Temperamental Ignition

A funny thing happened to me on a coast-to-coast trip from California to the East Coast in my Cessna 310. I was on a business trip that would take me from my home base in Santa Maria, California, to Frederick, Maryland, then to Atlanta, Georgia, and then home again.

I first became aware of the problem as I was climbing out of Santa Maria on the very first leg of what I expected to be a 30-hour round-trip. I had reduced to my usual 75% cruise-climb power, was climbing at my usual 130 KIAS cruise-climb airspeed, with the usual 110 pounds/hour fuel flow on each engine. This was all standard routine that I’d performed hundreds of times before.

The engines felt smooth. The airplane was climbing nicely at about 1,000 FPM despite being loaded right at max gross.  The air was smooth. My yoke-mounted SeriusXM satellite weather display indicated no significant weather all the way to Tulsa, Oklahoma, where I planned to make an overnight stop before continuing on to Frederick. The SeriusXM audio was tuned to the classical music channel, piping one of my favorite Bach Brandenburg Concertos into my stereo ANR headset. All seemed right with the world.

My reverie was interrupted by a flashing amber annunciator light that told me my digital engine monitor was trying to get my attention. Sure enough, when I looked over at the instrument on the right side of the panel, its display was flashing a high turbine inlet temperature (TIT) alarm on the left engine, and displaying the TIT as 1620°F. I knew from experience that normal TIT in this configuration is around 1570°F and I’d programmed the engine monitor to alarm any time the TIT exceeded 1600°F.

JPI EDM 760 engine monitor

On climbout, the left engine showed excessive TIT and excessive EGT on the #5 cylinder. However, #5 CHT was normal. This suggested that one spark plug might not be firing in the #5 cylinder. An in-flight mag check confirmed that indeed the bottom plug was not firing.

In-flight troubleshooting

Looking carefully at the engine monitor’s digital readouts, I noticed that the EGT on the left engine’s #5 cylinder was noticeably higher than the other cylinders, and definitely higher than what I was used to seeing. The high #5 EGT suggested to me one of two possible problems: (1) a partially clogged #5 injector; or (2) a #5 spark plug that wasn’t firing.

If a clogged injector was causing cylinder #5 to run too lean, I would have expected that cylinder also to have elevated CHT while operating ROP during climb. However, the engine monitor did not indicate that the #5 CHT was elevated; if anything, it seemed to be a bit lower than usual.

That suggested to me that a non-firing spark plug was the most likely cause of the elevated #5 EGT. To confirm this theory, I performed an in-flight mag check. When I shut off the righthand magneto switch for the left engine, the left engine started running quite rough and the #5 EGT bar on the display dropped out of sight.


My problem was definitely a non-firing spark plug in the #5 cylinder.

Which plug wasn’t firing? Because the cylinder went cold when I shut off the right magneto, the non-firing plug had to be the one connected to the left magneto. On my engines (as with most big-bore TCM engines), each magneto fires the top plugs on its side of the engine and the bottom plugs on the opposite side of the engine. Since cylinder #5 is in the right bank of cylinders, its top plug is fired by the right mag and its bottom plug is fired by the left mag. Therefore, I reasoned, my non-firing spark plug had to be the bottom plug on cylinder #5.

(Bottom plugs tend to misfire much more often than top plugs, because the bottom ones are so vulnerable to oil-fouling and contamination with debris.)

That’s odd, I thought. I had done a thorough runup prior to takeoff, including the usual preflight mag check at 1700 RPM. All 24 spark plugs appeared to be working just fine. Why would one decide to quit working now? Definitely odd.

I leveled off at my cruising altitude of 13,000 feet and did the “big mixture pull” to transition to LOP. The engine monitor continued to show elevated DIFF on the left engine, and elevated #5 EGT. During the next couple of hours, I repeated the in-flight mag check a couple of more times and got exactly the same result: The bottom plug on cylinder #5 was definitely not firing.

Sometimes fouled plugs clear themselves spontaneously. But not this time. Darn!

Not to worry, that’s why I always carry a couple of spare spark plugs in the emergency toolkit I keep in my left wing locker, together with the necessary tools to change out a plug on the ramp if necessary. So I knew what I had to do.

