P-lead question

Matthew

Touchdown! Greaser!
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Matthew
A month or two ago I was preflighting a Warrior. Being able to see both sides of the engine seems like a good thing.

This was very early in my training, maybe only the 4th or 5th time I'd done a preflight on my own.

Since I didn't know any better, I was checking a little more than just the oil and something didn't seem right. I lifted the insulating boot off the p-lead of the left magneto. The nut had come off, and the only thing holding the terminal to the stud was gravity (and the weight of a star-washer and the boot).

I squawked it and took a different aircraft.

I've come to learn that the p-lead grounds the magneto and disables it. The safety issue of having a 'hot' magneto when parked seems apparent.

My questions:

What symptoms would show on a mag check that would indicate a loose connection?

Would there be any indication in flight that this had happened?

The oil had been changed earlier in the day - would there have been any reason to disconnect/reconnect the lead when changing the filter?

Thanks,

Matt
 
Matthew said:
What symptoms would show on a mag check that would indicate a loose connection?

It depends. If it were loose but temporarily making good contact, you would get no abnormal symptoms. If it were loose and temprorarily making no contact, there would be no rpm drop when you turned that mag off. If it were making partial contact you would get a smaller mag drop than normal and/or the engine would vary its rpm as contact was made/lost/made etc.

Matthew said:
Would there be any indication in flight that this had happened?

Not unless you did a midair mag check. The P-lead grounds the mag out, effectively turning it off. While the switch is in the "both" position, neither P-lead is grounded. Therefore, normal operation.

Matthew said:
The oil had been changed earlier in the day - would there have been any reason to disconnect/reconnect the lead when changing the filter?

I doubt it, but the specific routing of the wires would have to be seen to verify this.

-Skip
 
During a mag check (and yes, I should know this) when the switch is set to 'L', is the engine running on the left magneto or has the switch grounded the left mag and is the engine then running on the right?

Matt
 
Matthew said:
During a mag check (and yes, I should know this) when the switch is set to 'L', is the engine running on the left magneto or has the switch grounded the left mag and is the engine then running on the right?

Matt

There's only one way to know, and thats doing a continuity check from the p-lead to ground. If you have the switch in the L position and have continuity on a lead, that is the lead that is NOT running, ie it's points are grounded.
 
I'll look it up tonight - Intuitively (to me, at least) the mag switch has 'L', 'R', 'Both'. On 'Both' , they both run. I'll assume that for consistency that 'L' will ground the right mag and run only on the left.

But I agree, the only way to KNOW if the connections are properly made is to test continuity.
 
Matthew said:
I'll look it up tonight - Intuitively (to me, at least) the mag switch has 'L', 'R', 'Both'. On 'Both' , they both run. I'll assume that for consistency that 'L' will ground the right mag and run only on the left.

But I agree, the only way to KNOW if the connections are properly made is to test continuity.

Right and it doesn't matter to the pilot, but never make that assumption or tell mechanic which "Mag" is failing check without KNOWING, they might take you at your word and it may cause an hour+ of unnecesary work, time and billing. Just tell the mechanic "When the switch is in this position it does...", they should check anyway, but... some of the things I've seen:hairraise: . Mechanics don't mark em when they're changing them out or for whatever reason, so it's a 50/50 chance that you are right free for all. It doesn't really matter. The pilot will squawk what key position has the problem and the mechanic will ascertain which mag it's connected to. So unless you are the person doing the work, which lead is which is irrelevant.
 
I've come across broken or disconnected P leads several times. Because of the high potential for injury I tell the office no one is to touch that airplane until I return. Then, keeping the keys, I go immediately for an A&P. If I cannot find that person I go to their subordinate. I keep going until I find someone that can resolve the problem right away. Think about a rental fleet on a busy flight line with students or low time pilots/CFIs mucking about.


It's a different case if only a select few have access to the a/c.
 
Richard said:
I've come across broken or disconnected P leads several times. Because of the high potential for injury I tell the office no one is to touch that airplane until I return. Then, keeping the keys, I go immediately for an A&P. If I cannot find that person I go to their subordinate. I keep going until I find someone that can resolve the problem right away. Think about a rental fleet on a busy flight line with students or low time pilots/CFIs mucking about.


