The Savvy Aviator #50: Lessons From A Geriatric Jug

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The Savvy Aviator

I got the idea for this column as I was finishing up this year's "annual ordeal" on my airplane. Because I do all my own maintenance, I tend to get a bit grumpy that time of year (or so says my wife). This year, I had to pull the #3 cylinder off of my right engine, which has made me just a tad grumpier than usual (so she says). I fly a 1979 Cessna T310R. It has two TSIO-520-BB engines with a dozen cylinders between them. Most all are still the original, 1979-vintage, TCM cylinders that were installed when the airplane originally rolled out of the factory, so I guess you'd have to say that they're fully depreciated. This was only the fourth time in 28 years and 4,050 hours that I've had to yank a jug. That averages out to one cylinder problem every seven years and 1,000 hours. So far, two-thirds of my 12 jugs have never had a problem (knock on wood). So I really can't complain. Before I explain why I had to pull the #3 cylinder off of my right engine at this year's annual, let me tell you a little bit about its service history.

My Jugs: Their Life And Times

My two TSIO-520-BB engines have a ridiculously stingy published TBO of 1,400 hours. I frankly cannot understand what TCM was thinking about when they came up with that number. On their first run, both of my engines made it to TBO without any problems whatsoever. In fact, they were running so well that I decided to keep flying them for another 500 hours. At 1,900 hours, I felt I'd reached the edge of my comfort zone, so I removed both engines for major overhaul. Neither engine had ever had a cylinder off until that time. After tearing down the engines, the overhaul shop inspected all 12 cylinders and measured their bores to determine whether they were still serviceable after 1,900 hours. Here's an excerpt from the table of limits from the TCM engine overhaul manual:
TCM Cylinder Diameter Overhaul Limits
"D" Diameter (inches)
New Parts
Service Limits
0.005" Oversize (P5)
0.010" Oversize (P10)
0.015" Oversize (P15)
Note that TCM specifies both new limits and service limits, and that they also authorize three approved oversize dimensions (P5, P10, and P15) in addition to standard. What the overhaul shop found -- much to their surprise and mine -- was that all twelve cylinders were still within new limits after 1,900 hours (albeit just barely). "These cylinders are in such good shape," the shop owner told me, "that it would a crime to retire them." So on the shop's advice, I decided to continue them in service. The shop honed the cylinder bores lightly, installed new valves, valve guides, pistons and rings and bolted the 12 jugs back on for another run. I was understandably skeptical that these 1,900-hour cylinders had another TBO's worth of life left in them. My skepticism proved unfounded. At 1,400 hours SMOH, the engines were running great and all 12 jugs had compressions in the mid-70s. So once again, I kept flying the engines. At this year's annual, my engines were at 4,050 TT and 2,150 SMOH (750 hours past TBO), and those geriatric jugs still had compressions in the 70s ... except for the #3 cylinder on the right engine (RE #3).

Call Me Sherlock

Engine Inspection

Like any good detective story, the first omens of the impending demise of that cylinder first appeared about a year and a half earlier, when a routine oil-analysis report showed an uptrend in nickel in my right engine. Since my cylinder bores are steel, not nickel-carbide, there was only one place the elevated nickel could be coming from: an exhaust valve guide. But which one? The answer to that question came about two months later at the start of my 2006 annual inspection, when I performed a hot compression check. Eleven cylinders were in the 70s as usual, but RE #3 measured 50/80 with obvious leakage past the exhaust valve. Aha! Now, 50/80 is still airworthy according to TCM's compression testing bible (SB03-3), but it's certainly worrisome. So I borescoped that cylinder, with special attention to its exhaust valve, but couldn't see even the slightest evidence of a hot spot on the valve. I debated about pulling the cylinder anyway. Had it been someone else's airplane, or had it been a single-engine airplane, I might well have done that. But because it was my own plane, and a twin, and since I'm an A&P/IA and a student of powerplant gerontology, I decided to continue the cylinder in service to see what would happen. I also vowed to keep a very close eye on RE #3 and yank it at the very first sign of trouble. (NOTE: Don't try this at home, kids!)

Fast Forward One Year

I flew about 160 hours over the next 12 month, keeping a close eye on my JPI EDM-760 engine monitor, with special emphasis on RE #3. From time to time, I'd download the engine monitor data and graph it on my PC. Everything looked perfect. However, I did three oil changes during that year, and each time my oil analysis came back with ever-increasing nickel. Somehow I had a sneaking suspicion that it was coming from the exhaust-valve guide in RE #3. I'd just returned home from a 5,000 nm trip that took me from Santa Maria, Calif., to Ft. Lauderdale, Fla., then to Dallas, Texas, and then back home to Santa Maria. The airplane performed flawlessly. It was just about time for me to start my 2007 annual inspection. Before downing the airplane for annual, I flew a quick day trip to Southern California and back -- less than an hour each way. On the flight home, I noticed a tiny amount of "jitter" in one of the EGT bars on my engine monitor (see graphic below). You guessed it: cylinder RE #3! The jitter was very slight ... only about 20 or 30 degrees Fahrenheit. Most pilots wouldn't even have noticed it. But I'd been actively watching for precisely this symptom for 160 hours, and I knew exactly what it meant: The exhaust valve in RE #3 had finally started to leak.

