The Savvy Aviator #4: Debunking TBO

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Engine TBO (time between overhauls) seems to be one of the most misunderstood concepts in aviation maintenance. There are lots of TBO-related old wives tales that are widely believed by owners and mechanic alike, and they can cost owners a great deal of money. Mike Busch endeavors to clear up these misconceptions, and explain what TBO really means.

The Savvy Aviator

Ask any aircraft owner what the TBO is for the engine(s) on his aircraft and you'll almost always get the correct answer without hesitation: "My engine has a 1,700-hour TBO." But ask that owner to explain the significance of that TBO figure and you'll get all sorts of answers, most of them flat wrong. Here are a few of the most common misapprehensions about TBO:

"It's illegal to fly an airplane if the engine is past the TBO established by the manufacturer."

Nonsense. The TBO figures published by Lycoming and TCM are not airworthiness limitations. An engine may be long past TBO and still be legally airworthy. (An engine may also become unairworthy long before reaching TBO.)

"While it's true that manufacturer's TBO isn't compulsory for non-commercial (Part 91) operators, commercial (Part 121/135) operators are required to overhaul an engine when it reaches TBO."

Not so. Both Lycoming and TCM publish engine TBOs in the form of non-mandatory service bulletins. Some Part 121/135 operators have Operations Specifications that require them to comply with all manufacturer's service bulletins (even non-mandatory ones), while others have Op Specs that require compliance only with mandatory service bulletins. Those in the latter group are no more obligated to comply with published TBO than are Part 91 operators. Those in the former group might theoretically be required to overhaul at published TBO, but most such operators request TBO extensions from their FSDO and these are routinely granted, often for as much as 50% over the engine manufacturer's published TBO. So, in actual practice, published TBO is hardly ever compulsory for any operators -- commercial or non-commercial.

"Continuing to fly an engine beyond TBO could void your aircraft insurance."

Poppycock. I've yet to see any aircraft insurance policy that requires compliance with non-mandatory service bulletins as a condition of coverage. Most policies only require that the aircraft be airworthy and in compliance with FAA inspection requirements.

"Continuing to fly an engine beyond TBO is dangerous because doing so increases the chance of an in-flight engine failure."

To the contrary, an engine is much more likely to fail during the first few hundred hours after major overhaul than during the first few hundred hours after passing published TBO. If you exclude fuel starvation or exhaustion (i.e., pilot error), most engine stoppages involve mechanical failure of some "top end" engine component like a cylinder, exhaust valve, piston, magneto, turbocharger, exhaust stack, etc. Such bolt-on components are routinely replaced during normal maintenance without any need to overhaul the engine. The purpose of a major engine overhaul is to inspect, recondition and or replace the engine's "bottom end" components -- crankshaft, camshaft, crankcase, gears, bearings, etc. -- that cannot be accessed without splitting the case. But these "bottom end" components are seldom implicated in catastrophic engine failures. Furthermore, in those rare cases when these components do fail (e.g., crankshaft fracture), the failure is almost never correlated with time since overhaul. (If a crankshaft is going to fail, it's most likely to fail during the first few hundred hours after manufacture, or after a prop strike.)

"Continuing to fly an engine beyond TBO is false economy, because doing so just makes the inevitable major overhaul more expensive."

This old wives tale probably originated back in the days when new cylinders were very expensive and most engines were field overhauled using reconditioned (chromed or oversized) jugs. In those days, if you pushed an engine to the point that its cylinders could not be reconditioned, you'd have to spend more at overhaul to buy new ones. Nowadays, however, the cost of new cylinders has come down to the point where most major overhauls include all new jugs as standard procedure. Consequently, there's no longer any real advantage to overhauling sooner rather than later. The only things that will impact the overhaul cost are an unserviceable crankshaft or a cracked crankcase, and neither of those items are any more probable for an engine operated beyond TBO.

By the way, it's not just owners who hold these misconceptions. Plenty of A&P mechanics believe these things, too.

TBO From The Horse's Mouth

The definitive word on the subject of TBO for engines manufactured by Teledyne Continental Motors is TCM Service Bulletin M91-8 -- "Recommended Overhaul Periods for All Teledyne Continental Motors Aircraft Engines" -- dated July 10, 1991. This is the document in which TCM publishes a table of recommended TBOs for all TCM engine models.

TCM service bulletins come in three different grades: recommended, mandatory, and critical. Critical service bulletins are typically reserved for items that are considered so urgent that TCM asks the FAA to issue an Airworthiness Directive to mandate compliance. Mandatory service bulletins are less urgent, normally not accompanied by an AD, and normally "mandatory" only for commercial operators. Recommended service bulletins are used for conveying helpful hints to owners and mechanics, but they are merely suggestions and compliance is strictly up to the individual operator.

M91-8 is one of these lowest-priority service bulletins. It offers recommendations, but they are not intended by TCM to be obligatory for any operator. To underscore this point, let's take a look at exactly what TCM says in M91-8 (emphasis mine):

Thousands of hours of operating experience indicate that Teledyne Continental Motors (TCM) aircraft engines, when operated within prescribed limitations, instructions and recommendations, can be operated between overhauls for the number of hours listed in the following table. The overhaul periods listed are recommendations only. They are predicated on the use of genuine TCM parts, compliance with all applicable Service Bulletins and ADs, as well as all required preventive maintenance, periodic inspections, manufacturer's specifications, and the determination by a qualified mechanic that the engine is operating normally and is airworthy. The accomplishment of cylinder leakage checks and spectrographic oil analysis may be helpful in making this determination. Any operation beyond these periods is at the operator's discretion and should be based on the inspecting mechanic's evaluation of engine condition and operating environment. Calendar time also affects this condition and should be taken into account.

