Sputtering
Several months ago, Connie and I took off out of Pearland, had leveled off at thirteen hundred feet, and were just a couple of minutes out of the airport when the engine started vibrating. It only lasted about fifteen or twenty seconds. I had just started turning back toward the airport when it quit. The rest of the flight was uneventful.
About two weeks ago, I had flown over to La Porte after getting the airplane back from its latest repair when it happened again. I had been doing touch and go’s there, climbed to twelve hundred feet to cruise back to Pearland under the Class B airspace, when the engine started vibrating again. I applied full carb heat and the vibration quit. Too quickly. I later talked it over with several pilot friends of mine and we agreed this latest incident could have been due to carb ice. But I was unconvinced that was the problem. I felt that it had cleared too quickly, and that my application of carb heat and the engine running smoothly again had been coincidental.
Last Sunday, Connie and I flew the Cheetah down to Galveston to talk with Bill (my mechanic) about an Airworthiness Directive I had received from the FAA. The A.D. directed that a certain part number and type of engine cylinder manufactured by a certain company had to be replaced once they acquired about 800 hours of operating time. I had stepped through the logbooks and was fairly convinced that the AD didn’t apply to us, but it still looked like we might need to have Bill pop some cylinder heads to take a look. So, with the A.D. in hand, we manned up around 3:30 in the afternoon and flew down to Galveston to find out what the story was.
The flight down was nice and uneventful. I asked the tower for a touch and go and then a full stop and they cleared me for a left downwind for runway 35. We heard an airplane call from off to the east, and the tower cleared him for a right downwind for the same runway. I started through the landing checklist, pulled the power back, and started the approach toward the landing spot, turned onto base and then final, rolling in with all four VASI lights white, telling me I was high on glideslope. That’s simply not a problem in my airplane. I simply slowed us down to the recommended 75 mph and we started sinking harder. Even though the lights said I was high, I was not high enough to need flaps, so I left them up. As we flew down the last hundred feet or so and across the dark grey ribbon of Seawall Boulevard, the airplane hit a patch of sink and really started down. I added a little power to cushion the landing, touched down, and then pushed the throttle in and closed the carb heat. We lifted off again, something that caught Connie by surprise. She had missed me telling the tower I wanted a touch and go.
I climbed straight ahead to 500 feet and then turned for crosswind. I hit my downwind distance a little early at 800 feet and turned onto downwind anyway to finish out the climb to 1000. Tower had asked me to call when I was on downwind and I did. He instructed me to “make a right 360 for spacing” which I immediately and smoothly rolled into. My friend in the Cessna was still a bit far out. As I completed the turn, the tower asked me if I had my traffic to which I responded “no joy”, followed a few seconds later by a “Tally ho!” as I picked him up on a still long right base. I pulled the throttle back, performed the landing checklist again, and, taking interval on the Cessna, flew my left hand pattern to a touchdown not far beyond the runway threshold. Tower instructed me to turn right at the next taxiway, which I did, following the Cessna that was now taxiing toward the Terminal. A radial-engine Texan taxiing opposite to us offered to take taxiway Delta to get out of everyone’s way and off the runway sooner. The tower approved that and then cautioned me not to hit the Cessna Cardinal RG taxiing toward us. The guy’s nosewheel was right on the taxiway’s centerline and was not budging; I swung us off the centerline as far right as I could without crossing into a Movement Area. I said something to Connie about how it would help if the Cardinal’s pilot wasn’t so anal about staying on centerline; just seconds before we finally crossed paths, as if he had just seen me, he swung his RG to his right and helped improve passing clearances.
We taxied over to Bill’s hangar, swing the airplane around so its nose was pointing back out the taxiway, and shut down. Bill met us as we got out and we chatted for a few moments, saying “hello”, before looking at the A.D. and the airplane’s engine logbooks. We raised the airplane’s hoods and looked under the cowling at the cylinders. Bill said right away I didn’t have to worry with the A.D. I had chrome cylinders and the ones in the A.D. were of a different material, not to mention there was no evidence that the company involved had even made the things. With the major business of the day completed, I went after what was the next foremost airplane issue on my mind, i.e., the engine roughness I had experienced twice in the last few months.
