What does “TBO” stand for? What does it mean? What does it REALLY mean? Is the TBO a mandatory number? How can I tell whether I need to do an engine overhaul prior to TBO, at TBO, or whether I can operate beyond TBO (exceed TBO)?
Technical Editor:
Depending on the reference material you are using, “TBO” means “Time Before Overhaul” or “Time Between Overhauls”. It’s a fine distinction, but I favor the former definition; simply because a new engine in a new airplane would seem to be using up Time Before Overhaul; it would not seem to be Time Between Overhauls until after the first one. Pick the one you like.
Lycoming Service Letter No. L201C (3/9/90) lists revised TBO times and caveats. There may be a later version by now; I couldn’t get a response from the Lycoming site to verify currency. For example, the IO360-A1B6 used in the mid-70’s Sierras has had its TBO revised from 1600, to 1800, and now to 2000 hours. Some of these revisions are based on the introduction of upgraded parts, for example 1/2″-diameter valve stems, that must be installed as a condition of the TBO extension (or some related Service Bulletin/Service Instruction/Service Letter). Sometimes you will read that the TBO value is an ironclad rule; and the FAA enforces it in revenue service (Part 121 and Part 135 operations), unless the operator has received FAA approval on an alternative approach to assuring engine reliability for revenue passenger operations.
Some people have opined that insurance companies may view TBO as regulatory; others have opined that the number is a manufacturer’s recommendation only. The latter view seems to be at odds with the FAA’s position for revenue operators. The companies that finance aircraft purchases will usually reduce their level of financing on a plane with an engine running at or beyond TBO. In their case it is almost certainly a value issue, not a safety issue.
For all practical purposes, the service history, engine condition indications, and wear trends will establish the real TBO of an individual engine in Part 91 (private) aircraft service. Many engines are likely to require overhaul prior to the published interval, for one reason or another. While it is my personal inference and belief, as opposed to clearly published fact by the major engine manufacturers, in my opinion the intent of TBO is widely misunderstood. Most people take it to mean that the engine will probably be “worn out” by the TBO interval. If that is someone’s understanding, the next rational assumption is that if the engine doesn’t seem to be worn out, it should not have to be disassembled. In reality, considering the way TBOs are developed and periodically extended, I think that their intent is very different.
If Lycoming says that the IO360A1B6 has a 2,000 hour TBO, what they are really saying is this: If the engine is disassembled at the TBO point, and is inspected and overhauled to the standards published by Lycoming, then that engine should not experience a catastrophic failure during the next 2,000 TBO interval. In other words, Lycoming has determined that if no cracks are found during the various non-destructive testing, if all the required parts are replaced, if the engine is reassembled using new engine tolerances, and is properly reassembled to torque and other specifications, then Lycoming expects it to live up to the durability standards for the FAA certification and for Lycoming’s published TBO interval. This may seem to be a fine point, but there is a very big conceptual difference between “it’s not worn out yet”, and “there were no known potential flaws that could lead to a catastrophic failure within the next 2,000 hours, but now I’m past that time”.
Note that this definition applies ONLY to engines that are truly and properly overhauled to new limits, with all the mandated parts replaced; all bets are off on a field overhaul to “service limits”. While the FAA allows the latter to be called an “overhaul”, in reality it is a poor “dip-strip-reassemble-paint” substitute for the real thing; a “dip-n-strip” job so someone can claim an “overhaul”. Often even the “paint” part is omitted. Also note that ANY overhaul by any shop other than the original engine manufacturer CANNOT result in a “zero-time engine”. Any other form of overhaul results in contined engine hours with an SMOH value (Since Major OverHaul). The engine manufacturers typically offer two levels of engine repair, in addition to brand-new engines. Their overhauled engines typically include all new cylinders and cylinder parts, all the replacement parts required in their bulletins, and some number of reused parts that meet their standards for limits. These engines must also continue their operational hours, with an SMOH value. However, the OEMs typically also offer a true zero-time engine that they term a “rebuild”. These engines cost somewhat more, and are intended to be the exact equivalent of a new engine, but with some parts reused. Those parts must typically meet new-part limits, to be used in the zero-time rebuild. Some of these practices and definitions are hotly contested by field overhaulers and the high-end engine shops, but they are well established at this point. Some of the high-end shops contend that their product exceeds the factory’s standards and performance. There is plenty of anecdotal evidence for successes and failures to go around. One irony is that in some cases the factory has actually adopted some of the practices of their competitors, offering custom work and services. My personal conclusion is that the well-known quality shops do a far better job of dealing with warranty issues, compared to the manufacturers. Caveat Emptor still applies. Educate yourself on anyone or any operation you are considering to overhaul your engine.
If you are stretching a TBO, be aware that as clearances increase, wear can accelerate dramatically due to the “hammer effect” of the widening gaps, even though rotating friction may decline and power output may seem fine. A very expensive crankshaft may be completely reusable if bearings are changed at 2000 hours and the case is properly inspected for fretting, reconditioned if needed, and properly torqued during reassembly. The crank may become expensive junk, along with the case, if operation is stretched to 2500 hours in a fretted case with loosened bearings (particularly if the engine has no “real” oil filter). You cannot safely prevent this via periodic re-torquing of the case; if you try, and fretting has occurred, you can seize a bearing. Too much fretting and the resulting loss of preload can “rock” a bearing, and perhaps even “spin” it. Either of these bearing effects will be called “catastrophic engine failure” in the subsequent accident investigation. Spun bearings virtually always result in a dramatic loss of oil pressure, and accompanying destruction of at least the involved journal. The debris usually takes out a lot of other parts as well.
One initial sign of probable case fretting is leakage at the case mating joint (parting line). I have seen attempts made to seal such leaks by dribbling epoxy on the joint. The epoxy soon develops a hairline crack that follows the case joint. This implies relative movement of the case halves, and is not simply a sealant failure. Don’t just keep cleaning the engine; overhaul it.
It is neither safe nor economical to run an aircraft engine until it shows major wear symptoms similar to those in a totally clapped-out junker car engine. TBOs can be, and are, successfully stretched in private service; but be sure you know what you are doing. Adequate compression and oil pressure should not be the only decision factors.
One final thought. Check with your insurance company on the terms of your coverage. I have heard rumors of denial of coverage if you suffer an engine failure, and the engine is found to be past TBO. However, I have yet to find a verified instance of this happening.