It's a different case if only a select few have access to the a/c.

Some FBO's have big "Hot Mags" signs to hang on the prop.
 
Matthew said:
I'll look it up tonight - Intuitively (to me, at least) the mag switch has 'L', 'R', 'Both'. On 'Both' , they both run. I'll assume that for consistency that 'L' will ground the right mag and run only on the left.
Correctamundo. And, unless you're flying a Grumman, there's one other position that grounds a mag. Who besides TD can tell what position that is and why?
 
Gotta be "start". One of the mags gets retarded by the coupling. The other is shutoff.....

The operational fix for the occasional not-yet detected P lead break is throttles and mix to cutoff at the kill. Not foolproof but at least you'll only get one blade....maybe.
 
What Bruce said. But that is assuming that the switch has a "Start" position. Many airplanes have a separate starter switch (Button).
 
bbchien said:
Gotta be "start". One of the mags gets retarded by the coupling. The other is shutoff.....QUOTE]

Thats one what's the other?
 
Richard said:
I've come across broken or disconnected P leads several times. Because of the high potential for injury I tell the office no one is to touch that airplane until I return. Then, keeping the keys, I go immediately for an A&P. If I cannot find that person I go to their subordinate. I keep going until I find someone that can resolve the problem right away. Think about a rental fleet on a busy flight line with students or low time pilots/CFIs mucking about.


It's a different case if only a select few have access to the a/c.

Tell ya what, if you have a 24" jumper lead with aligatotor clips on both ends (everybody carries a couple of these right? very handy things for the bag), you can just clip that to the post and ground, and that mag is now off. If you have a hot mag, I don't see what good taking the keys will do you.:dunno:
 
Henning said:
Tell ya what, if you have a 24" jumper lead with aligatotor clips on both ends (everybody carries a couple of these right? very handy things for the bag), you can just clip that to the post and ground, and that mag is now off. If you have a hot mag, I don't see what good taking the keys will do you.:dunno:

Keeping the keys is to prevent someone from going out there and tampering with the plane. The missing keys hopefully would tell them something's up. It did jog the memory of the office girl once when a guy wanted the plane.
 
Greg Bockelman said:
What Bruce said. But that is assuming that the switch has a "Start" position. Many airplanes have a separate starter switch (Button).

Correct and I like to refer to my start button as an anti-theft devise as most folks can't find it in my Tri-Pacer. How many other aircraft besides the Tri-Pacer have a start button?:dunno: ;) :dunno:
 
sere said:
Correct and I like to refer to my start button as an anti-theft devise as most folks can't find it in my Tri-Pacer. How many other aircraft besides the Tri-Pacer have a start button?:dunno: ;) :dunno:

Larry,

The Grumman has a starter button, too. At least, the Cheetahs I've been in have one; not hard to find, though.

But, you are right...to the uninitiated, a starter button could be an anti-theft device.

Jim
 
Jim Chumley said:
Larry,

The Grumman has a starter button, too. At least, the Cheetahs I've been in have one; not hard to find, though.

But, you are right...to the uninitiated, a starter button could be an anti-theft device.

Jim

I don't remember the Tiger I just flew having a seperate button, but folks keep talking about them. I turned the key and pushed it in, IIRC. Maybe the new ones are different?
 
sere said:
How many other aircraft besides the Tri-Pacer have a start button?

Cherokee 180 and 140

Personally I like the separate start button. It seems a more natural thing to use to me.
And there's no chance of engaging the starter when turning the mags to BOTH.
 
sere said:
Correct and I like to refer to my start button as an anti-theft devise as most folks can't find it in my Tri-Pacer. How many other aircraft besides the Tri-Pacer have a start button?:dunno: ;) :dunno:

Most original early model Cessna's, as in early 60's and older. Not sure about the Cherokees. Most if not all twins, even today use some sort of button, not a keyed swithch.

Actually there are quite a few.
 
Greg Bockelman said:
Most original early model Cessna's, as in early 60's and older. Not sure about the Cherokees. Most if not all twins, even today use some sort of button, not a keyed swithch.