For some time now, I had been wanting to change out the spark plugs. My sojourns through the airplane’s engine logs did not show me where the plugs had last been changed, and I had a feeling they had been treated like too much else on this airplane, not replaced until there was a problem. I had tried to get Bill to change out the plugs earlier, but he didn’t seem interested. He wasn’t convinced it was needed. This time, when I suggested we change out the plugs as the first step in chasing the engine roughness, he agreed it was the thing to do. I wanted to do the job myself but at his hangar with his tools and expertise in my hip pocket not only as a safeguard against something going wrong but to get a Federal Air Regulation Part 43 sign-off that would let me do the job myself from then on. We agreed I would fly down the following Saturday and swap out the current set of plugs with new ones he would order. Connie and I wanted to take the airplane to Missouri in two weeks, and Bill thought it best if we did the job before that cross-country.
So, Connie and I hopped in the Cheetah to take her home.
Not so fast.
The tower cleared us to taxi out to runway 35 for takeoff, and we followed a Cessna 172 along the long, curving taxiway that crossed runway 31 to a spot just west of the hold short. I started going through my Takeoff Checklist which first called for an engine runup and magneto check. When I switched to running only the left mag, the engine began shaking like crazy and the engine RPM dropped 250 or so, far beyond the 175 maximum and the normal 100 RPM drop I usually saw. Cursing my airplane once again, still in denial and hoping it was just carbon fouling, I ran the throttle up to 2000, leaned out the mixture, and sat there for thirty seconds before putting the throttle back at 1800 with a full rich mixture and trying again. Left mag….shake,shake,shake,shake,shake,
…250 rpm drop..we weren’t going anywhere. Throttling back and returning the Magneto Switch to Both, I called the tower on the radio and told him I needed to taxi back to Texas Flight Line. He told me to “hold my position” and that he was working on getting me a taxi down runway 35 ahead of me. The Cessna ahead of me took off, and I had a brain fart and released my brakes anticipating the “taxi down 35” call when the tower reiterated for me to hold my position and I cranked the brakes back on. He then called and approved me for a one hundred eighty degree turn and taxi back to the hangars. So, I rotated the airplane around and went back the way I came.
When I turned off the taxiway onto the ramp, I ran the engine up to 1800 RPM again and switched to the left mag, confirming that the problem was still there. I throttled back down and taxied up in front of Bill’s hangar and shut down. He was working on a sawhorse, sawing some wood. As we got out, I bet Connie the Tower had a wager going me and my Cheetah visited about whether I would take off again on the first try, at all that day, or any that week.
“Three hundred RPM drop on the left mag,” I said to him as we approached.
“Probably a plug,” he said. “You’re sure it’s on the left mag?”
“Yep.”
Bill grabbed an open-end wrench and a spark plug ratchet and opened the left hood on the airplane’s cowl.
“It’s usually the bottom plugs that have a problem,” he explained. “The left mag fires the bottom plugs on this side of the engine and the top plugs on the other.”
Bill removed the bottom plugs on my engine’s left two cylinders, and I followed him like a puppy dog into his hangar. I watched him push a plug into a “cleaning” machine that sounded like a ginder and then mount a plug in a Champion tester and saw two little blue-white arcs as the plug fired. He repeated the procedure with the other plug, coated both of them with anti-seize compound and then mounted them back in the airplane.
I hopped back in the airplane’s pilot seat and tried starting the airplane and the engine started hard, meaning it took a couple of tries to get it going. Once she roared into life, I checked its oil pressure and temperature and, seeing them in “the green”, pushed the engine up to 1800 rpm and switched to the left mag. Shake!Shake!Shake! 300 rpm drop! I shook my head “no” as I shut the engine down.
