I recently removed a magneto from a Piper Cherokee, and my curiosity about why the magneto had failed got the best of me.
I took the mag home, sat down at the kitchen table, put my grandson Spencer on my lap, and took the magneto apart! It was great fun having him help me 🙂
Before long, we had the case apart, and it quickly became obvious why the magneto wasn’t working… there were 6 broken teeth on the distributor gear! And 3 of the broken teeth were together in a row, making it impossible for the distributor gear to mesh with the drive gear… this mag was completely dead.
It had been more than 600 hours since the last 500 hour magneto inspection.
I wonder how long it had been since the first tooth broke… we’ll never know.
But it does remind me of the importance of doing 500 hour magneto inspections. For Part 91 airplanes, it’s a recommendation and not a requirement, but in this case, I think the recommendation is a really good one.
You can listen to the podcast episode for more details about the troubleshooting process on the magneto, and here are some other items also included in this episode:
Some King Air flying this week (I never get tired of the “view from the office!”)
Our complex ATC clearances we received on the way to the Boston area.
The annual inspection I’m currently doing on an A36 Bonanza… here’s the overhauled alternator drive coupling I installed because the old one failed the slippage torqe test:
Our IA renewal seminar at Blue Ridge Community College… it’s always a good time to see friends, learn some things, and satisfy the FAA requirements to keep my IA certificate current.
And finally, Maria and I are getting very excited about our trip to Hawaii! We’re going to visit John and Leslie Caubble on Maui, and I’ll be working with them on the flight school airplanes at FlyMauiHI.com It’s going to be a grand adventure!
That’s a good question, and although I don’t have the full story, we will toss around some possibilities in this episode. Thanks to all of you who responded with some really good guesses about the cause.
I received responses from 33 people after I sent out that email recently! The prevailing guess about the cause had to do with improper magneto timing which then caused the engine to kick back in the reverse direction and break the starter housing. That was my guess as well.
Be sure to listen to the audio to hear about all the responses and ideas.
In a future episode real soon, I’ll be releasing an interview with Adam Sipe from the Airplane Intel Podcast and AirplanePrebuy.com
Adam has a wealth of money-saving, stress-reducing wisdom and knowledge about buying an airplane and conducting prebuy investigations. Be sure to look for this episode real soon.
I have a story for you today that I wish I didn’t have. But it’s true, so we might as well learn from it. Thankfully, everything turned out well on this one.
At Moody Aviation where I went to school years ago, we had a concept called “SYE.” It stood for “Share Your Experience.”
It was a concept to encourage us to take our mistakes and make a learning opportunity from them.
Listen to today’s podcast for the story of how I accidentally left a red shop towel inside an airplane engine… I hope I never do that again! Definitely an opportunity to learn “what not to do!”
Ok… it’s time to ‘fess up 🙂 I know I’m not the only one out here who’s done a dumb thing or two in airplane maintenance… so, if you’re willing to share your story, leave me a message here on the website, or shoot me an email dean{at}airplaneownermaintenance{dot}com
I’ll assume it’s ok to share the stories, without names (unless you ask me to share your name.) Thanks!
Another item we discussed in this episode is a Cessna 172 that would not start. It turned out to be a problem with the left magneto, but in the process of troubleshooting, we discovered the spark plugs had gone far too long without being cleaned.
Take a look:
And here’s the loose distributor electrode inside the magneto:
Other items we discussed in this episode:
An oil leak on a Twin Cessna (you might have seen the email I sent out about this.)
The items we used to fix that oil leak: Aviation Permatex, Continental silk thread P/N 641543 (If you only need a small amount, check with Fresno Airparts… they sell it for 20 cents per foot) and the third item used was Loctite 515. These 3 components, used properly, make an excellent seal for the oil pump cover.
Yesterday, I had an opportunity to present a talk at our local IA renewal seminar about exhaust valve borescoping.
I shared some of my favorite pictures – the ones that demonstrate how critically important borescoping is, and that compression testing alone is not sufficient.