Actually there are quite a few.

Some if not all Barons have the start switch built into each of the two mag switches. Only the master switch is key operated. Keys for starting a twin would mean two keys.
 
sere said:
Correct and I like to refer to my start button as an anti-theft devise as most folks can't find it in my Tri-Pacer. How many other aircraft besides the Tri-Pacer have a start button?:dunno: ;) :dunno:

If the door is unlocked, there is no need for a key at all on our club Archer III. The master, mags, and start are all on the overhead...no key needed.

The Citabria I flew for aerobatics had no key, master and mag swtiches on the overhead, starter button on the dash.
 
Richard said:
Keeping the keys is to prevent someone from going out there and tampering with the plane. The missing keys hopefully would tell them something's up. It did jog the memory of the office girl once when a guy wanted the plane.

Won't keep from having whomever needs to handle your plane for whatever reason from grabbing it by the prop. Keys not in a plane doesn't come into play with a ground handler. Real Hazards need to be marked obviously, not with clues. You either eliminate the problem or tape "Don't Touch, Hot Mags" signs to the prop unless you are leaving it in the direct and immediate care of a mechanic.
 
Henning said:
Won't keep from having whomever needs to handle your plane for whatever reason from grabbing it by the prop. Keys not in a plane doesn't come into play with a ground handler. Real Hazards need to be marked obviously, not with clues. You either eliminate the problem or tape "Don't Touch, Hot Mags" signs to the prop unless you are leaving it in the direct and immediate care of a mechanic.

Agree. But I'm not touching the prop until the problem is corrected. That includes hanging anything on it. I'm thinking most pilots are triggered to find the keys first so I make them unavailable. I agree that can be construed as a not too obvious clue but other than carrying a roll of red tape I don't know what the best prevention would be and time is of the essence when it's a busy flight line. OTOH, I've seen people walk through construction fencing/barriers without a clue of the consequences.

Lineman...if they can fog a mirror, they're hired. Most folks know to not stick their toes under the mower, but when it comes to propellors--on boats too--that kind of caution just isn't there. Unfamiliarity.
 
Joe Williams said:
I don't remember the Tiger I just flew having a seperate button, but folks keep talking about them. I turned the key and pushed it in, IIRC. Maybe the new ones are different?
1969-79 American/Grumman-American/Gulfstream-American aircraft of all models have separate mag swithces and starter buttons. The AG-5B's built by American General and Tiger Aircraft have a "push and turn" combined mag/starter switch. That's why you have to start all the 1969-79 Grummans on "L" rather than both -- to prevent kickback due to the right mag firing too soon for starting, the impulse coupler on the left mag retarding the spark properly. If you start on "Both" you stand the chance of a kickback which will trash the ring and starter gears. The only exception is if you have an impulse coupler added to the right mag, too, so you can start on "Both" with more spark, and still start the engine if the left mag impulse coupler gives up or gets weak. And yes, there's a Lycoming Service Instruction that approves the installation of the impulse coupler on the right mag on most of their engines.
 
Ron,

I wondered, seriously, about why we had to start the Cheetah using the L mag position. Thank you for the neat info. Sometime when the right thread is developed, I have a Grumman tale for you. It has to do with emergency exit.

By the way, I tried to give you a positive stroke for the helpful information. The system told me I had to spread around the praise before I could give you any more.

Jim
 
I recently (last annual) found a similar problem on my Cherokee 140. The nut on the P lead to the right mag had backed off. The ring lug was still captive on the bolt but bouncing around making intermittent contact. One symptom as an intermittent kick back when starting because the right mag, with no impulse coupling was occaisionally firing while cranking (symptom went away after tightening the nut). Another symptom was that when performing shutddown by turning off the key, the engine would take a long time to quit, but it would eventually quit. Another symptom that I attributed to it at the time but now am not so sure is that the mag drop on left mag was very little (because the right mag was still intermittently helping). After fixing the problem, the left mag drop remained less than the right mag which I am now attributing to replacement of the coil, condenser, and points in the left mag, and not the right, a couple of years ago. Since I don't have before and after data, I can't be sure exactly how the before and after left mag drops compare.
 
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