The Savvy Aviator #49: Reliability-Centered Maintenance Q&A

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The Savvy Aviator

My recent columns about reliability-centered maintenance (RCM) have elicited some interesting questions from readers. Here are some questions I've been asked, along with my answers.

Quarter-Century-Old Engines?

Mike: I read with very great interest your latest article regarding operating engines beyond the recommended TBO. My personal observation is that many engines are needlessly overhauled way too early for things like cylinder problems that may have a much less costly but still airworthy repair available. It just so happens that a friend just bought a D55 Baron with IO-520s that were factory rebuilts installed in 1982 -- 25 years ago -- and have about 1,650 since reman. They have early heavy cases that are not of the seven-stud type and non-VAR crankshafts. [Editor's Note: VAR is vacuum arc remelt; see sidebar on right.] As such, at overhaul time they will need replacement of these two expensive parts. My question to you is this: Do the comments you make about engine overhaul times in your article also apply to these engines, given the case-cracking tendencies of IO-520s and the crankshaft AD? Your view is most refreshing and I greatly appreciate your desire to help owners reduce their ownership cost while not compromising safety. -- Steve Johnson, ATP-CFII-A&P-IA

Inspecting Engine

Absolutely applicable to your friend's D55 engines, Steve! The TSIO-520 engines on my 1979 airplane are 28-year-old originals. They have 1979-vintage cases and no seventh stud, just like the engines on your friend's D55. My engines do have the larger crankshafts (-xB), which are VAR. I took these engines to 1,900 hours on the first run (TBO is 1,400), discovered on teardown that the engines were absolutely pristine and all 12 jugs still within new limits! So I did a "minimalist overhaul," which reused all the jugs. I am now at 2,200 hours SMOH (4,100 TTIS) with zero problems. When time comes for me to overhaul these puppies, I have no intention of changing cases or adding the seventh-stud mod. Ten of my 12 jugs are still originals with 4,100 hours on the bores, and I just pulled one to replace a burned exhaust valve and its bore was still within service limits after 4,100 hours! So I re-valved it, installed P5 rings, and put it back on for another 1,000 hours. But when it comes time to overhaul, I won't attempt to recondition these 28-year-old cylinders because I think 5,000-hours is about as far into the cylinder-head fatigue life as I consider prudent. I wouldn't be surprised to make it to 2,500 hours on my minimally-overhauled, 28-year-old, TSIO-520s and I see no reason your friend couldn't get 3,000 hours on his normally aspirated, first-run, 25-year-old IO-520s -- if the force is with him and if he resists the urge to euthanize the engines before their time. I would seriously doubt that your friend's 1982-vintage humpback crankcases will have a cracking problem if he operates the engine properly. He will have to replace the airmelt cranks at overhaul, of course -- don't get me started about that VAR crank AD travesty! -- and that's a great reason not to split the case before absolutely necessary. I think you and I both know that the TCM airmelt crankshaft AD was a complete boondoggle. We've had many more crankshaft problems with VAR crankshafts than we ever did with the old airmelt crankshafts. The FAA did owners a real disservice on that one, in my opinion, costing owners big bucks with no demonstrable safety benefit. Your friend's IO-520s will surely develop a problem someday that will require splitting the case, but the odds are extremely high that the problem won't be a safety-of-flight item, just a safety-of wallet item: a spalled cam-lobe, a case crack, an oil leak or something like that. The kinds of catastrophic failures that can get you hurt or killed are far more likely to occur shortly after overhaul than beyond TBO. One concept I try to drill into owners during my weekend Savvy Owner Seminars is that the decision to overhaul should never be influenced even slightly by anything that happens to cylinders, because cylinders are just bolt-on accessories like alternators and magnetos. As you point out, Steve, so many engines are needlessly torn down because they developed one or two bad jugs and the owner's knee-jerk reaction is that "the engine is tired" and needs to be overhauled. (Or sometimes that's the mechanic's knee-jerk advice to the owner.) To which I say, "If you had a bad alternator, would you overhaul the engine? No? Well then why do it for a bad cylinder? They're both simple, bolt-on units!" Your friend's biggest problem in taking those engines to the ripe old age they probably deserve may well be finding a cooperative IA willing to keep approving them for return to service each year. (Unless he's lucky enough to have you signing off his annuals!) In this post-GARA age of litigation against shops and mechanics, it's hard to find an IA willing to stick his neck out for a customer any more. After having talked with a few aviation plaintiff's attorneys, I find it hard to blame them.

Engine Monitor? Oil Analysis?

Mike: I first want to thank you for taking the time to respond to my query with a complete and thought-provoking answer. It sounds like we're on the same wavelength; namely if it isn't broke, don't fix or replace it! Your comments lead to a couple more questions: My friend's D55 Baron currently has a single-probe EGT and single-probe CHT as delivered from the factory. The local hangar rats are telling him he needs a Chinese television engine analyzer system installed. I'm telling him those engines have run just fine for 1,650 hours with what's installed now, and that the engine analyzer maybe is an item for him to consider installing when he gets around to replacing the engines -- hopefully several hundred hours from now. I say that if something goes wrong with an engine, he'll know it by roughness, loss of power, bad mag checks, compression checks, etc. What do you think? Second, the same hangar rats are saying he needs to do oil analysis samples. I can see the value of doing it, but would caution him not to put too much emphasis on the results and not make a hasty decision on spending a pile of money without fully exploring all the alternatives if he has a high number on an oil sample. The old guys used to say it wasn't a serious problem until you got to where you could read the part numbers on the pieces of metal in the screen! Thanks again for your help, and best regards. -- Steve Johnson, ATP-CFII-A&P-IA

Engine Monitor

Steve, I'm a big fan of both engine analyzers and oil analysis. I'm afraid that puts me in agreement with the hangar rats. For one thing, I believe that the minute you decide to toss fixed-interval TBO out the window and maintain the engine strictly on-condition (i.e., RCM), you've obligated yourself to employ the very best condition-monitoring tools available. In my view, those definitely include spectrographic oil analysis, regular borescope inspections and the use of a digital engine monitor. I agree with you that it is never appropriate to use an oil analysis report in isolation as the basis of some expensive maintenance decision. The same is true of a compression test or an anomaly in engine analyzer data. All these tools must be used together and big-ticket maintenance decisions should always be made using a "preponderance of the evidence" approach. Only then can you be absolutely sure that you're doing the right thing at the right time. A perfect illustration is a recent experience involving one of the engines on my 1979 Cessna T310R. In February 2006, I started seeing elevated nickel in my oil analysis. Since my cylinders are steel (not nickel-carbide), I knew the elevated nickel could only be coming from an exhaust-valve guide. I didn't know which one yet, but the oil analysis report started me thinking that I should expect an exhaust-valve problem pretty soon. In April 2006, at my annual inspection, the compression test showed 70s on all cylinders except for right-engine cylinder #3, which showed 50/80 with air leaking past the exhaust valve. I staked the valve and the compression immediately climbed into the 60s. I inspected the valve with a borescope and saw no evidence of hot spots. So I continued the jug in service but now I was pretty sure that the combination of oil analysis and funky compression was telling me that the right-engine cylinder #3 exhaust valve was very likely to start giving me trouble in the future. I continued to fly the engine with a very close eye on the right engine cylinder #3 EGT on my engine monitor, but it was solid as a rock, which told me that the valve wasn't leaking -- at least not yet. In March of 2007, 160 hours later, I started to see the first tiny indications of EGT instability on cylinder #3 EGT. The EGT wiggles were very, very tiny -- just 20 degrees F to 30 degrees F out of 1,500 degrees F -- but I'd been watching for them and I knew exactly what they meant: The valve was just starting to leak a little. Upon landing, I borescoped cylinder #3 and found two well-defined hot spots on the exhaust valve, just as I expected. I then did a hot compression test and cylinder #3 measured 0/80! I pulled the jug, had it re-valved and honed, and put it back on with a set of new piston rings. The old exhaust valve guide was extremely sloppy and the valve was starting to burn, but it was not remotely close to failing -- I'd estimate the valve would have lasted at least another 50 hours before being "swallowed" and causing the cylinder to cease combustion. In short, I used all available tools -- oil analysis, borescope inspection, engine monitor and compression test -- to gain an excellent insight into what was happening inside this engine. With that knowledge I was able to make a well-educated judgment about just how far to push before downing the engine and fixing the problem. The advantage of having and using all these tools is that you wind up making far better decisions and not having to guess or worry. If I didn't have the engine monitor, I'd have worried about the valve; but because I did have it and knew exactly how to interpret the data, I felt confident that I could continue to fly and would get a clear heads-up the moment the valve actually started leaking. And indeed that's exactly what happened. My vote, therefore, would be that your friend run, not walk, to install a JPI EDM-760 in his Baron, and that he put his engines on oil analysis with Blackstone Laboratories in Indiana at the next oil change. Then he can fly those mothers to 3,000 hours!