Moving to the other side of the airplane, Bill pulled the top two plugs on those cylinders and headed back into the hangar. He had already put one in the tester.
“It’s not firing at all,” he said.
We looked into the spark plug’s firing end and could see small blobs of a grey material down in the plug. It was “lead fouling”, he said. And suddenly, it all made sense.
Lead fouling is caused when aviation gasoline burns but at a temperature too low to also consume the lead additive it contains. These temperatures correspond with the RPM the engine is operated at and a fuel/air mixture that is too rich. Most pilots are taught to lean (adjust the fuel/air mixture with the Mixture knob by pulling it out) according to the Pilot’s Operating Handbook for the airplane, and the one for my Cheetah says leaning is not necessary until 5000 feet. While I know for a fact that’s too high, the point is that most of the time I run around with the mixture in the full rich position. Yet, a service bulletin from my engine manufacturer encourages leaning at any altitude at engine power settings of 75% or below. So, I’ve been doing what I’ve been taught for years to do, and it’s close but not quite correct.
So, let’s assume that lead deposits built, as they obviously had. Most lead deposits still got burned out. But if a particular plug gummed up for whatever reason and the engine temperatures never quite got rid of all of the lead accumulation, then it would only take a little more to cause the plug to quit firing. A single plug misfiring would cause quite an engine vibration and RPM drop.
The engine roughness I had experienced in flight was exactly like that I had experienced on the ground when the plug had misfired.
Once the engine vibration starts, there was now a mechanism that would help clear the plug, i.e., the vibration would shake the contaminants loose. This would be especially true on a top-mounted plug where gravity would help pull the material out and into the cylinder. As soon as the lead was shaken loose, the plug started firing again. The problem became “self-clearing”. That exactly matched what I had experienced. A bottom-mounted plug that points upward might be another story; but in this case, all the pieces fit; and I was sure we had found the problem.
Bill finished cleaning the plugs and put them back in. I fired the engine up and ran the mag check again. It worked like it was supposed to.
When I fly back to Galveston again, not a single old spark plug will return alive.
About two weeks ago, I had flown over to La Porte after getting the airplane back from its latest repair when it happened again. I had been doing touch and go’s there, climbed to twelve hundred feet to cruise back to Pearland under the Class B airspace, when the engine started vibrating again. I applied full carb heat and the vibration quit. Too quickly. I later talked it over with several pilot friends of mine and we agreed this latest incident could have been due to carb ice. But I was unconvinced that was the problem. I felt that it had cleared too quickly, and that my application of carb heat and the engine running smoothly again had been coincidental.
Last Sunday, Connie and I flew the Cheetah down to Galveston to talk with Bill (my mechanic) about an Airworthiness Directive I had received from the FAA. The A.D. directed that a certain part number and type of engine cylinder manufactured by a certain company had to be replaced once they acquired about 800 hours of operating time. I had stepped through the logbooks and was fairly convinced that the AD didn’t apply to us, but it still looked like we might need to have Bill pop some cylinder heads to take a look. So, with the A.D. in hand, we manned up around 3:30 in the afternoon and flew down to Galveston to find out what the story was.
The flight down was nice and uneventful. I asked the tower for a touch and go and then a full stop and they cleared me for a left downwind for runway 35. We heard an airplane call from off to the east, and the tower cleared him for a right downwind for the same runway. I started through the landing checklist, pulled the power back, and started the approach toward the landing spot, turned onto base and then final, rolling in with all four VASI lights white, telling me I was high on glideslope. That’s simply not a problem in my airplane. I simply slowed us down to the recommended 75 mph and we started sinking harder. Even though the lights said I was high, I was not high enough to need flaps, so I left them up. As we flew down the last hundred feet or so and across the dark grey ribbon of Seawall Boulevard, the airplane hit a patch of sink and really started down. I added a little power to cushion the landing, touched down, and then pushed the throttle in and closed the carb heat. We lifted off again, something that caught Connie by surprise. She had missed me telling the tower I wanted a touch and go.
