The Savvy Aviator #11: Ignition TLC

Mags and spark plugs don't need a lot of maintenance, but they do require some and it's often neglected. You may get away with such neglect for a while, but eventually it'll bite you.


It all started when my friend and longtime AVweb columnist Rick Durden emailed me that he was going to be in the Bay Area for a few days, and suggested that we try to get together while he was on the Left Coast. (Rick is based in Grand Rapids, Mich., so I don’t often get the chance to see him except once a year at Oshkosh.) After an exchange of emails, we agreed that I’d fly up to Livermore Municipal Airport (LVK) and meet Rick at the terminal at 2:00 p.m.I cranked up my Cessna 310 and launched for the hour-long flight from SMX to LVK, arriving in front of the terminal building at almost precisely the appointed time. But there was no sign of Rick. I tried calling his cellphone. No answer. Strange, I thought; I’ve never known Rick not to be on time for an appointment. I took a seat in the terminal lobby and started to read a magazine to pass the time until Rick arrived.About 10 minutes later, I heard a female voice calling my name. It was a lady from the airport administration office, and she said someone was on the phone for me. It was Rick and it was obvious from his tone of voice that he wasn’t having a great day.


Rick explained apologetically that he was stranded about 40 nautical miles northwest of my position at Napa County Airport (APC). He’d been meeting that morning with two LVK-based pilots and aircraft owners, one of whom suggested that the three of them fly up to APC in his Piper Cherokee 235 for the proverbial $100 hamburger. Rick said OK, so long as he could get back to LVK by 2:00 p.m. The three of them flew up to APC and had lunch at the airport restaurant, but when they climbed back in the Cherokee after lunch to fly back to LVK, the engine wouldn’t start.I told Rick that I’d fly up to APC. About 20 minutes later, I shut the 310 down next to the cantankerous Cherokee and its three frustrated occupants. Rick introduced me to the Cherokee’s owner (whose name was also “Mike”) and to the other LVK-based pilot named Jerry, who owns a Cessna 340A.”You’re an A&P,” Rick said to me, “do some of your magic A&P stuff!” I explained that I knew next to nothing about Pipers, and I had hardly any tools with me, but that if Mike would remove the top cowl from the engine of his airplane, I’d at least have a quick look.

What’s Wrong?

As Mike uncowled the Lycoming O-540, I asked him to describe the problem. He explained that the engine cranked briskly but would not fire. “I don’t think it’s getting fuel,” he said, explaining that he’d tried using the primer, tried pumping the throttle to prime the engine with the carb’s accelerator pump, and even tried a flooded-engine start just in case the problem was too much fuel instead of too little. But no matter what he tried, the engine wouldn’t fire.I asked Mike to crank the engine while I watched. He stroked the primer a few times and cranked the engine. Nothing. He continued cranking while pumping the throttle. Still nothing. Drawing my index finger across my throat, I motioned at Mike to stop.”You definitely don’t have a fuel problem,” I said as Mike exited the cockpit. I pointed to the fuel dripping from the induction system drain, and the fresh blue fuel stain on the nosewheel fairing. I explained that combustion requires three things: fuel, air, and spark. There was obviously plenty of fuel, and apparently no shortage of air. “I suspect you’re not getting spark,” I ventured. “Let’s see if we can verify that theory.”I opened the wing locker on my 310 to see what tools I could find. No spark plug socket, nor even a 7/8-inch combination wrench — rats!I did come up with a pair of channel-lock pliers, and used it to remove the ignition lead B-nuts from the top spark plugs on the #1 and #2 cylinders. I asked Mike to crank the engine once again — with the mixture at idle cutoff — while I held each ignition lead contact spring close to the grounded spark-plug body and looking for a spark.

Diagnosis Confirmed

The #1 lead yielded only the most pathetic little spark, hardly more vigorous than what you might expect to produce from a flashlight battery. Clearly not nearly enough to fire a spark plug.The #2 lead yielded no spark at all, but I was pretty sure this was normal — this engine is set up to start on just one magneto, and only that one mag has an impulse coupling enabling it to produce a spark at cranking RPM.I gave Mike the “cut” sign again, and explained to him what I’d found. “How long since those mags were off the engine?” Mike didn’t know, and didn’t recall having any mag maintenance done since he bought the airplane.I did a few more checks, using an ohmmeter to verify that the P-leads to the mags were not grounded. They weren’t, although I couldn’t help but notice that the insulation was badly cracked and deteriorated — not good, but nothing that would prevent the engine from starting.I told Mike that I doubted his airplane would be going anywhere until the mags were repaired or replaced. It was Saturday, but there was a mechanic on duty at the local FBO. I explained to the mechanic what I’d found, and he said he’d arrange to have the Cherokee pulled into the maintenance hangar, and have the mags looked at on Monday morning.Since there wasn’t anything further I could do with the limited tools I had available, I suggested that Mike, Jerry and Rick pile into my 310 and I’d fly them back home to Livermore. They agreed, and 20 minutes later we were all back at LVK, and I finally had my tte–tte with Rick about three hours later than we’d planned.(I also made a mental note to start putting together a better traveling toolkit for my airplane, so I wouldn’t be so helpless next time I encountered a mechanical away from home.)


A few days later, I got an email from Mike thanking me for giving him a lift home. He said that the shop at APC had gotten his airplane running, and that my “no spark” diagnosis had been spot-on. The shop had opened up his Slick mags and found the breaker points severely worn and the point gap way below specifications. They also pulled his spark plugs and said they were “shot.”The shop cleaned up the breaker points, adjusted the point gaps, and installed a new set of spark plugs. The engine started up just fine. Mike got a lift up to APC and flew the airplane home. Problem solved, and Mike indicated he was a happy camper.”Not so fast,” I told him in my email reply. “You’re not out of the woods yet.”I pointed out that while the mechanics at APC had done enough to get the engine running — as they’d been asked to do — Mike’s ignition system was still not up to snuff. The mags still had not received the major preventive maintenance that Slick calls for every 500 hours, which includes inspecting the coil, lubricating internal gears and bearings, and adjusting the internal mag timing (“e-gap”) to restore maximum spark output. These things can’t be done without removing the mags from the engine and disassembling them.And then there was the matter of those crumbling P-leads — since they carry about 400 volts AC during engine operation, it just doesn’t make sense to let them deteriorate.

Wake-Up Call

“This was a wake-up call,” I told Mike. “Your airplane has not been getting the preventive maintenance that it needs. If it had, your points and plugs would never have been worn to the point of failure, and those P-leads would have been repaired long ago. If all these ignition problems were missed at your last annual, I wonder what else might have been overlooked.”Mike was clearly taken aback by my comments. He pointed out that he’d never penny-pinched the maintenance on his airplane, and that he even had 100-hour inspections done despite the fact that they were not required, since his airplane was used strictly for non-commercial operations.Top-notch maintenance is not cheap, but I learned early in my tenure as an aircraft owner that spending money is no guarantee of getting top-notch maintenance. You also need a persnickety inspector with high standards. Using a different inspector each year also helps, since each pair of eyes sees different things.


There are two manufacturers of magnetos for horizontally opposed piston aircraft engines: Unison/Slick (owned by General Electric) and Bendix (owned by Teledyne Continental Motors). Both manufacturers call for minor mag maintenance every 100 hours, and major maintenance every 500 hours.100-hour mag maintenance consists of removing the distributor cap, inspecting the breaker points for pitting and burning, checking the point gap and adjusting if necessary, cleaning the inside of the distributor cap, and checking the ignition timing and adjusting if necessary. All this can be accomplished without removing the mags from the engine, and normally doesn’t require more than a half-hour per mag.500-hour mag maintenance requires removing the mags from the engine and disassembling them. It entails a detailed internal inspection, lubricating the internal gears and bearings, and adjusting the internal mag timing (“e-gap”) to ensure maximum output voltage. Figure a couple of hours per mag, plus parts.My airplane uses TCM/Bendix mags, and I normally perform the 500-hour maintenance on them myself. Although it’s not required, I routinely replace the points, the distributor block, and the carbon brush assembly every 500 hours. The parts cost is less than $100 per mag, and I consider it money well spent. It takes me a full day of work to do all four mags on my twin Cessna.For aircraft that use Unison/Slick mags, I generally recommend replacing the mags every 500 hours with new or overhauled/exchange units, rather than disassembling them in the field. The reason is simple economics: Compared with TCM/Bendix, Slick mags are cheaper, but Slick replacement parts are more expensive. For example a brand new Slick 6300-series mag costs about $400, and a replacement distributor block costs $150. (By contrast, a TCM/Bendix rebuilt S-1200 mag costs $600, and a replacement distributor block costs $60.) At these prices, it just doesn’t pay to mess with Slick mags in the field; just swap ’em out every 500 hours.