Spark plug transplant

I landed at my first planned refueling stop, Saint Johns, Arizona. KSJN is a frequent fuel stop for me going eastbound because it consistently has among the lowest 100LL prices west of the Mississippi. Also, KSJN has a field elevation of 5,736 feet MSL, which shortens the descent for landing and the subsequent climb back to altitude. All in all, it’s one of my favorite places to refuel.

After topping off the tanks, I retrieved my emergency toolkit and proceeded to remove the bottom spark plug from cylinder #5 of the left engine.

The plug had been in service for about 100 hours, and it looked okay to me. But since it clearly wasn’t firing, I decided to swap it out anyway. I installed a brand new spark plug in the bottom plug hole of left engine cylinder #5, torqued it to 360 in.-lbs. using the torque wrench I carry in my emergency toolkit, and reattached the ignition lead.

After closing up the left engine nacelle and stashing my emergency toolkit back in the wing locker, I fired up and taxied out for departure. At the runup area, I performed an extra diligent runup and mag check to verify that all plugs were firing properly—they were. I then took off and turned eastbound toward Tulsa.

Climbing out of KSJN, I tuned the SeriusXM audio to the ‘60s oldies channel and was just getting into the groove when it happened again: The amber light started flashing and the engine monitor started complaining about high TIT on the left engine. A quick cycle of the instrument and a quick in-flight mag check confirmed that the bottom plug on #5 was once again not firing. Yes, the very same brand new spark plug that I’d just installed!


Plug transplant, part deux

I continued on to Tulsa, taxied to the FBO, and broke out my emergency toolkit once again. This time, I removed the newly-installed plug and installed my one remaining spare. I wasn’t sure it would solve my problem, but figured it was worth a shot.

The next day, climbing eastbound out of Tulsa, I actively monitored the engines looking for signs of trouble. Everything seemed to be working fine. After leveling off in cruise and switching to LOP, I tried another in-flight mag check. The left engine continued to run smoothly on each magneto  individually, and the engine monitor confirmed that everything was operating normally now.


I flew nonstop to Frederick at FL210 (to stay above a bunch of rather nasty frontal weather). High-altitude LOP operation is pretty demanding on the ignition system, but the engines didn’t miss a beat and another in-flight mag check at altitude confirmed that all was well.

After completing my business in Frederick, I flew to Charlotte, North Carolina to spend a few days with my in-laws who live there, then proceeded on to Atlanta for another business meeting. After that, I headed home to the west coast with stops in Memphis and Denver. The engines continued to run perfectly, and I pretty much forgot about the earlier ignition problem.

It’s baaaack!

After returning home to Santa Maria and resting up a bit, I decided it was time to do some preventive maintenance on the airplane. I changed the oil, sent oil samples to the lab for analysis, replaced the oil filters, and cut open the old filters for inspection. (No metal.)

Since the spark plugs had over 100 hours on them, I pulled them and sent them to Aircraft Spark Plug Service in Van Nuys for cleaning, gapping, and bomb testing. All of my cleaned/gapped spark plugs passed the bomb test with flying colors and came back a week later, whereupon I reinstalled them in the engines.

After closing up the engine nacelles, I took the airplane out for a post-maintenance test flight. A thorough pre-flight runup indicated that everything was working fine. But the test flight once again revealed elevated DIFF and elevated #5 EGT on the left engine, and an in-flight mag check showed the bottom #5 spark plug was once again not firing. Arggghhh!!!

It was finally starting to dawn on me that the ignition problem must be something other than a bad spark plug. It had to be either a problem with the magneto itself or a problem with the ignition harness.

I tried replacing the insulator (“cigarette”) and contact spring on the bottom #5 ignition lead, but another test flight showed that this did not solve the problem. I pulled the left mag and opened it up, but couldn’t find anything wrong. The distributor cap was clean inside, the contact springs looked good, the point gap was correct and the internal and external mag timing was spot-on.