The following three examples are especially startling, since they showed a compression test of at least 60/80, but were in imminent danger of in-flight failure:
The FIRST ONEis from a Continental engine in an A36 Bonanza. Notice the green area at 9:00, indicating that area was in danger of a piece breaking off the edge.
The SECOND ONEis from another Continental engine, also an A36 Bonanza. The compression of this one was 61/80, but there was a chunk missing from the edge, with a crack toward the center of the valve!
The broken edge still had enough area to seal for a compression test:
And the THIRD ONE was from a Lycoming engine in a Cessna 152. Take a look:
In the previous three examples, the borescope was absolutely necessary to discover these dangerous and unairworthy exhaust valves.
For comparison purposes, the following two pictures are examples of healthy exhaust valves:
I also shared some pictures of valves that other people have removed from their engines.
The following is from Sylvan Martin’s Grumman Tiger with a Lycoming O-360 engine. Sylvan was able to land safely after his engine ran slightly rough. He said, “My opinion is that cylinder 1 would have been noticeable at a minimum of 150 hrs before it deteriorated to the point of cracking that far across the face… Compression check will help w/ ring problems, but our family’s experience in industrial diesels (power generation) is that rings fail slowly/predictably while valves fail suddenly/catastrophically…..”
He later had to change two other cylinders, after the borescope indicated they were burning:
(Previous three photos courtesy of Sylvan Martin.)
Jason Figueroa had to change a cylinder on his Cherokee 235’s Lycoming O-540 engine due to a burning exhaust valve that was found with a borescope.
(Photo courtesy of Jason Figueroa.)
Jason said, “Borescoping told us we had an impending problem, and led to catching it before it failed.”
Here are two more exhast valve pictures from an unidentified source:
I’m convinced this broken valve could have been detected with a borescope LONG BEFORE the edge broke off.
My hope is that all of us who have the responsibility of determining the airworthiness of piston engines in airplanes, will use a borescope along with the compression tester for higher levels of safety and reliability.
Do you have a dream to fly? If so, you might be encouraged by the poem I shared at the end of my talk yesterday. My daughter sent it to me one day last Summer… it’s called, “The Dream Does Not Die!” If you’re interested, be sure to listen to the last part of the audio.
Note: scroll to the bottom of this post for a special announcement.
Have you ever experienced an inflight magneto failure? If not, you might some day… but if you take action on the recommendation in today’s episode, you can dramatically reduce the likelihood of that happening.
The left mag on this Cessna 172RG failed in flight, and after opening the mag, it became obvious what happened.
The finger electrode that is part of the distributor gear inside the magneto, had gotten loose from its mounting position, and was able to rotate freely 180 degrees, while still occasionally resting in its proper position, which made it somewhat intermittent. Take a look:
The finger electrode should stay firmly in it’s mounting point as in the following picture:
The LAA, or Light Aircraft Association, put out an excellent, one page Airworthiness Alert February 21, 2019, about this very issue. Here’s a link for that document:
The bulletin recommends changing the copper electrode in certain Slick magnetos to a monel electrode, which is much stronger. This is definitely a worthwhile service bulletin to comply with for the affected Slick mags.
Now back to the Cessna 172RG… it also needed a new ignition switch kit. The Gerdes ignition switch is subject to AD 93-05-06 which requires inspection and lubrication each 2000 hours, although I sometimes wonder if this interval is frequent enough. Take a look at the carbon tracking and wear inside the ignition switch:
This condition can cause the switch to malfunction… it’s easily remedied by installing an A-3650-2 Ignition Switch Kit, which includes replacement of this contact plate, as well as the 3 contact cups in the switch housing. Notice the difference between the old cups and the new cups… any contacts that have the silver plating worn off, must be replaced… most of the switches I’ve inspected, have needed a new kit.
Replacing the switch plate is fairly simple, but slightly tedious, by transferring one wire at a time, from the old switch plate, to the new one:
The second story we talked about in today’s episode is about an inflight magneto failure in an Aerostar. The pilot did some excellent inflight troubleshooting, and landed safely after determining the right engine left mag had failed, and was able to fly to a safe destination with the right engine selected on right mag only.
