When it comes to what to do when your engine reaches TBO, your choices range from doing “everything” to doing “nothing.”
The following is an excerpt from Bill Ross’ 144-page book “Engine Management 101.” Published by Superior Air Parts, Inc., the book is a compilation of what Ross has learned during his 36-plus years of experience as a pilot, aircraft owner, piston aircraft engine industry leader and FAA A&P/IA.
TBO —what does it really mean?
Today, there are many in the industry making a case for flying the aircraft until it breaks and not necessarily adhering to manufacturer’s recommendations for continued airworthiness. To me, that’s just asking for trouble.
How do you know when the aircraft will “break?” Will it be on short final at your home airport, or at night, in the clouds, with your family on board? Is the risk worth it to you? I certainly hope not.
My opinion is you can safely fly past TBO without consequences if you and your maintenance provider follow the engine manufacturer’s recommendations.
The ABCs of TBOs
Let’s look at the term “time between overhauls” (TBOs). The FAA requires manufacturers to publish a TBO for each of their engines. These aren’t numbers that are pulled out of a hat. The engine manufacturers establish these recommendations based on typical maintenance and typical engine operation.
Engines are required to have both an accumulation of actual operating time and calendar time recommendations. Mistakenly, many pilots try to “extend” their engine’s overhaul—and the cost thereof—by not flying as often as they should. The fact is, lack of consistent use is probably one of the worst things you can do to an aircraft engine.
Aircraft engines that are sedentary for many months, and sometimes years at a time, are more likely to have internal damage than those that are maintained and flown regularly. Most engine manufacturers recommend that if the engine is going to be inactive for six months or longer, it should be preserved in accordance with their respective instructions.
TBO: Your time has come
One of the questions I get asked most frequently about TBO is whether an owner should overhaul or replace their engine. It’s a good question, and it has more than one answer. Fact is, when your engine reaches overhaul time, you basically have six choices:
1) Purchase a new engine.
2) Purchase a rebuilt engine.
3) Have your engine overhauled.
4) Patch the engine of leaks, address any low compressions and accessory issues.
5) Wait too long.
6) Do nothing. (Really. That is an option.)
OPTION 1: Purchase a new engine.
When you purchase a new engine, every part is new and meets factory-new specifications with zero time, a new serial number and a factory-new warranty.
Aside from those points, there is really no technical benefit to the new engine. It will not necessarily provide you with any more power, smoothness, better performance or longer service life than any of the other options we will discuss.
OPTION 2: Purchase a rebuilt engine.
Rebuilt engines are different from overhauled engines, even though people often use the terms interchangeably. The rebuilt engine is assembled at the engine’s original manufacturer using various parts from the manufacturer’s used/reclaimed stock.
When rebuilding an engine, the manufacturer is not required to disclose the total hours (i.e., total times) on those “stock” items. Therefore, you could have a crankshaft or crankcase that has a lot of hours or several previous TBO intervals on it.
Nevertheless, rebuilt engine components must meet factory-new fit tolerances, but specific parts can be machined undersized to meet specifications. There is nothing inherently wrong with this practice, but it could result in an unusable crankshaft at the next overhaul.
Rebuilt engines are issued new serial numbers and granted zero-time status by the manufacturer.
OPTION 3: Have your engine overhauled.
Historically, an engine overhaul has been the most economical option for aircraft owners. During an overhaul, your original engine is sent to a third-party overhauler, where it typically receives new cylinder assemblies, hardware, gaskets, bearings and other piece parts.
The overhauled engine is not granted zero-time status. The engine’s total time is continued from the point of overhaul. For example, the engine may have 2,000 hours and zero time since major overhaul. The engine still has 2,000 hours total time.
An overhauled engine, if worked up by a reputable facility, can greatly enhance the aircraft’s value. Why? Simply because it keeps the engine/airframe serial numbers as matching pairs. That means a lot to some owners.
OPTION 4: Patch engine leaks; address low compressions and accessory issues.
Patching the engine should be only considered if the internals of the engine are determined to be in good condition. That means no metal in the filter, and oil consumption is within the limits set forth by the manufacturer.
Most aircraft engine failures that I have investigated or have performed analytical inspections after the fact, failed due to either a malfunction in the fuel delivery or ignition system. Therefore, always follow the manufacturer’s recommendations for continued airworthiness for critical items including the magnetos, alternator and the fuel delivery system. Like the engine as a whole, each of these components have specific service intervals that should be followed.
For example, ignition systems typically have 500-hour inspections, and some have requirements for overhaul after four years in service or five years from date of manufacture.
