mags grounding so you know if you have a hot mag or elec ign that is not OFF! Mixture location means nothing if you have no fire. Many times an engine will fire a cyl or two when the prop is moved and the key in your pocket and the mixture at full cutoff, but you had a hot mag and didn't know it.

So my question is..... what bad thing will happen if we shut down by turning off the ignition and watch the engine die and then pulling the mixture control to full lean. :eek:
 
I don't know specifically about our aircraft engines in this regard, but a lot of air cooled engines will let you know the exhaust is still hot by making a loud "bang" in the exhaust if you are putting unburned fuel into it after shutting off the spark.
 
We don't shut the fuel off in our cars........... The fuel is in vapor form and there is not much of a oil film left after combustion. Would not it be safer to shut down with ignition than fuel?
 
The engines in the Champs and Cubs never had mixture controls in them to worry about. Shut off the mags and the engine shut down.
 
Cars don't have magnetos that are alive if not grounded and you can't carelessly turn the prop either.

While you could shut off the engine with the mag switch, it is feasible that the p-lead could break at some point afterward (during maintenance or an oil change). Then the mag would be hot AND have a fuel charge... a very bad combination. The only way to guarantee you won't have both a spark and fuel is to eliminate the fuel until the engine starves.

I think the broken p-lead was a much bigger issue back in the '30s. It seems I've seen uninsulated p-leads on some antiques... must have been easier to break.

Karl

P.S. I used to rent a Cub and one day I was the first to fly it after it came out of 100 hr inspection. I taxied to the end of the runway and did a mag check. To my surprise, one of the mags was totally dead. I taxiied back and told the owner about it. He couldn't believe it because he had flown it 30 minutes before and it was fine. We ran the engine and the mag was dead (internal ground?). While this is the opposite of a broken p-lead, I will alway shut down with mixture and only turn the prop with extreme caution.
 
gasman said:
We don't shut the fuel off in our cars........... The fuel is in vapor form and there is not much of a oil film left after combustion. Would not it be safer to shut down with ignition than fuel?
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Why would it be safer to leave a combustible mixture in the cylinders? If someone moves the prop, and a mag is live, a spark could cause a cylinder to fire, and the prop could do significant injury. Cars don't have props, nor mags, so they don't have this problem.
 
Mixture ICO

gasman said:
We don't shut the fuel off in our cars........... The fuel is in vapor form and there is not much of a oil film left after combustion. Would not it be safer to shut down with ignition than fuel?
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Modern automotive fuel injection does, in fact shut off the fuel as soon as the switch is turned off. The fuel continues to flow into the cylinders when the mags are turned off in most aircraft engines. But not very much. So my answer to the question is "because we can." Why take the risk on washing the oil into the sump? On the hot mag issue, you should shut down with the mags (I do it about once a week) to check that everything is working properly. I usually do it when the engine is going to be started again, like after fueling with a taxi to the hangar.

John Clark
RV8 N18U "Sunshine"
KSBA
 
Mag Ground Check

I was taught to always do a ground check before shut-down.

Mixture Rich
Throttle to Idle
Mags OFF momentarily and right back on BOTH
Engine should stop firing, then resume normal Idle
Mixture ICO
Mags OFF


It demonstrates that immediately before shut-down, the mags will ground as expected, but leaves no fuel in the intake that could fire (or diesel) later.
 
It is for safety

The reason Cubs, Champs, and the like are shut down with the mag switch is that their Stromberg carbs have no mixture control. The few carbs that have a mixture control (Cessna 140) do not have an idle cutoff position, in other words, the mixture can be leaned for altitude but not leaned enough to kill the engine on the ground.

The proper technique is (1) switch the mags off and (2) simultaneously open the thottle wide. The wide open throttle eliminates the suction in the carburetor throat so that fuel is not sucked through the idle passage. The engine induction system and cylinders therefore fill with plain air and not air/fuel as the engine spins down.

You can't do this with a larger engine because they have a carburetor equipped with an accelerator pump. The accelerator pump shoots fuel into the carburetor throat when the throttle is opened, thereby negating the reason for the whole process, which is to fill the cylinders with plain, non-combustible air.

You don't need a hot mag to fire a hot engine. Simply moving the prop can be enough to set off a combustible charge in a cylinder. I have seen this happen twice. Each time a pilot was moving the propeller on a hot engine. The mags were not "hot". Each pilot (both experienced) got away uninjured but shaken. They each had suddenly found themselves standing right next to a spinning propeller. One guy flattened his back against a hangar door while the engine coughed and farted right in front of him!