Engine Monitor Data (click here for larger version -- 68 KB)
Upon landing, I put the airplane in the hangar and borescoped the cylinder. Sure enough, the exhaust valve in RE #3 had two very prominent hot spots where the normal exhaust deposits were gone. It had the classic appearance of a burned exhaust valve. I knew what I had to do. A few days later, I flew the airplane for about 15 minutes to warm up the engines, then grounded the aircraft for its annual inspection and started off with the traditional hot compression check. Once again, eleven cylinders had compressions in the 70s. The exception was RE #3, which now measured 0/80!

Surprise, Surprise

This was quite a surprise for me -- in fact, the first of several surprises. Given the appearance of the exhaust valve under the borescope, I certainly expected the cylinder to have lousy compression. But I certainly did not expect its compression to be zero! The engine was running smooth with all cylinders clearly making full power, and the telltale EGT jitters on RE #3 were just barely detectable. Needless to say, I pulled the cylinder. Once I'd wrestled it off the engine, I got my second surprise. Given that the cylinder had been in service for 28 years and 4,050 hours, I anticipated that it would be worn well beyond service limits and would live out the rest of its days as a lamp base in my office. But when I inspected the cylinder bore, it looked astonishingly good. Although the crosshatch hone pattern was gone, I couldn't see or even feel the usual wear step in the ring-reversal area at the top of the stroke. Had I not known with certainty that this cylinder had been in service for more than 4,000 hours, I would not have believed it. I don't own a cylinder-bore gauge, and my attempts to borrow one proved fruitless. So I packed up my geriatric jug and shipped it to TCM engine guru Bob Moseley at SkyTEC in Fulton, Mo. Mose inspected the cylinder and reported back that it was still within standard service limits (albeit just barely) and that there were no exhaust-port cracks or other flaws that would make the cylinder unairworthy. He replaced the valve and valve guides, honed the barrel to restore the crosshatch, and shipped it back to me with a set of new P5 piston rings. (Note that standard service limits are the same as minimum new P5 limits. Consequently, if a cylinder is near standard service limits, using P5 rings will provide tighter ring gaps and better compression.) I'd asked Mose to send me back the old, burned, exhaust valve for use as a training aid in my Savvy Owner Seminars. When I looked at the valve, I got my third surprise. Although slightly burned in two places, the valve was still in astonishingly good shape. It had very little metal erosion and no obvious warping, and was clearly nowhere remotely close to the point of failure. After inspecting the valve, I have to believe that RE #3 would have continued to develop full power for another 50 hours before the valve deteriorated to the point of shedding a sliver of metal (so-called "swallowed valve"), at which point the cylinder would no longer be able to sustain combustion. (Mose agreed with my assessment.)

Lessons Learned

Engine Monitor

Exhaust valves and guides have proven to be by far the least-robust components in our piston aircraft engines and often cause problems prior to engine TBO. But by using all available engine-condition monitoring tools -- including oil analysis, borescope inspections, digital engine-monitor data and even the venerable, old, compression test -- actual in-flight exhaust-valve failures are almost entirely preventable. Looking back, my valve started telegraphing warnings at least 200 hours before the point that it would have failed. (That's a 200-hour P-F interval for you RCM buffs.) The oil analysis gave me a first early warning that I had an impending valve problem. Last year's compression test told me which cylinder had the worn valve-guide, but the borescope told me that the valve was not yet in jeopardy of failing. At that point, it probably would have been a good idea to initiate 50-hour borescope inspections of that cylinder, but careful attention to the engine monitor told me precisely when the valve deteriorated to the point that it started to leak exhaust gas in flight. The borescope confirmed the engine monitor's diagnosis, and this year's compression-test goose egg left no doubt. The rest of the cylinder assembly is far more robust, and with some basic pilot TLC -- mainly proper leaning procedure and religiously keeping CHTs below 400 degrees F -- they can provide reliable service for an astonishingly long time. My 28-year-old, 4,500-hour-old, #3 cylinder is back on the right engine with new valves and rings, and the airplane is back in the air and performing beautifully. I'm betting that this geriatric jug has a decent shot at making another 1,000 hours or so before starting its second career as a lamp base. I just hope I can keep my FAA medical that long. See you next month.
Want to read more from Mike Busch? Check out the rest of his Savvy Aviator columns.
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