Harness transplant

By elimination, that left the ignition harness. I examined the #5 bottom ignition lead and couldn’t spot any visual anomalies. But since I was running out of ideas, and since a brand new full harness (for both mags) cost less than $500.00, I decided to order one and install it. Even though the existing harness looked fine, it did have nearly 2,000 hours on it, so presumably it was fully depreciated.

Ignition Harness

A new full harness (for both magnetos) costs only about $500. (A harness for just one magneto is called a “half harness.”) Figure on four hours of labor to install. I prefer the Slick-brand harnesses (shown above) because of their superior construction and flexibility.

There are a variety of ignition harnesses that are PMA approved for my engines, including Champion, Kelly, Skytronics, Continental, and Slick. I have always preferred the Slick harnesses because of their superior construction and flexibility, so I ordered a new Slick M1740 harness to mate with my Continental/Bendix S-1200 magnetos.

Removing the old harness and installing the new one was more time-consuming than I expected. Doing the job correctly involves considerable Adel clamping, grommeting, and tie-wrapping to ensure that the ignition leads cannot vibrate or chafe on anything and have no tight bends. It took me about six hours to complete the job, including retiming both mags.

I am, of course, the world’s slowest mechanic. I imagine a professional A&P could do it in three or four hours.

Finally, it was time to do yet another post-maintenance test flight. This time, I was overjoyed to find that everything was perfect. The engine monitor readings were just as they should be, and a high-power in-flight mag check showed all systems go. Success at last!

Lessons learned

I learned some important lessons as a result of this experience. One is that the usual pre-flight mag check is a laughably inadequate test of ignition system performance. While trying to track down my problem with a non-firing #5 bottom plug, the ignition system repeatedly showed no problems whatsoever during the pre-flight mag check, only to fail immediately and repeatably as soon as the aircraft was in flight.

Clearly, the pre-flight mag check is not a very demanding test of the ignition system, and won’t detect anything but the grossest ignition anomalies. An in-flight mag check is a far more demanding and revealing test. The most demanding ignition system test is a high-power in-flight mag check with the engine leaned aggressively (preferably LOP).

Many pilots have never done an in-flight mag check, and many are afraid to perform one. I’ve even known some experienced A&P mechanics that discourage pilots from shutting off a magneto in flight. Obviously, I don’t agree with that advice. In fact, in the wake of my experience, I now make a point of performing an in-flight mag check on almost every flight, and I heartily recommend that you consider adopting the same practice.

Another lesson I learned here is the tremendous diagnostic value of a modern digital probe-per-cylinder engine monitor. If it hadn’t been for my JPI EDM 760, I’d never have known that my #5 bottom plug was not firing. It’s quite possible that this situation could have gone on for months and hundreds of hours without being detected. Once again, my engine monitor proved that it is worth its weight in gold.

Finally, I learned that ignition harnesses have a finite useful life. They may look perfect upon visual inspection, yet develop internal electrical leaks that seriously compromise ignition system performance. Since a new harness is relatively inexpensive (at least as aircraft parts go), it probably wouldn’t be a bad idea to replace the ignition harness every 1,000 hours or so just on general principles. In fact, I decided to order another new harness and installed it on my right engine, so now both engines have new harnesses.

Mike Busch is arguably the best-known A&P/IA in general aviation, honored by the FAA in 2008 as National Aviation Maintenance Technician of the Year. Mike is a 8,000-hour pilot and CFI, an aircraft owner for 50 years, a prolific aviation author, co-founder of AVweb, and presently heads a team of world-class GA maintenance experts at Savvy Aviation. Mike writes a monthly Savvy Maintenance column in AOPA PILOT magazine, and his book Manifesto: A Revolutionary Approach to General Aviation Maintenance is available from in paperback and Kindle versions (112 pages). His second book titled Mike Busch on Engines was released on May 15, 2018, and is available from in paperback and Kindle versions. (508 pages).