Some engines, when hot, will start firing while being cranked even though the mags are off and the mixture is still in idle cut-off. This was very common on a DC-3 I used to fly. Broken p-lead not required. There was enough residual heat in the engine and enough residual fuel in the induction system to start it running just by cranking, although usually not enough fuel to allow the engine to accelerate. But plenty of energy to kill you if you moved that propeller on the ground and got in the way. I have heard that hot engines can "cook off" on the ground, in other words, the prop can move all by itself after the engine has been shut down. That I have never seen, but I believe it could happen.
 
Mag check

Steve is on the money with his advice. Remember guys, what you should have been taught pre-solo.......that when you do your mag check before flight, is that if you DO NOT get a mag drop, that mag P-lead could very likely be broken/disconnected.

Regards,
 
gasman said:
We don't shut the fuel off in our cars........... The fuel is in vapor form and there is not much of a oil film left after combustion. Would not it be safer to shut down with ignition than fuel?
Click to expand...

Actually, we do shut the fuel off in today's fuel injected cars. This is one reason why today's car engine are lasting 200+K miles. The fuel air mixture is not being sucked into the engine and washing the oil off the cylinder walls on shut down.
 
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Toggle Switches

This topic illustrates one of the main reasons that I use toggle switches instead of "key" type mag switches in my aircraft.
When you do a normal "mag check" with a key type switch, you aren't checking the "off" position. With toggle switches, you ARE checking each off position every time.
Key type switches do fail, and more often than you might think. And unless you routinely check the off position, you may never know.
 
AlexPeterson said:
I don't know specifically about our aircraft engines in this regard, but a lot of air cooled engines will let you know the exhaust is still hot by making a loud "bang" in the exhaust if you are putting unburned fuel into it after shutting off the spark.
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I'm familiar with this, my first car having been a Porsche 914. :D
 
The M14-series radials as delivered have a pressure carb with no mixture control. You shut 'em down with the mags. As they are prone to hydraulic lock in the lower cylinders, you have to pull the prop through before the next start....after as little as a 1/2 hour. If it doesn't scare you, it should.

I know of at least one report of an M14 firing a cylinder w/ mags grounded and spinning backwards for a moment. More likely it was a broken P-lead, corroded connection or bad switch. There are no impulse couplings, so with M9-F mags it fires at full advance and thus the backwards start. An energetic pilot may well be able to spin the mags fast enough to get a spark without impulse couplings. It is a geared engine (.658), so pulling the prop spins the engine faster than prop speed.

The M9-F start system has another trap. The mag rotor has two fingers, one advanced (run), the other retarded (start). A separate start coil powers the retarded finger; you actually start the engine with the mags grounded. If the start spark system is "on" due to defect or pilot error, fuel in a cylinder may get you whacked with a prop.

AFP has developed a throttle body for the M14. Not as restrictive as the carb and allows leaning in flight, but best of all allows you to shut it down with the mixture. No fuel in the cylinders is good insurance.

I know M14's are not an RV item, but they have become pretty common in US sport aviation. Spreading the above info may keep somebody from getting hurt. The P-lead check Neal described is a very good habit with your RV. Checking before shutdown AND killing it with the mixture is as good as you can do. Well, there is one more thing.....always toss your keys on the glareshield.
 
<<When you do a normal "mag check" with a key type switch, you aren't checking the "off" position. With toggle switches, you ARE checking each off position every time.>>

Mel, that is one heck of a good point.
 
SOP for the Rotax 912/912S is to turn off the mags with the throttle at idle. No mixture to pull, no opening the throttle during shutdown. With the gearing and typically light prop, the Rotax does tend to stop right now with just a few prop rotations - it's more like your lawnmower than a Lyclone - so perhaps there is not much issue of removing oil.

(Oh, no, I can already hear the lawnmower comments coming ... why did I use that word? :D )

TODR
 
DanH said:
<<When you do a normal "mag check" with a key type switch, you aren't checking the "off" position. With toggle switches, you ARE checking each off position every time.>>

Mel, that is one heck of a good point.
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Why not go to off briefly to confirm engine starts to quit?
I do this on every runup on my toggle switches.
Kill left. On left,
Kill right, on right,
Kill both, on both
I do this cause the EI drop can be almost imperceptable depending on mixture setting and altitude. But killing both is not imperceptable. Its immediate and obvious. Takes only a brief split second in the off position.
This has been my process for years and a couple thousand RV hours.
Also on the M14 thing, I got a buddy a couple hangers up who lost a knee cap from an M14 accidental firing with mags off while hand turning blades. Only took one blade to swing by and whammo, knee cap across the ramp.
Best,
 
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Yes, this is a valid test.

Kahuna,
You are right of course. The 2 problems I have with this is, #1, most people don't do it. And #2, some people who DO do it won't do it properly and can cause after-firing which is not good for the exhaust system or valves.
This method, as you know, should be done ONLY at idle rpm.
 
THAT'S WHAT HAPPENED........