Remember that doing a preflight runup and magneto check at the end of the runway is not always an indicator of good ignition system health. Preventive maintenance can go a long way in providing enhanced reliability and safety.
OPTION 5: Wait too long.
One of the airplanes my father and I own had the original engine it was delivered with back in 1966. When we purchased the aircraft, it only had about 1,200 hours total time. Many of you might think, “Wow, it had lots of time left on the engine.” Which it did…
…but we were continually repairing oil leaks, reworking accessories and worrying about becoming stranded away from home base.
This continued for a number of years until one day I was performing an oil change. I invited my daughter to ride with me around the patch in order to warm the engine oil. We came back, landed and removed the cowling for draining the engine oil.
When the oil began to flow, I noticed what looked like pieces of metal flowing from the drain hose. That’s not good. I pulled the oil screen and we could almost read the part numbers for the metal coming from the engine.
I remember the look on my father’s face. It wasn’t a look of financial distress, but rather, relief. Now we had a valid excuse to overhaul the engine. The trouble here is we could have waited too long.
Not only was it a risk to our safety to push the envelope on this engine, but also by continuing to fly it, we could have done irreparable damage to the engine that would have been more costly to repair than a standard overhaul.
OPTION 6: Do nothing.
You read it right: the last option is to do nothing. By definition, TBO is just a “recommendation.” It’s not a law. Many engines go beyond TBO and perform very well.
But before going that route, you need to have a thorough evaluation of the engine’s current state. Questions that I ask of owners that question me about going beyond an engine’s TBO are:
1) What is the calendar time since the engine was new or last overhauled?
2) What is the oil consumption?
3) Do you have any persistent oil leaks?
4) Have you discovered any wear material in the oil during analysis?
5) What is the reliability of the accessories including the magnetos, alternator, carburetor or fuel injection system?
6) What price do you put on your peace of mind?
7) What price do you put on the safety of you and your passengers?
While there are plenty more questions I could ask, these seven hit the high points when it comes to an engine’s overall health—or lack thereof.
My engine has plenty of life left!
It may well, but you should also ask yourself this question: what is the calendar time since the engine was new or last overhauled?
Why does that matter? Remember, inactivity of an engine can be very damaging due to internal corrosion. That fact that an engine has low operational hours does not mean a thing to me.
For example, look at the ads that list aircraft for sale. I have never once seen an ad that actually stated the engine’s calendar time. You see many with 500 or 600 hours since major overhaul or since new all the time.
What buyers should be asking is, “What is the engine’s actual calendar time?” and “How long has the engine been sedentary?” Generally, you would like to see the aircraft flown at least a couple of times per month.
“Oil, that is...black gold. Texas tea...”
Another critical thing that helps determine the overhaul health of an engine is its oil consumption. Does it have any persistent oil leaks? Or have you discovered any wear material in the oil during analysis? Regular oil analysis can be a very helpful diagnostic tool.
With regard to oil consumption: are you constantly filling the oil every time you fill up with fuel? If so, your engine is likely a candidate for replacement or overhaul. Increased engine oil consumption could be the result of actual engine usage, leaks or a combination or both.
An old engine that is beginning to show these symptoms is probably getting close to needing overhaul, replacement or significant repair. The question to ask yourself is: how long can I continue to bale the engine together?
To TBO and beyond
I am not advocating that you rigidly adhere to your engine’s recommended TBO numbers, but the manufacturer is required to provide one as a point of reference.
Clearly, you can safely fly past TBO, provided you follow the manufacturer’s recommendations for determining continued airworthiness of the engine. Those owners and maintenance providers that follow the proper maintenance guidelines will beat others to the engine’s TBO numbers and beyond through overall lower operating costs and improved safety.
Basically, if your engine is not producing abnormal wear metal, has good compressions, does not consume oil at an alarming rate and is maintained in accordance with manufacturer’s instructions, more times than not, you can safely fly past TBO without any issues. And there are folks who do it day in and day out.
My point here is that you can expect long life out of your aircraft’s engine if you take care of it and do the maintenance and inspections when and how the manufacturer recommends.
Bill Ross is a graduate from the University of South Alabama and was employed by Continental Motors for 15 years holding positions in engineering, analytical, air safety and technical product support. Ross is now Vice President of Product Support for Superior Air Parts and committed to the company goal of making flying affordable. When not working at Superior, Ross can be often found flying his family’s 1941 Boeing Stearman, working on antique aircraft or exposing young people to the joys of flight and potential careers in aviation. Send questions or comments to .