I climbed straight ahead to 500 feet and then turned for crosswind. I hit my downwind distance a little early at 800 feet and turned onto downwind anyway to finish out the climb to 1000. Tower had asked me to call when I was on downwind and I did. He instructed me to “make a right 360 for spacing” which I immediately and smoothly rolled into. My friend in the Cessna was still a bit far out. As I completed the turn, the tower asked me if I had my traffic to which I responded “no joy”, followed a few seconds later by a “Tally ho!” as I picked him up on a still long right base. I pulled the throttle back, performed the landing checklist again, and, taking interval on the Cessna, flew my left hand pattern to a touchdown not far beyond the runway threshold. Tower instructed me to turn right at the next taxiway, which I did, following the Cessna that was now taxiing toward the Terminal. A radial-engine Texan taxiing opposite to us offered to take taxiway Delta to get out of everyone’s way and off the runway sooner. The tower approved that and then cautioned me not to hit the Cessna Cardinal RG taxiing toward us. The guy’s nosewheel was right on the taxiway’s centerline and was not budging; I swung us off the centerline as far right as I could without crossing into a Movement Area. I said something to Connie about how it would help if the Cardinal’s pilot wasn’t so anal about staying on centerline; just seconds before we finally crossed paths, as if he had just seen me, he swung his RG to his right and helped improve passing clearances.
We taxied over to Bill’s hangar, swing the airplane around so its nose was pointing back out the taxiway, and shut down. Bill met us as we got out and we chatted for a few moments, saying “hello”, before looking at the A.D. and the airplane’s engine logbooks. We raised the airplane’s hoods and looked under the cowling at the cylinders. Bill said right away I didn’t have to worry with the A.D. I had chrome cylinders and the ones in the A.D. were of a different material, not to mention there was no evidence that the company involved had even made the things. With the major business of the day completed, I went after what was the next foremost airplane issue on my mind, i.e., the engine roughness I had experienced twice in the last few months.
For some time now, I had been wanting to change out the spark plugs. My sojourns through the airplane’s engine logs did not show me where the plugs had last been changed, and I had a feeling they had been treated like too much else on this airplane, not replaced until there was a problem. I had tried to get Bill to change out the plugs earlier, but he didn’t seem interested. He wasn’t convinced it was needed. This time, when I suggested we change out the plugs as the first step in chasing the engine roughness, he agreed it was the thing to do. I wanted to do the job myself but at his hangar with his tools and expertise in my hip pocket not only as a safeguard against something going wrong but to get a Federal Air Regulation Part 43 sign-off that would let me do the job myself from then on. We agreed I would fly down the following Saturday and swap out the current set of plugs with new ones he would order. Connie and I wanted to take the airplane to Missouri in two weeks, and Bill thought it best if we did the job before that cross-country.
So, Connie and I hopped in the Cheetah to take her home.
Not so fast.
The tower cleared us to taxi out to runway 35 for takeoff, and we followed a Cessna 172 along the long, curving taxiway that crossed runway 31 to a spot just west of the hold short. I started going through my Takeoff Checklist which first called for an engine runup and magneto check. When I switched to running only the left mag, the engine began shaking like crazy and the engine RPM dropped 250 or so, far beyond the 175 maximum and the normal 100 RPM drop I usually saw. Cursing my airplane once again, still in denial and hoping it was just carbon fouling, I ran the throttle up to 2000, leaned out the mixture, and sat there for thirty seconds before putting the throttle back at 1800 with a full rich mixture and trying again. Left mag….shake,shake,shake,shake,shake,
…250 rpm drop..we weren’t going anywhere. Throttling back and returning the Magneto Switch to Both, I called the tower on the radio and told him I needed to taxi back to Texas Flight Line. He told me to “hold my position” and that he was working on getting me a taxi down runway 35 ahead of me. The Cessna ahead of me took off, and I had a brain fart and released my brakes anticipating the “taxi down 35” call when the tower reiterated for me to hold my position and I cranked the brakes back on. He then called and approved me for a one hundred eighty degree turn and taxi back to the hangars. So, I rotated the airplane around and went back the way I came.