Mel said:
This topic illustrates one of the main reasons that I use toggle switches instead of "key" type mag switches in my aircraft.
When you do a normal "mag check" with a key type switch, you aren't checking the "off" position. With toggle switches, you ARE checking each off position every time.
Key type switches do fail, and more often than you might think. And unless you routinely check the off position, you may never know.
Click to expand...
TO ME..... with my PA28151 Piper. It was shut down for two days and I used the mixture to do that. I had done a mag short test during the shut down, the keys were in my pocket. But when I went to move the prop from 9 to 6, THREE cyls fired :eek:.

Checking the ignition switch, I found that as the switch wears, the drum can be turned past OFF by about 1/16" and stick there. At that point, ALL MAGS are hot again even when you pull out the key.

A key switch is like a yoke, there is no reason to have them in an aircraft. Most stolen aircraft have key switches.
 
Mel said:
Kahuna,
You are right of course. The 2 problems I have with this is, #1, most people don't do it. And #2, some people who DO do it won't do it properly and can cause after-firing which is not good for the exhaust system or valves.
This method, as you know, should be done ONLY at idle rpm.
Click to expand...

Not sure I understand "after firing" and why only at idle.
I perform this at mag check rpm with no ill effects that Im aware of.
 
Kahuna said:
Not sure I understand "after firing" and why only at idle.
I perform this at mag check rpm with no ill effects that Im aware of.
Click to expand...
Great thread! I just learned the advantages of the separate mag switches my -4 has. So far I've had no problem confirming that the Lightspeed was switched off during my mag check.

I've heard something negative about momentarily switching off the mags at high rpm years ago, but I have no idea why.

Does anyone know if this is true or an old wives tale?
 
Low n Slow said:
I've heard something negative about momentarily switching off the mags at high rpm years ago, but I have no idea why.
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If you switch both ignitions off at high RPM, no big deal imho. If you flip them back ON, however, that's when you might hit a bump.
 
Backfire

Low n Slow said:
I've heard something negative about momentarily switching off the mags at high rpm years ago, but I have no idea why.

Does anyone know if this is true or an old wives tale?
Click to expand...

When you shut off the ignition unburned fuel fills the exhaust system, which will ignite and possibly damage the muffler or stacks when you switch back on again. Only do it at idle, and then only for an instant.
 
YEP!

Stephen Lindberg said:
When you shut off the ignition unburned fuel fills the exhaust system, which will ignite and possibly damage the muffler or stacks when you switch back on again. Only do it at idle, and then only for an instant.
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What Steve said!
 
"backfire"

i know it's not really a backfire -- that's what goes up through the carby, but that's what we called it when i was a teenager. i had a friend who had a '53 kaiser (this was in mid-60's) who used to love to turn off the key (at about 30-35 mph), pump the accellerator for about 15 seconds, then turn the key back on for a loud "bang".

sure enough, one day he left his muffler and part of his tailpipe on the road.:eek:
 
n00b alert!

OK, how do you guys that use toggle switches for mags key the airplane? Is the main electrical switch a key? Isn't that just as scary as a mag switch key?

... forgive my ignorance ;)
 
Only key is the canopy lock. If someone wants to steal your airplane, a keyed ignition will not slow them one bit.
Actually you could use a keyed master. In this case, you will test it every time you turn it on or off. The problem with the keyed mag switch is the off position is not used during the mag check.
 
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Yup, I can see that point. Anyone who knows enough to steal it and has the desire could certainly hot wire it.

How would you secure an airplane that was stored at a tie-down? Some sort of chain like a motorcycle?
 
You must be mental to steal and Experimental!

I don't worry about someone stealing my plane. The instruments maybe, but not the plane. Thieves may be dumb but most are not suicidal.

I have to figure that anyone stealing a plane has had some basic flight training and they probably know that experimentals are all different. Besides, it is much easier to sell a 172 South of the boarder than an RV. Unless it a contract theft, in which case, the key wouldn't matter.
 
N941WR said:
I don't worry about someone stealing my plane. The instruments maybe, but not the plane. Thieves may be dumb but most are not suicidal.

I have to figure that anyone stealing a plane has had some basic flight training and they probably know that experimentals are all different. Besides, it is much easier to sell a 172 South of the boarder than an RV. Unless it a contract theft, in which case, the key wouldn't matter.
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Exactly, RVs don't carry enough cargo to be attractive targets for theft.

In fact, I don't even lock my canopy (though I do take the head$ets and 496 with me). If a thief has already removed the tight cover from my plane, they have already decided they want something. I'd rather let them steal it and deal with insurance rather than spend three months redoing my canopy _and_ dealing with insurance.
 
I just want to thank all that responded to this thread. It is always amazing what can be learned by just asking. It just goes to show that no matter how you shut down your motor, NOTHING IS FOR SURE!!!..... as we see here.
 
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