How About Low-Usage Engines?

Mike: Could you comment on how RCM would apply to low-usage engines (e.g., 50 to 75 hours a year) that are typical of much of the piston-GA fleet? How does one decide when to overhaul such an engine? What if all you have is stock factory instrumentation, as is the case with many older Bonanzas, and adding an engine monitor would cost up to 25 percent of the value of the airplane? Is there a reason to follow TCM's 12-calendar-year overhaul recommendation regardless of hours SMOH? -- Tom Turner, ABS Mgr. of Technical Services

Tucson International Airport Boneyard

Tom, let me answer your last question first: No, in my judgment, there is never any reason to follow TCM's ridiculous 12-calendar-year overhaul recommendation. Of course we know perfectly well why TCM made that recommendation: It's because they're concerned about corrosion issues in low-usage engines that fly so few hours annually that it might take 50 years to reach the TBO engine-hour recommendation. Indeed, such a low-utilization engine is quite likely to develop corrosion problems before accumulating that many operating hours. But a one-size-fits-all, 12-year limit makes no sense at all. A low-utilization engine tied down in Tampa might be a corroded mess in less than 12 years, while a similar low-utilization engine hangared in Tucson might be totally corrosion-free after 50 years. (It is not for nothing that the Air Force and airlines keep their no-utilization hardware in Tucson!) It's just silly to pick some number out of the air and say that it applies to all engines across the board. We need to treat each engine individually, and maintain it based strictly on its actual condition -- one of the major findings of RCM research. So to answer your first question: The way you decide when to overhaul a low-utilization engine is exactly the same as for a high-utilization engine. You implement a rigorous program of surveillance using all available condition-monitoring tools -- oil filter inspection, spectrographic oil analysis, compression tests, borescope inspection, spark-plug inspection, digital engine-monitor analysis, trend monitoring of oil pressure, oil consumption, etc. -- and then use a "preponderance of the evidence" approach that combines all these tools to evaluate engine condition and decide when maintenance actions (including overhaul) is necessary. Finally, concerning Bonanzas that still have only stock, factory, engine instrumentation: I think you need to redo your calculations, because there's no way an engine monitor installation could cost 25 percent of what a Bonanza is worth, even if it's a really old one. Let's do the math. The street price for a JPI EDM-700-6C digital engine-monitor system is $1,830. Installation time is approximately 10 man-hours according to JPI, so if the shop rate is $80/hour that's another $800. So the total installed price for the system is about $2,600. The very cheapest Bonanza I could find listed was a 1951 C35 with a run-out E225-8 engine and electric prop and a market value of $40,000. Adding an engine monitor to that bucket of bolts would cost 6.5 percent of its value. And that's a really extreme case. Most 1960s-vintage Bos are changing hands at between $80,000 and $140,000, so adding an engine monitor costs about two or three percent of value. In my mind, the aircraft value is really irrelevant, because the operative question is: Will the engine monitor pay for itself? Assuming a shop rate of $80/hour, a $2,600 engine-monitor installation would need to save 32.5 hours of A&P labor to pay for itself. If the instrument saved eight hours of troubleshooting labor each year (which strikes me as a reasonable estimate based on my own experience), it would pay for itself in four years -- regardless of what vintage airplane it is installed in. And the first time it allows the owner to detect a leaking exhaust valve, a mistimed magneto, destructive detonation or an incipient pre-ignition event ... priceless! So whether you fly a C35 or a B36TC, my advice is the same: Just do it! See you next month.
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