When I turned off the taxiway onto the ramp, I ran the engine up to 1800 RPM again and switched to the left mag, confirming that the problem was still there. I throttled back down and taxied up in front of Bill’s hangar and shut down. He was working on a sawhorse, sawing some wood. As we got out, I bet Connie the Tower had a wager going me and my Cheetah visited about whether I would take off again on the first try, at all that day, or any that week.
“Three hundred RPM drop on the left mag,” I said to him as we approached.
“Probably a plug,” he said. “You’re sure it’s on the left mag?”
“Yep.”
Bill grabbed an open-end wrench and a spark plug ratchet and opened the left hood on the airplane’s cowl.
“It’s usually the bottom plugs that have a problem,” he explained. “The left mag fires the bottom plugs on this side of the engine and the top plugs on the other.”
Bill removed the bottom plugs on my engine’s left two cylinders, and I followed him like a puppy dog into his hangar. I watched him push a plug into a “cleaning” machine that sounded like a ginder and then mount a plug in a Champion tester and saw two little blue-white arcs as the plug fired. He repeated the procedure with the other plug, coated both of them with anti-seize compound and then mounted them back in the airplane.
I hopped back in the airplane’s pilot seat and tried starting the airplane and the engine started hard, meaning it took a couple of tries to get it going. Once she roared into life, I checked its oil pressure and temperature and, seeing them in “the green”, pushed the engine up to 1800 rpm and switched to the left mag. Shake!Shake!Shake! 300 rpm drop! I shook my head “no” as I shut the engine down.
Moving to the other side of the airplane, Bill pulled the top two plugs on those cylinders and headed back into the hangar. He had already put one in the tester.
“It’s not firing at all,” he said.
We looked into the spark plug’s firing end and could see small blobs of a grey material down in the plug. It was “lead fouling”, he said. And suddenly, it all made sense.
Lead fouling is caused when aviation gasoline burns but at a temperature too low to also consume the lead additive it contains. These temperatures correspond with the RPM the engine is operated at and a fuel/air mixture that is too rich. Most pilots are taught to lean (adjust the fuel/air mixture with the Mixture knob by pulling it out) according to the Pilot’s Operating Handbook for the airplane, and the one for my Cheetah says leaning is not necessary until 5000 feet. While I know for a fact that’s too high, the point is that most of the time I run around with the mixture in the full rich position. Yet, a service bulletin from my engine manufacturer encourages leaning at any altitude at engine power settings of 75% or below. So, I’ve been doing what I’ve been taught for years to do, and it’s close but not quite correct.
So, let’s assume that lead deposits built, as they obviously had. Most lead deposits still got burned out. But if a particular plug gummed up for whatever reason and the engine temperatures never quite got rid of all of the lead accumulation, then it would only take a little more to cause the plug to quit firing. A single plug misfiring would cause quite an engine vibration and RPM drop.
The engine roughness I had experienced in flight was exactly like that I had experienced on the ground when the plug had misfired.
Once the engine vibration starts, there was now a mechanism that would help clear the plug, i.e., the vibration would shake the contaminants loose. This would be especially true on a top-mounted plug where gravity would help pull the material out and into the cylinder. As soon as the lead was shaken loose, the plug started firing again. The problem became “self-clearing”. That exactly matched what I had experienced. A bottom-mounted plug that points upward might be another story; but in this case, all the pieces fit; and I was sure we had found the problem.
Bill finished cleaning the plugs and put them back in. I fired the engine up and ran the mag check again. It worked like it was supposed to.
When I fly back to Galveston again, not a single old spark plug will return alive.
posted by Andy Foster at 5:53 AM
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