You know me. I'm the "To TBO and Beyond" guy. I believe in running engines as long as they're demonstrably healthy, even if that means going beyond the manufacturer's recommended TBO. Nothing disturbs me more than when I hear about owners who are talked into tearing down an engine that's running just fine (or talk themselves into doing so).
Case in point: Not long ago, I received an email from an owner seeking a second opinion on what to do about the TCM IO-520 engine in his Bonanza:
"The engine is now at 1,500 hours (TBO is 1,700) and it seems to be running very well. But here's the bad part: It's using a quart of oil every 3.5 to 4 hours, and putting a lot of oil on the belly of the aircraft, even with an air/oil separator installed.
"So what should I do? Should I get a field overhaul, or opt for a factory rebuilt engine? (The engine does not have a VAR crank.) Should I consider an STC upgrade to an IO-550? I'm leaning toward using Superior Millennium cylinders, do you agree?"
I took a deep breath and counted to 10. This owner just told me that he has a fine-running engine, yet he's already concluded it needs to be torn down or replaced. What was he thinking? It wasn't obvious to me that this engine had any real problem at all, much less one requiring immediate euthanasia.
So what if it's using a quart in four hours? Is that so terrible?
No, it isn't. TCM SID97-2A says not to be concerned until "oil consumption exceeds a quart every three hours or if any sudden change in oil consumption is experienced." Even when that threshold is reached, TCM simply calls for a borescope inspection to determine if there's really a problem. If the cylinders look OK under the borescope, the engine can remain in service despite the high oil consumption.
SID97-2A also indicates that in February 1997 TCM actually reduced the tension on the oil control rings in its cylinder assemblies in order to increase oil consumption, in order to achieve improved lubrication of the cylinder bore. A certain amount of oil consumption is essential for maximum cylinder life. When it comes to oil consumption, less is not necessarily a good thing.
Bottom line is that it's quite likely that there's nothing at all wrong with the engine in this owner's Bonanza. At worst, perhaps it has a couple of worn cylinders that might need to be replaced eventually. (Even that's not clear, since the owner didn't mention low compressions.)
Even if a borescope inspection reveals that the engine has a worn-out jug or two, so what? Both TCM and Lycoming cleverly designed their engines so that the cylinders were bolt-on accessories that can be repaired or replaced without removing the engine from the airframe or splitting the case. If the engine actually does have badly worn cylinders, that's a reason to repair or replace the jugs, not to tear down the whole engine.
Think about this for a moment. If some other bolt-on engine accessory went bad -- say an alternator or vacuum pump or magneto or prop governor -- would you let your mechanic remove the engine and have it major overhauled? Of course you wouldn't.
If you had a hangnail, would you let your doctor perform an amputation and hand transplant? No, I didn't think so!
|This TCM cylinder head developed a fatigue crack after about 2,500 hours of service. We don't overhaul engines because of a top-end problem like this; we change cylinder and keep flying.|
Why would an aircraft owner even consider major overhaul or engine replacement just because one or two cylinders might be worn out? To my way of thinking, it doesn't matter whether an engine is at 100 hours since new or 100 hours past TBO -- a sick cylinder calls for cylinder replacement, not engine replacement.
Here's what I emailed back to the owner:
"I spend about four hours of my weekend seminars on this very subject, but let me try to give you the five-minute version:
"First, I would never consider overhauling an otherwise good-running engine just because it has high oil consumption. There's nothing wrong with burning a quart in four hours, so long as your sparkplugs aren't oil-fouled and your compressions are within acceptable limits. If things get bad enough and you find one or more cylinders with unacceptably low compression, you may want to consider replacing them. That's why TCM makes its engines with bolt-on cylinders: so you can change them without having to overhaul the engine. The only valid reason for overhauling an engine is a problem with the "bottom end" (crankcase, crankshaft, camshaft, gears, main bearings, etc.) that cannot be cured without splitting the case.