  1. There are many problems that cannot be duplicated on the ground or at less than full power. Doing an in-flight mag check is not a bad idea but is probably a good idea to be near a suitable airport in case a critical problem, i.e. engine failure, happens.
    Problems with fuel tank venting and check valves might only show up at altitude. Problems with ignition will vary with temperature, density and humidity.
    Ignition voltage needed to jump the gap in a plug increases with spark gap wear and at some point that voltage will short out in the wiring harness.
    Visual inspection may look perfect. Bench testing may not detect a problem unless a symptom detected in-flight is used to guide the testing process. A good, new, spark plug will fire at less than 10,000 volts. A high performance ignition system might support much higher voltage in the harness. But at some point, the voltage rise can short in the lead before the plug fires.
    If you rent or fly a club airplane you have to trust your life to on maintenance controlled by third parties you may not even know. If you own your own airplane, some maintenance can be deferred safely, some cannot.
    Fifty years ago Flight Engineers were part of the cockpit crew on all the airlines and still part of the KB50 tankers. The engineers panel allowed testing and monitoring the big radial engines to monitor performance and schedule maintenance before failures.
    Today a few pounds of electronics can do as much but the private pilot has to know how to interpret the changing or steady read-outs. To my knowledge none of the current engine monitors monitor each spark plug firing voltage and flag when it gets to be too high for the leads.

  2. Mr. Busch, this seems to be a reversal of your philosophy of replace-on-condition. What’s up with that?

  3. An ignition lead test is easy and cheap…why was that not done? Leads could “leak” because of low air density at altitude. That is why the old bombers had low voltage ignition systems with the coils right at the plug. A lead tester with higher-than-magneto voltage output could have found this at ground elevation.

  4. Good article! Thanks. I occasionally do in-flight mag checks, but will now make a point of doing one at altitude and LOP from time to time.

  5. To find a cold plug just spit on it if you do not have a spray bottle. If you are out in the middle of nowhere, your spit is valuable for troubleshooting. Lets assume all your plugs are good and you have a 300 mag drop in flight but your mag drops were normal on the ground – you most likely have a bad magneto coil. As the mag gets hot, the coil resistance increases and the voltage output decreases resulting in huge mag drops. This exact scenario happened to me years ago out in the middle of nowhere! On one occasion, I had an inflight 175 mag drop and at the next fuel point did find that a bad plug developed and my spit proved valuable. Its an old radial engine method because of so many cylinders. Bad ignition leads are rare, but can happen in flight – very rare. 2 cents!

  6. Be very careful when using this method. Do not do this with a suspected bad mag (not firing at all). The afterfire experienced after turning the mag switch back to the good mag can do significant exhaust system damage.

    I worked for an engine shop for over two decades doing the test flights and engine setups on all the aircraft that came in for engine repair/overhaul. One of our Aerostar customers had a suspect mag in flight and did an in-flight high power mag check to investigate further. The ensuing afterfire, when he switched back to the good mag, distorted nearly every exhaust collector and header from the engine to the turbocharger. The pressure wave also blew a number of turbine blades clean off the turbine wheel.

    The compromised exhaust system turned an annoying situation into an emergency. The repairs were very expensive.

    • This is really a non-problem, Tom. In the rare case where the engine quits when switching to single-mag operation (and that’s a very low-probability event indeed), the correct procedure is to pull the mixture to idle cutoff, then turn the mag switch back to BOTH, then advance the mixture control until the engine restarts. I agree that if you just turned the mag back on without first shutting off the fuel flow, you could risk an afterfire. But that’s simply the wrong restart procedure, and it’s no reason to avoid doing in-flight mag checks.

      • Unfortunately, it is a problem, as the Aerostar pilot found out. Possibly, a problem of training (or lack thereof) on the part of those who wish to use the procedure. Those of us who have made it our profession to know engines inside and out can deal with the situation as you state. The average pilot, however, is trained to “undo” whatever he/she did just before the engine quit and likely will do so when it does on his/her in flight mag check gone bad. If one wishes to do an inflight check, they should know what to expect and train accordingly.

        Unfortunately, mag failures in flight are not a rare as we would hope. I have had at least five such inflight failures so far in my career and am acquainted with others who have experienced the same. As Jim said above, “Doing an in-flight mag check is not a bad idea but is probably a good idea to be near a suitable airport in case a critical problem, i.e. engine failure, happens.” Amen Jim.

        Oh, and don’t forget to brief the passengers LOL.

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