You know me. 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 get talked into (or talk themselves into) euthanizing engines that are running just fine.
Case in point: Here’s an email I received from a Bonanza owner seeking a second opinion on what to do about his Continental IO-520 engine:
“The engine is now at 1500 hours (TBO is 1700) and it seems to be running very well. But here’s the bad part: it’s using a quart of oil every 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 ten. This owner just told me that he as a fine-running engine, yet he’s already concluded it needs to be overhauled or replaced. What was he thinking? It sure wasn’t clear to me that this engine had any major issues, much less anything requiring immediate euthanasia.
Where’s the beef?
So what if it’s using a quart in 4 hours? Is that so terrible?
No, it isn’t. Continental SID97-2B is the bible when it comes to determining the airworthinss of Continental cylinders, and here what it has to say about oil consumption:
Oil consumption can be expected to vary with each engine depending on the load, operating temperature, type of oil used and condition of the engine. A differential compression check and borescope inspection should be conducted if oil consumption exceeds one quart every three hours or if any sudden change in oil consumption is experienced and appropriate action taken.
This guidance indicates that the Bonanza’s oil consumption of a quart in four hours is perfectly acceptable. Even when Continental’s oil consumption threshold of a quart in three hours is exceeded, Continental simply calls for a borescope inspection to determine if there’s really a problem. If the cylinders look okay under the borescope, the engine can remain in service despite the high oil consumption.
SID97-2B also indicates that in February 1997, Continental actually reduced the tension on the oil control rings in its cylinder assemblies to increase oil consumption 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 always 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 compression readings. Maybe all he needs is some new piston rings.
A worn jug is like a hangnail
Even if a borescope inspection reveals that the engine has a worn-out jug or two, so what? Both Continental 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 not.
If you had a hangnail, would you go to a surgeon for an amputation and hand transplant? No, I didn’t think so!
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.
Euthanasia is a bit much
Here’s what I emailed back to the owner:
“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 4 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 Continental 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.
“Have you simply tried running the engine at a lower oil level on the dipstick? Big-bore Continental 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 8 quarts on the dipstick.
“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 and your oil will get dirty very quickly after each oil change). 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?) or a leaky front crankcase seal (which is not difficult to change).
“It sounds to me as if you may be a long way from needing to major-overhaul this engine. If you do decide to overhaul 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 just a suggestion, not a requirement or a life limit. (The engines on my Cessna T310R are made it well past 200% of TBO and were still running magnificently.) When your engine is ready for overhaul, it’ll let you know by starting to make metal or to leak oil or to crack their crankcases or spall their cam lobes or something else obvious to let you know that “it’s time.” That’s the time to overhaul them. Doing it earlier always strikes me as being a capital crime.