Massive Mag Drop

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Piper introduced the PA-28-235 Cherokee Pathfinder in 1963 as its competitive answer to the Cessna 182 Skylane. Powered by a 235-hp Lycoming O-540 engine with a gross weight of 2,900 pounds, it could “lift anything you could close the doors on.” In 1972, Piper lengthened the cabin by five inches, lengthened the wing and tail surfaces, and increased gross weight to 3,000 pounds. Finally, in 1978 Piper replaced the constant-chord “Hershey Bar” wing with a tapered wing and changed the model designation to PA-28-236 Dakota.

Ray’s plane was a 1965-model. He based it at a small but very busy GA airport in the desert Southwest, where the flying weather is VFR 360 days a year by local statute. So, on a beautifully clear afternoon in early November, Ray decided to go flying. But he never got further than the runup area.

What the !@#$%?

Ray ran his engine up to 2,000 RPM as always, then started into his usual mag-check ritual. He typically saw a drop of 75 RPM on each mag. But this time when he switched to the left mag, the RPM dropped all the way to 1,000 RPM—that’s a 1,000 RPM drop!—and he feared the engine was going to quit, though it didn’t.

Ray knew he was definitely NOT going flying with a problem like this. So he taxied back to his tiedown and pulled out his smartphone. Ray is a SavvyMx managed maintenance client, so he used his phone to open a trouble ticket for his Savvy account manager Dave Pasquale A&P/IA. Ray described his symptoms and even attached a short video clip of the stumbling engine that he’d taken with his phone.

Watch Ray’s video of the stumbling engine

Within 15 minutes, Dave had jumped on Ray’s ticket.

“The most likely cause would be one or two fouled spark plugs,” Dave said. “Does your plane have an engine monitor? Did you notice which cylinders were registering no EGT?”

“No engine monitor,” Ray said. “Still trying to get quotes for an install.” He added that with the mixture set to full-rich, he saw the RPM drop from 2150 to 900 when he switched to the left mag.

Which Plug?

With an engine monitor installed, Ray would have been able to instantly pinpoint exactly which spark plug was the culprit. Without one, Dave was going to have to make an educated guess.

“On your Lycoming engine, the left mag fires the top plugs on the left side of the engine and the bottom plugs on the right side of the engine,” Dave explained. “That means the top plugs in cylinders 2, 4 and 6, and the bottom plugs in cylinders 1, 3 and 5. Bottom plugs are much more likely to foul than top plugs, so we should check the bottoms in cylinders 1, 3 and 5 first.”

“Is this something you’d like to check yourself,” Dave asked Ray, “or would you like me to contact a local mechanic?”

“Please reach out to a mechanic,” Ray replied. “I’ve never changed a spark plug, though I’d like to learn how. Maybe the mechanic can show me?”

Which mechanic?

Dave pulled up Savvy’s service center map on his computer screen. This is a proprietary digital map of the United States displaying  hundreds of little icons that represent the various maintenance facilities that Savvy has worked with over the past 13 years. The icons are colored green, yellow or red based on Savvy’s past experience working with each of these shops. There are also some white icons representing shops that are in Savvy’s database but with whom Savvy has no experience.

Dave quickly zoomed in on Ray’s home airport and saw that there were several icons representing shops on the field. Several were red (“avoid”). There was a green one, but it was an avionics shop which wouldn’t help in this case. There was also a yellow one, and that seemed like the best bet to help Ray resolve his issue.

Dave phoned the shop’s Director of Maintenance (DOM), who said that they wouldn’t be able to look at Ray’s airplane until the middle of next week. Ray said that would be fine. Dave gave the DOM the plane’s tiedown number, and the DOM said they’d be glad to tow the airplane to the shop’s maintenance hangar.

Maintenance Lesson

The following week, Dave touched base with the A&P at the shop who would be working on Ray’s plane. (The DOM was out of town.) He briefed the A&P on what needed to be done (check the bottom plugs on cylinders 1, 3 and 5), and asked him if it would be okay for the owner to come watch and learn. The A&P said that would be fine, and Dave let Ray know.

“I got the spark plugs cleaned!” Ray told Dave a few days later. “The A&P showed me how, and I think I should be able to do it myself next time. I just need an invoice and a logbook entry at this point.”

“I ran the engine up,” Ray continued, “and the mag check was fine, so I flew a few laps around the pattern and the plane seemed to fly great!”

“I spoke to soon,” Ray told Dave the next day. “It’s dying again on the left mag.”

“They need to take another look at the airplane,” Dave said. “Did it run correctly after the spark plugs were cleaned?”

Ray confirmed that it did run well right after the plugs were cleaned, but that the mechanic remarked that the plugs exhibited an unusually large amount of lead on them.

“It’s helpful to know that this is a lead fouling problem,” Dave said..”Some engines are more prone to lead fouling than others. Do you lean aggressively during taxi, idle, and other ground operations? If not you should, that will help a lot.”

Ray confessed that he usually didn’t lean the engine while on the ground, and resolved to start doing so going forward. Dave said that if the problem persisted, it might be worth switching to projected nose “BY” plugs or fine-wire plugs, both of which are far more resistant to fouling than are standard massive-electrode plugs. 

Dave referred Ray to Mike Busch’s webinar “All About Spark Plugs” on Savvy’s YouTube channel (one of over 100 webinars there on almost every imaginable maintenance topic). Ray promised to check it out.

Several days later, Ray advised Dave that the airplane seemed to be flying fine after a second plug cleaning and more careful attention to ground leaning by Ray.

“I guess we can close this ticket now,” Ray said.

SavvyMx clients like Ray rely on Savvy’s experienced account managers to help them through all sorts of maintenance problems ranging from major to routine. This was a routine one—a simple case of fouled spark plugs—but it was a great learning experience for Ray. He wound up learning:

  • Which shops on his field were trustworthy
  • Why he really should install an engine monitor
  • The importance of aggressive leaning during ground ops
  • Which mags fire which spark plugs
  • Which spark plugs are most likely to foul
  • How to remove, clean, and reinstall spark plugs
  • Fouling-resistant spark plugs he might consider installing

Education is an integral part of Savvy’s mission. We love working with maintenance-knowledgeable and maintenance-involved aircraft owners, but we also love educating owners like Ray who aren’t especially maintenance-knowledgeable but aspire to be. Which kind of owner are you?

You bought a plane to fly it, not stress over maintenance.

At Savvy Aviation, we believe you shouldn’t have to navigate the complexities of aircraft maintenance alone. And you definitely shouldn’t be surprised when your shop’s invoice arrives.

Savvy Aviation isn’t a maintenance shop – we empower you with the knowledge and expert consultation you need to be in control of your own maintenance events – so your shop takes directives (not gives them). Whatever your maintenance needs, Savvy has a perfect plan for you: