Editor’s Note: This is the second article in our new series called “I Can’t Believe I Did That,” where pilots ‘fess up about mistakes they’ve made but lived to tell about. The aim is not to embarrass or demean the author, but to offer lessons that other pilots can learn (and hopefully avoid). If you have a story to tell, email us at: firstname.lastname@example.org.
Why a Mag Check?
By Tom Winter
I lost a cylinder last time up. Here’s the story, with all details which I can recall, followed (figuratively, thank goodness) by a postmortem.
Typically flying the Cessna 150 in Cub-emulation mode about 1000 feet AGL, I cruise at 2450 rpm, and I always lean. My co-owner never leans. He’s an actual Cub flyer, and there is no mixture adjustment in his Cub. So I figure it’s on me to keep the plugs from fouling up with lead in our 150. Plugs for magneto ignition have narrow gaps, and there are a couple grams of lead in every gallon of 100LL.
The first abnormal sign was a bad mag check. Three guys, first me, then one of Lincoln’s most experienced pilots, then an older pilot, all thought plug fouling:
- On return to Lincoln from the National KR fly-In at Red Oak, I got a bad mag check–RPM drop over 100 right, less than 25 left–and figured it was fouling and what my engine needed was exercise, at higher than my normal RPM range. So I took off and flew home. Flew fine and smooth. Nice flight.
- Next flight, same bad mag check. Ran at full throttle a bit, then tried mag check at full throttle. Drop difference was 200 RPM! I aborted, came back from run-up, and asked an old hand at the FBO. “Nine out of ten, plug fouling,” he said. “You can’t hurt it on the ground. Lean until it coughs, then richen a bit. Even take off with some lean.” Tried it, and everything seemed OK. Flew fine. 200 miles. Ran strong as ever or stronger; I was happy. This was my first flight involving cloud avoidance. Sometimes under, and sometimes around fair weather cumulus. Joy! Hoped problem was over.
- Next flight, leaned on ground for taxi. Had line guy add a quart of oil (was at 4.5 quarts). It was cold, so we waited in the plane for the lineman to find the oil and return with it.
On the apron, I smelled a mild burnt oil smell. I imagined the line guy had spilled some oil, or that it was cabin heat. (My experienced pilot pax had opened cabin heat. I had never touched it before.) In retrospect, maybe I should have shut down on the chance the burnt smell was an oil leak. But the oil pressure gauge looked normal, and stayed there.
At runup, I got the same bad mag check. “What do you think?” I asked my more experienced pilot pax. “Fouling. It’ll clean out at cruise.”
Took off at full rich. Leaned while settling into cruise (leaned out to max RPM, then in some). More experienced pax showed me how to run the LORAN, which was a main reason for the flight. (Good news, the LORAN works!) Suddenly the arpies dropped from 2500 to 2200, all at once, nothing gradual about it. Carb heat did nothing, so we turned around and made it back to LNK. Full throttle, full rich, still just 2200 rpm. And it was shaking almost like a wet dog. But it got no worse.
Tower was great.
“Lincoln Approach, Cessna 2885S has lost 300 RPM. We’re coming back.”
“Cessna 85S, report airport in sight.”
“Lincoln Approach, Airport in sight”
Tower cleared us on the spot to 35R.
“Affirmative, 35R. But if you need something else, we can deal.”
Wind serene, I had been considering a downwind on 14, which was closest and straight ahead, but no need–steady at 2200 rpm all the way. Flew straight for the numbers, not descending at all. Still several hundred feet over traffic pattern altitude until the numbers were there in the windshield. Used full flaps to “divebomb” to the runway. (Nice landing!)
Pax, an IFR pilot, congratulated me on staying calm throughout. Mechanic at LNK said zero compression in cylinder #2, and non-ferrous flecks of metal were all over the screen. The screen was crushed. He wondered out loud if the wrong ring had been installed. I wondered out loud if I’d been leaning too much. Our mechanic in Crete came up and took the engine off. I’m on the ground waiting…
Fortunately, our larder is already well-stocked with beans and rice. Looks like I’ll be living on beans and rice for a while–again.
Saw my disassembled engine Saturday.
The cause of my sudden 300 rpm drop in flight was a hole in the piston. The hole was at the edge, next to the upper plug. I asked to see the plug. The plug looked like any other plug except it had no porcelain around the stem. The A&P didn’t blame it on the plug, but said it could have been a pit in the original piston becoming a hotspot. “Piston’s probably been in there since 1967. Nothing different you could have done to prevent it.” Bless him, he covered me in front of my partner–and to some extent his explanation may even be true!
Other possible causes? Old savvy pilot guys hear the tale and just opine “You leaned too much!” If so, the stock oil temperature gauge is useless. Oil temp says nothing about a hotspot. Another old hand related that a magneto getting loose and advancing would burn a piston.
They tell us in school to check the mags, but they don’t tell us what to do if you don’t like the mag check. OK. Here it is and my experience is what may happen to you if you don’t: Don’t take off if the drop difference is too big. Go back and have the plugs pulled no matter what the old hands tell you. It’s not their engine, it’s not their mortgage, it’s not their life. Even if it’s not the plugs, the plugs will have a story to tell, and through the plughole, they can even look at the piston and the sleeve.
I HAD gone back from runup the flight before, but bought the advice I got, namely to try unfouling the plugs by taxiing lean and flying lean, and then bought the advice of my pax that it would clean up!
So there. Go back, don’t take off until you see the plugs. If it’s just fouling, cheap. If it’s the one time in ten it’s something bigger, you find out on the ground. Still cheap.
Ben Franklin said it. “Experience is a harsh school, but a fool will learn in no other.” Not a fool? Good. Learn from my experience.
Tom Winter retires this year from the University of Nebraska after 45 years of college teaching — mostly Latin. He got his pilot’s license on March 17, 2000, at age 56. In his club plane (he helped start a flying club) and as a co-owner, he averaged 28 hours a year: since becoming sole owner of his 1967 Cessna 150, six years ago, he’s averaging between 50-60 hours a year. Tom’s retirement plan: fly to small-town strips, pull out the folding bike from behind the seat, and tour small town Nebraska. Also spend a week or so at Estrella and get a sailplane rating.