"Second, have you simply tried running the engine at a lower oil level on the dipstick? Big-bore TCM engines are famous for throwing out excess oil if the crankcase is overfilled. The TSIO-520s on my T310R have a 12-quart sump, but I typically run them at eight quarts on the dipstick.
"Third, excessive oil on the belly is usually caused by excessive crankcase pressure. Sometimes this is due to worn cylinders that permit excessive blow-by past the rings (in which case your cylinders will show low compression readings). But it can also be due so something as simple as an oil filler cap that isn't sealing properly. (When did you last check the oil cap gasket?) Another possible cause is a leaky front crankcase seal, which is pretty easy to change, although it does require pulling the prop.
"In any case, it sounds to me as if you may be a long way from needing to major-overhaul this engine. If you do decide to rebuild it anyway, drop me another email and I'll offer some suggestions. But I really think that any consideration of rebuild/overhaul at this point is way premature.
"Don't obsess about the manufacturer's published TBO. It's only a suggestion, not a requirement or a life limit. The engines on my Cessna T310R are now 600 past TBO and running magnificently. I have no intention of overhauling them any time soon. Last time around, I flew them to 500 past TBO, and then got nervous, chickened out, and had them torn down. The teardown inspection revealed that they were near-perfect internally, and it was clear they could have gone hundreds of hours more.
"I'll try not to make that mistake again. When my engines are ready for overhaul, I'm confident that they'll let me know by starting to make metal or to leak oil or to crack their crankcases or something like that. That's when I'll overhaul them. You might consider a similar approach."
I don't know what started this owner thinking about doing a premature overhaul on his IO-520, but it very well might have been something his mechanic said. I've often heard mechanics advise owners of high-time engines with one or two soft cylinders that it makes more sense to "bite the bullet" and do a major overhaul rather than continue to spend money trying to nurse another few hundred hours out of the engine.
When I do the math, this invariably turns out to be bad advice. Let me show you what I mean.
If you compute the hourly "reserve for overhaul" for your engine, you'll probably come up with a figure in the $15 to $25 range. That's how much each extra hour we can get out of our engine is worth to us.
For instance, if the IO-520 has a 1,700-hour TBO, a factory rebuilt from the "$300 over factory invoice" place costs $25,000, and the cost of R&R labor, prop and governor overhaul, new hoses, engine mounts and exhaust stuff adds another $9,000, then we calculate an hourly reserve of $34,000 divided by 1,700 hours, or $20/hour.
Now suppose one of our cylinders goes south and needs to be repaired or replaced. What does that cost? A new IO-520 cylinder costs about $1,200. A reconditioned or continued-time jug might cost $600 to $800. The R&R labor would come to about $400. So the total cost of repairing or replacing the cylinder will come to $1,000 to $1,600 depending on whether we opt for repair, continued-time or new. (Personally, I consider it wasteful to install brand new cylinders on a near-TBO engine, since all we really need is to get another 500 hours or less out of them.)
At $20/hour, that means that the break-even point for replacing the cylinder is only 50 to 80 hours. In other words, if we can replace the cylinder and get another 100 or 150 hours out of the engine, we're definitely money ahead.
What if we have two problem cylinders? If those cylinders are on opposite sides of the engine, then just double the previous figures, and figure a break-even time of 100 to 160 hours. If both bad cylinders are on the same side, however, the R&R labor is less for the second cylinder (by about half) than for the first one, since we only have to drop the induction and exhaust system once to replace both jugs. Replacing two same-side cylinders with continued-time jugs might cost $1,200 in parts plus $600 in labor, for a total of $1,800 and a break-even point of 90 hours.
So if your IO-520 is just 200 hours from reaching TBO and you find yourself with one or two bad cylinders, does it make economic sense to replace those cylinders and keep on flying? You bet it does! If you do it and then fly the engine to TBO, you're money ahead. And if you get lucky and make it to TBO+500, you might be $14,000 ahead!
See you next month.
Want to read more from Mike Busch? Check out the rest of his Savvy Aviator columns.