Getting the lead out
Why it matters and why we are working on it
Not long ago, I experienced how even a tiny amount of lead in 100LL avgas can wreak havoc on piston aircraft engines.
It was a beautiful fall afternoon, and I was flying my Cessna 140 from Zanesville, Ohio, to Frederick, Maryland. I’d been aloft for a little over an hour as I approached Morgantown, West Virginia, and was excited to take in the vibrant fall colors on the Appalachian Mountain trees just coming into view.
In an instant my previously happy four-cylinder Continental engine began to run rough. Having owned the airplane for more than 11 years, I knew this typically meant ice had formed in the carburetor, so I applied carb heat. No change. Could there be water in the fuel? I thought. Switching fuel tanks brought no relief to the high-frequency engine shake, so I performed an in-flight magneto check. Right mag, no rpm drop. Left mag—whoa—400 rpm drop.
I was fortunate to find myself seven miles north of Morgantown Municipal Airport (MGW) and made a beeline to the runway. Although the engine was clearly not producing full power, the airplane was light and the air was cool, so I could maintain altitude. I called the tower and said I had a rough running engine and needed to land right away. When asked, I told the tower I did not need emergency services and made a normal landing.
Now I was AOG (aircraft on ground) 120 miles away from my home airport on a Sunday afternoon. I still had a few hours before sunset to troubleshoot the issue and potentially continue my trip. My hope was to find a local mechanic who could check my magnetos and spark plugs. The helpful FBO personnel provided a few phone numbers to call for maintenance but, unsurprisingly, my calls went straight to voicemail.
I then called my regular mechanic and, luckily, he answered. I described my plight, and he suggested a high-power ground run with the engine mixture aggressively leaned to see if lead fouled spark plugs were the culprit. “Enough lead can build up on spark plugs while flying at cruise power to stop them from firing?” I asked incredulously. “Sure, especially if you’re not properly leaned,” he said. I’d experienced lead fouled plugs plenty of times in my Cessna 140 during ground runups, but never while airborne.
The technique to burn lead off spark plugs is straightforward: Brakes on, power to 2,000 rpm, lean the mixture until the engine is running slightly rough, wait one minute, and then perform a normal mag check to see if the engine rpm drop is within the acceptable range. Sometimes it takes several attempts to burn off all the lead.
My mechanic’s advice worked, and my engine was running smoothly again. I took off and climbed in a circle near the airport until I was at a reasonable cruise altitude, and then I requested flight following to remain in constant contact with ATC. I headed east toward the front range of the mountains and performed an in-flight mag check to ensure every spark plug was firing properly. The remainder of the flight was uneventful. The next day, I removed my spark plugs and found the top plugs perfectly clean and two bottom plugs with some remaining lead build up.
Beyond leaning the mixture during ground and air operations, there are a few common practices to prevent lead fouling. Some aircraft owners use a fuel treatment called TCP to scavenge the lead from 100LL fuel and prevent a buildup on spark plugs, but it’s pricey. Others use Marvel Mystery Oil, although it is not approved for use in certified aircraft engines. There are also spark plugs available from Champion and Tempest that burn hotter and have a different profile to reduce the possibility of lead buildup.
The best way to prevent lead fouling of spark plugs is to get rid of the lead in our aviation fuel—and the GA industry is progress on that front. One-hundred-octane unleaded fuel is available through a GAMI supplemental type certificate at two airports in California, and that number is expected to grow. Ninety-four-octane unleaded fuel is available through a Swift Fuels STC at more than 40 airports, and the company is also working on STCs for its 100-octane unleaded fuel. And LyondellBasell and VP Racing Fuels continue to pursue a 100-octane unleaded fuel through the PAFI process.
Unleaded fuel reduces pollution and may offer additional maintenance benefits such as the ability to use synthetic engine oil, longer duration between oil changes, and more time between engine overhauls. Our industry has committed to nationwide availability of 100-octane unleaded aviation fuel that works in every piston aircraft engine by 2030, and my Cessna 140 and I can’t wait. 
