Preventative Maintenance......Plug wires/Plugs

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Monty

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Jul 15, 2010
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Fayetteville, AR / USA
My fishing trip was OK. Not the best, not the worst. We caught about 50 fish in two days of hard fishing. Weather wasn't too bad. Usually we would have caught 50-75 per day, but that's fine, it was a good time all round until......

We got in the plane to head home. My first clue that something wasn't quite right was when the engine died during my first start attempt. Hmmm.....that's unusual. My second clue was when I had to use the primer to keep it going even though it was warm outside. That's odd....must be fouled plugs.....but I did my usual lean burn the crap off the plugs before shutdown....so what the heck.

200 rpm mag drop on the right mag....three attempts at clearing the plugs and no dice. Rough as a cob and getting worse...CRAP!! Luckily there is an A&P on the field. All plugs test good...mag is removed and tests good...two cold cylinders.

Final diagnosis is the plug wires. One is shot, several are intermittent. CRAP, CRAP, TRIPLE CRAP:mad2::mad2:.

It's Friday afternoon, the shop is closed on Sat. and Sun., I have oddball Bendix mags, and you can't just go down to Nappa and get a set of plug wires for a 1958 C172. Plus I have cooler full of fish fillets that need to get to the freezer 500 miles away. There is a NO Saints/LSU bowl game double header and every freaking rental car is spoken for...except Some ginormous GMC Yukon. So I get to drive a gas guzzling thing the size of school bus home. $%#@#%$%#$%^%$ These are some expensive fish.

The parts just got there this Friday, a week later, plane is still 500 miles away. Now I have to see if this really fixes the problem and if so find a way down to retrieve the AC.

I keep the airplane in the hanger with a heater on a thermostat to keep the engine warm. That prevents condensation and keeps everything nice and dry. Because of this, the plug wires were probably marginal, but tested OK at annual. It was cold when we landed, and got colder. The airplane was parked on the ramp. Then we had a warm front with humid air, and several 100% humidity mornings. I'm sure the engine was dripping wet, and water would have been in every crevice or flaw. So any weak point in the plug wires became a short circuit waiting to happen. Once a path to ground is formed....Ugh...

The thing that really gripes me is for the cost of all this inconvenience, I could have replaced all the plugs, wires, and had the mags rebuilt or changed to slick mags.

So how often do you guys replace your plugs/plug wires as a preventative thing? Would you do the Slick mag conversion?
 

BBerson

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Dec 16, 2007
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Port Townsend WA
So how often do you guys replace your plugs/plug wires as a preventative thing? Would you do the Slick mag conversion?
I would replace parts as needed... you have two mags anyway. The problem is your dry storage (as stated). Leave it in the rain a few times to find the fault at home first before a long trip.
The conversion will need lots of parts such as different impulse couplings.
 

skeeter_ca

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Jun 29, 2005
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1,027
Location
Yucaipa, Ca
I would replace parts as needed... you have two mags anyway.
So are you saying he should have flown home knowing that there was a problem with the right mag? And if he did know it has just a couple bad wires on the right side should he have flown it home?
 

TFF

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Apr 28, 2010
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Memphis, TN
The tough question is how old is the overhaul and were those components changed out then. The parts should make TBO as long as TBO is not 30 years away. I would say if those parts are 15 years old they are old. Slick mags are great but they will want the same care. Many budget overhauls will just bolt that stuff back on if they were working fine so that stuff will have 3000 hours and many owners never feel the degradation as they get use to the incremental loss in power. A local guy had to sit with his Aztec for 2 weeks when the mag went bad; one had to be put together for him. He was lucky he was retired and visiting family at the time, so his decision could be to that. Block heaters can also cause corrosion if they are run all winter without a bunch of flying; a noted engine rebuilder called it turning your engine into a terrarium. It is best to warm it up when it is time to go fly.
 

Dan Thomas

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Sep 17, 2008
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5,251
If one looks at the manufacturer's inspection requirements he will see that the plug wires are to be tested every 100 hours, the plugs removed and cleaned every 200 or annually, the magneto to be taken off and inspected internally every 500, the alternator is a 500-hour thing, and so on.

Most private airplanes don't get this sort of attention, and sooner or later the owner gets stuck in some inconvenient place or has his weekend or whole vacation upset. In the end, no money has been saved. There's nothing better than looking at the service manual to see what the maker thinks.

Dan
 
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BBerson

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Port Townsend WA
So are you saying he should have flown home knowing that there was a problem with the right mag? And if he did know it has just a couple bad wires on the right side should he have flown it home?
I never said that. Takeoff is almost always optional.
I said: "leave it out in the rain and find the marginal faults at home."

The point was, since he has two mags, no reason to change parts on a arbitrary time basis. Even parts that you have only one of (like oil pump, for example) are not usually replaced yearly. Why should mags be any different?

The AD's and airworthiness limitations prescribe the minimum time changes required for part 91 operations. As Dan said, periodic inspection according to service manual is needed, but not replacement.
 

Monty

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Jul 15, 2010
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Fayetteville, AR / USA
More info:

I had a weather window, a passenger...we were running out of daylight, and there was rising terrain ahead...taking off with an airplane less than 100% was not an option. If the weather had been stellar, it was just me, and the distance was only a couple hundred miles....maybe, but probably not. Static RPM was way off, and it was really rough even on both mags. My guess is there were some intermittent shorts in some of the wires on the left mag also.

The engine has about 500 since major. Airplane was just out of annual. Everything was fine on the way down. I have only been a partner in the airplane for a year, and would have replaced the wires at annual had I known how old they were. My partner and I are going to go through the engine log with a magnifying glass...to see what else might need attention.

RE engine heaters...I'm aware of the problem with corrosion. This is only a problem when part of the engine is warm, and part of it is cold. Really a problem if you heat the oil pan and boil out the water and other nasty stuff, only to have it condense on some vital part. That is not the case here. A warm engine is a happy engine, especially if you live somewhere that sees a lot of cold weather followed by warm humid weather. I use an electric heater with a duct that blows warm air into the cowl, and keep a blanket over the cowl and outlets. This keeps the entire engine about 70-80F in cold weather. I usually pull the dipstick out to allow water vapor to escape. The heater is on a thermostat so it only runs when needed.
 

TFF

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Apr 28, 2010
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Memphis, TN
I agree about the warm air instead of block heaters. Your also the only other person I know out of about 100 owners to do it that way.
 

PaulS

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Sep 6, 2011
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Location
Seattle, Wa., USA
I replace the plug wires, plugs, cap, and rotor every two years on my cars! On an aircraft I would do it every year simply because there is no place to "pull over" to fix it if something goes wrong. Do it yearly and then you at least have piece of mind (and a better chance of getting there and back).
 

TFF

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You got deep pockets. Most certified wire kits are $500-1000, plugs $25-90. I have found that there is a limit of remove and reistall; doing it just to do can shorten life as the tips are fragile. I also put some DC4 on the wire coil; dont put it on the insulator.
 
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