Spark Plug TLC

Spark plugs need regular maintenance, too. They need to be cleaned and gapped every 100 hours. For maximum life, they should also be reinstalled in rotated position, with each plug alternated between odd- and even-numbered cylinders, and between top and bottom position.The reason for rotation is that mags generate high-voltage energy with alternating polarity. Consequently, the plugs in even-numbered cylinders wear differently from those in odd-numbered cylinders. (On six-cylinder TCM engines like mine, the plugs in even-numbered cylinders wear the center electrode, while those in odd-numbered cylinders wear the ground electrodes.) By rotating the plugs every 100 hours, the plugs will wear evenly and last longer.Regular massive-electrode spark plugs will normally last 400 to 500 hours. When the center electrode — which starts out round and gradually wears into the shape of a football — has worn to half its original dimension (across the narrow axis), the plug should be scrapped and replaced. Champion sells a simple go/no-go gauge that takes the subjectivity out of determining whether or not plugs are worn out.When it’s time to replace the plugs, figure $17.00 street price for new Champion massive-electrode plugs — or a shade over $200 for a six-cylinder engine. Unison’s Autolite brand plugs are a bit cheaper — figure $14.00 each. Fine-wire iridium-electrode plugs generally last three times as long, but cost three times as much, so the cost per hour is about the same as for massives.For years I cleaned and gapped my own spark plugs. But I fly a twin, and doing preventive maintenance on 24 plugs is a lot of work. So I’ve started sending my plugs off to Aircraft Spark Plug Service (818-787-5680) in Van Nuys, Calif. They’ll clean, gap, bomb-test and recertify your plugs, and return them sealed in plastic with new copper gaskets, for $3.50 per plug, with turnaround typically a week or less. Given today’s hourly shop rates, I consider this a real bargain.

Another Ignition Problem

Remember Jerry, the Cessna 340 owner? Well, it seems that Jerry was paying close attention to Mike’s mag problems and my ensuing email dialog about mag maintenance (since I’d cc’d Jerry and Rick). Turns out that the RAM TSIO-520-NB engines on Jerry’s Cessna 340 were a bit over 500 SMOH, and the maintenance logs revealed that their pressurized Slick mags had never been off the airplane. After seeing Mike’s predicament, Jerry wisely decided it was time to do something about his mags.Jerry exchanged some email with me, asking whether I thought he should switch to TCM/Bendix mags or stick with his pressurized Slicks. I replied that while I much preferred TCM/Bendix mags for high-altitude operations (because they produce a much hotter spark), it probably didn’t make sense for Jerry to change from Slick to Bendix at mid-TBO, since doing so would require changing ignition harnesses, and he probably couldn’t trade in his Slicks for core credit against Bendix mags. I also discussed the economics of exchanging Slick mags at 500 hours rather than trying to overhaul them in the field.Based on our correspondence, Jerry ordered four pressurized Slick mags from RAM, and had them installed by his shop. A few days later, I received another email from Jerry. He’d just flown the airplane for the first time since the new mags were installed, and something was clearly wrong.”The plane starts fine and seems to run fine,” Jerry reported, “but the TIT is much higher and I can no longer run lean-of-peak. To maintain the TIT below 1600F, I have to run an additional 10-15 pounds-per-hour of fuel flow on each engine. I have GAMIjectors and previously my max TIT when leaning beyond peak was 1650F. Now the temps jump up to 1750F when attempting to go to LOP.”

What’s Wrong Now?

0)]I replied to Jerry that the symptoms he reported were consistent with either one mag not firing, or both mags firing late (retarded timing). I felt the one-bad-mag scenario could be ruled out because it would have showed up during the preflight mag check, and because the problem was occurring on both engines. Therefore, I was forced to conclude that the mechanic who installed the new mags had probably adjusted them all to the wrong timing spec — something significantly less than the 20 BTDC that is called for on Jerry’s TSIO-520-NB engines. Here’s how I explained my diagnostic reasoning to Jerry:

High TIT or EGT indicates that the combustion event is ending too late, so that there’s excess heat remaining at the time the exhaust valve opens. This can occur for one of two reasons:a. The combustion event is starting later than it should (as would be the case if the ignition timing was retarded); orb. The combustion event is starting at the right time, but is taking longer than it should to complete (as would be the case if only one plug was firing — one flame-front instead of two — or if the mixture were unusually lean). (This is why EGT/TIT goes up during a runup mag check.)Your note seems to suggest that you’re having these symptoms on both engines. That would seem to rule out an isolated bad mag (or plug or harness). On the other hand, if the mechanic who installed the mags timed them wrong, it’s easy to imagine him doing that to all four mags.Retarded mag timing would not create a problem with starting or idle (it’d probably improve them) or even runup, but would cause some loss of power and excessive EGT and TIT readings at high RPMs. The symptoms would be more serious at leaner (slower-burning) mixtures than at richer (faster-burning) ones. That sure sounds like what you described.First thing I’d do is to carefully re-check the mag timing. On a Permold-case TCM engine like yours, I prefer to do the timing using the timing reference marks on the prop governor drive gears that are visible through the inspection plug on the left side of the nose portion of the crankcase. This method doesn’t depend on using protractors or accurately establishing TDC on cylinder #1, so there’s a lot less potential for human-induced error.

A few days later, I heard back from Jerry:

Mike, you were spot-on about the mags-they were indeed mis-timed. They are timed properly now and everything seems to be working as it should.

See you next month.

Want to read more from Mike Busch? Check out the rest of his Savvy Aviator columns.