Particular attention should be paid to throttle response, power, smoothness of operation, oil consumption, to the proper use and maintenance of oil and air filters, and adherence to the recommended oil change periods. Emphasis should also be placed on recommended fuel management. These recommended overhaul periods in no way alter TCM's warranty policies.

The wording of TCM's service bulletin M91-8 makes it explicitly clear that:

  • Published TBO is strictly advisory, not compulsory.

  • Operation beyond recommended TBO is permitted at the operator's discretion.

  • Operation beyond recommended TBO does not void the manufacturer's warranty.

  • Engine overhaul should be performed "on condition" based on the inspecting mechanic's evaluation of engine condition, based on compression checks, spectrographic oil analysis, oil consumption, and subjective assessment of engine performance (e.g., throttle response, power, smoothness of operation).

Bottom line is that if an engine is still going strong when it reaches TBO, there's absolutely no reason to consider removing it from service for major overhaul, and every reason to continue flying until it starts showing signs that overhaul is warranted.

So What Good Is TBO?

Does this mean that the manufacturer's TBO is a worthless figure that should be ignored? No, not at all. In my view, the best way to think about published TBO is the way we think about human life expectancy statistics.

According to the National Vital Statistics Report published by the Centers for Disease Control, the current life expectancy at birth for a white male in the United States is 75 years. This statistic might be quite useful in figuring out what premium to charge for a life insurance policy, or how to plan for retirement.

Does this mean that white U.S. males should be euthanized (removed from service) when they reach age 75? I certainly hope not! In fact, the same CDC figures show that the current life expectancy for a 75-year-old white male in the U.S. is 11 years. In other words, if you're still kicking at age 75, you can expect on average to live until age 86. Furthermore, if you are still alive at age 86, your life expectancy is 6 years so on average you can be expected to live until age 92.

Similarly, according to TCM Service Bulletin M91-8, the "life expectancy at birth" (recommended TBO) of TSIO-520-BB engines (like the ones in my 1979 Cessna T310R) is 1,400 hours. This statistic might be quite useful in figuring out a suitable dollar amount for amortizing overhaul expense. Since it costs about $30,000 to overhaul one of these engines, a reasonable "reserve for overhaul" would be $21.43 per hour (i.e., $30,000 divided by 1,400 hours). This figure would also be appropriate for adjusting the "blue book" value of my airplane to account for higher- or lower-than-average engine time.

Does this mean that I should have euthanized my engines when they reached 1,400 hours SMOH (since major overhaul), despite the fact that they were running great, had excellent compressions, low oil consumption, no metal in the oil filters, and excellent oil analysis reports? No, I don't think so. Although TCM doesn't publish figures for "life expectancy at 1,400 hours" for these engines, it only stands to reason that TSIO-520-BB engines that are in good shape at 1,400 hours surely have a good deal of useful life left in them. (As previously noted, many commercial operators routinely run their engines to 150% of manufacturer's TBO with the FAA's official blessing.)

Some Real-World Experience

When I purchased my T310R in 1987, it had 1,300 hours total time on the airframe and engines. Since TCM's published TBO for its TSIO-520-BB engines is 1400 hours, those engines were pretty much "run out" when I acquired the airplane (and the price I paid was adjusted downward accordingly).

At 1,400 hours those engines were still running superbly, and all signs pointed to them being in great shape. I wound up flying those engines trouble-free to 1,900 hours (500 hours past published TBO), at which time I started getting nervous and pulled the engines for major overhaul.

As it turned out, my nervousness about flying those engines 500 hours past published TBO were completely unfounded. The overhaul shop reported that all 12 cylinders were still within new limits, as was pretty much everything else. It was clear from the results of the teardown inspection that those engines could have gone considerably longer -- at least another 500 hours -- with no problem.

Those engines received minimalist (i.e., el-cheapo) major overhauls in 1990. The cylinders had their valves replaced, their barrels lightly honed, new pistons and rings installed, and were bolted back on for another run. I saved about $12,000 by not replacing the cylinders at overhaul, but I figured that there was probably no way these jugs would survive another 1,400 hours.

I figured wrong. Those engines and cylinders now have accumulated another 1,600 hours since the overhaul, and so those cylinders have 3,500 hours on them. I am just wrapping up my 2004 annual inspection as I write this. The compressions are all 75/80 or better, the oil consumption remains about a quart in 15 hours, the oil filters are clean, the oil analysis is excellent, and the engines are running as well as they ever have. I imagine I'll be flying behind them for a while longer (knock on wood).

This time around, I'm not even the slightest bit nervous about continuing to fly past TBO. I know that so long as I continue to keep a watchful eye on compression, oil consumption, oil filter inspection, oil analysis, temperatures and performance, I'll know when the engines are getting tired and it's time to overhaul them. That could be next year, or it might be five years from now. I'm not even going to try to predict how much more useful life those engines have left, but when the time comes to major them, they'll tell me.

I haven't yet decided exactly what I'll do when that time comes. Will I have the engines field overhauled again, or exchange them for factory-rebuilt engines? Will I recondition the cylinders or install new ones? Install TCM factory cylinders, Superior Milleniums, or ECI Titans? These are complex decisions that I discuss at considerable length in my Savvy Owner Seminar. In my own case, I'll make those decisions when the time comes, based on the best information available at the time.

But I can tell you one thing for sure: When those freshly overhauled or rebuilt engines are installed back in the airplane and it's time for me to get back in the air, that's when I'll be nervous!

See you next month.


Want to read more from Mike Busch? Check out the rest of his Savvy Aviator columns.