So after today’s episode, I have several tips about magneto troubleshooting:
When you select one mag, if the engine immediately runs rough, it’s most likely a spark plug issue.
When you select one mag, if it’s a more smooth, but large RPM drop, it may be a magneto problem.
When you select one mag, if the engine dies immediately, it might be a mag, or a shorted P-lead, or the ignition switch, (the switch being the most unlikely, in my opinion.)
Causes of ignition system troubles, in the order of likelihood:
Spark plug issues.
Magneto issues.
P-lead wiring.
Ignition lead, or corrosion in magneto attachment.
Ignition switch.
In fact, I seriously wonder if both issues talked about in today’s episode could have been avoided. I don’t have information on the 172RG’s magneto, but the Aerostar’s failed magneto had about 1000 hours since overhaul.
Find a reputable magneto shop, and keep your mags in tip-top condition.
My favorite magneto shop to recommend is Aircraft Ignition Services LLC, operated by Kevin and Leah Herrington in Honey Grove, Texas. They’ve been doing excellent work for us at Classic Aviation LLC. Check out their website:
Announcement: Carl Valeri is the host of the Aviation Careers Podcast and the Stuck Mic AvCast. He recently interviewed me for an episode of each of these shows (ACP255 and SMAC229), and it was so inspiring to talk with him. You can find these here:
Corrosion in the towers of a magneto can cause high resistance, and the problem will be magnified as you go up in altitude.
This green corrosion could likely be cleaned up and it may improve some, but it may also indicate a good time to perform either a 500 hour inspection or an overhaul on the magneto.
Pressurized magnetos have an o-ring that seals the connection where the wiring harness attaches to the magneto. I found this one on a twin Cessna last week, while changing one of the magnetos.
Listen to today’s episode to hear about the details of this issue, and some things to think about for optimal magneto performance.
Thankfully, we got the twin Cessna running again, and the owner made 4 flights the very next day!
Here are some recent happenings, straight from the shop at Classic Aviation, LLC:
Birds in the cowling!
It’s bird nest season, and they are driving airplane owners crazy by building nests on top of engines.
Listen to today’s episode to hear about a Bonanza owner with 2 nests on his engine after a short stay at Shenandoah valley.
And here’s a Cessna 182 that came into the shop for some maintenance. When it’s nesting season, the birds don’t waste any time making a safe place for their eggs.
Corrosion on the battery terminals!
Here’s what the battery terminals looked like on a Cessna 182 that stopped at KSHD due to an intermittent electrical charging system:
The negative terminal was severely corroded.The positive terminal had some minor corrosion as well.
Reminder: As an airplane owner, you can help ensure reliable electrical system operation by keeping your battery terminals free of corrosion.
It’s a good idea to check these terminals at least once in between your annual inspection, and apply some anti-corrosion spray.
Rough engine: Is it spark plugs, or a magneto?
Here’s a spark plug from a Piper Apache that had a rough mag check.
I received an email from the flight instructor and he was suspecting a magneto problem.
However, after some troubleshooting by running the engine, watching the JPI engine monitor, and removing the specific, affected spark plugs, it turned out there were actually 3 fouled spark plugs!
One way to avoid fouled spark plugs, is to aggresively lean the mixture while taxiing around on the ground at low RPM’s.
It’s amazing how much you can lean the mixture on the ground at 1000 RPM and still have the engines run smoothly during taxi operations.
AND, with the mixtures this lean, there is little chance of forgetting to enrich them for takeoff, since the engines will likely not run well if advanced to takeoff power.
For this reason, during taxi operations, it’s good to lean until the engine almost quits, and then enrichen enough to run smoothly.
If you only lean slightly for taxi operations, you risk forgetting to set the mixtures for takeoff.
Here is how I was able to set the mixture controls in that Piper Apache on the ground at 1000 RPM’s, for smooth ground operations.
So, that’s all for this week.
Remember:
Keep the birds out of your cowling if your airplane is parked on the ramp.
Keep your battery connections free of corrosion for reliable electrical system operation.
And consider how you can lean your idle mixture for cleaner spark plugs during taxi operations.
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