If you spend any time reading our website, you'll know that we are strongly in support of operating your aircraft engine beyond TBO. Our Overhaul Savings program gives you the ability, through utilization of the "Fly-On" program, to fly up to 150% of your hourly TBO. Having discussions with both potential and current customers, however, we've noticed some confusion on exactly what we mean about operating beyond TBO and the implications of doing it.
Just like saving for your next overhaul, operating your current engine beyond TBO is not a black-and-white decision whose answer manifests itself in clear daylight after only a few moments of thought. A lot of factors come into play concerning the operation of your engine, and what the implications of your actions can be.
First off, what is TBO? We've covered this on our website in an in-depth manner. But for the sake of simplicity in regards to the content of this article, it's important to remember that there is both an hourly and calendar TBO criteria specified by the manufacturer. These criteria do not necessarily form a limitation (as discussed here), however.
If you operate your engine under Part 91, these numbers only act in an "advisory" role in your decision making process. In our context, when you have a freshly overhauled engine, the manufacturer reported TBO should act as a budgetary tool to allow you to effectively plan for your next overhaul. Your engine has no guarantee that it will make it all the way to TBO, but it provides a foundation upon which you can start making assumptions to make your savings plan a framework built upon real data instead of just guesswork.
As your engine ages, your trend monitoring information, oil analysis samples, and other information will start to dominate what you are realistically going to be able to expect from your engine. This relationship, however, is not linear. The manufacturer TBO is the dominating factor in determine your savings plan for the majority of the engine's lifespan. As you near the final few hours to TBO, and beyond, the real-time data becomes more important. The figure below reflects this weighting of manufacturer TBO numbers vs. operational data.
Although the graph appears to shows the manufacturer TBO figure trying reach a weight of 0 at some point beyond TBO, it is important to note that this figure does remains a small positive amount. We'll touch on this later in the article.
We've written about, and give each new customer to one of Gemco's programs a large amount of information about creating an effective engine trend monitoring system, as well as how to integrate oil analysis as a piece of the puzzle.
For each hour you fly, your engine slowly wears itself away. Piston rings slide along cylinder walls, valves slam into valve seats, and controlled explosions send the exhaust gas temperature to over 1300 degrees. Engine oils, grease, metallurgical engineering, and cooling airflow act as the most important tools to slow the wear-and-tear.
When seen through oil analysis reports, the failure of a component is usually accompanied by an exponential rise in the amount of wear metal seen. For example, an exponential rise in aluminum would signal the failure of a piston/piston ring.
A good way to look at the manufacturer TBO would be to take a large sample size of pistons, and take progressive oil analysis samples up to failure. A large sample size would represent the majority of engines produced. Then, outliers in the data are dealt with. An example of the results are shown below.
As the engine wears, metal contamination will manifest itself in a near linear relationship with time. At some point on this line, the manufacturer will position TBO well before the failure point for the component. This safety margin is variable for components and the FAA even mandates certain safety margins for some parts used in aircraft. However, there is a failure point where the level of wear metals present in the engine starts to exponentially rise. This rise is indicative of an imminent component failure, or a reflection of an already destroyed part.
In our aluminum/piston example, a steep rise in contaminant metal would now indicate a good time to reference your engine's compressions to help confirm the issue. Because we have been watching our engine, instead of being surprised by a low compression at the next inspection, we have an idea of what is happening with the engine and what to do about it.
As can be seen, however, just because you've reached what the manufacturer has deemed the TBO of the engine is doesn't mean your engine stops wearing away. By continuing to operate beyond TBO, you eat into that margin before component failure. This is what many experts claim is the "false economy" of operating beyond TBO. Even though you are getting "extra" hours out of your engine by putting off your expensive overhaul, you're wearing away material that could otherwise allow a certain component to "pass" its next overhaul inspection.
We here at Gemco can't argue with the validity of this argument. You are wearing away metal every hour your fly. No question about that. We don't necessarily agree with "false economy," however.
Many parts need to be replaced at every overhaul, regardless of their actual wear. Many other parts, such as camshafts, rarely escape the inspection phase without having to be repaired. Use what you know about how your engine did during its last overhaul to get an idea how it will make it through the next one. If your case barely scraped by last time, it probably is not going to make it through another overhaul, no matter when you do it.
What we want owners to do, instead of bending to flat statements that are broad generalizations, is to use the information they have to make informed decisions. It is possible to operate beyond TBO to a certain point without incurring any real additional cost by closely monitoring their engine's wear metals and how it is running. Through monitoring compression, EGTs, CHTs, oil consumption, and accessory performance, an operator can get a clear picture of an engine's health, and on what point on the component failure curve the aircraft is at.
Furthermore, we encourage pilots to always keep the listed TBO for their engine in mind when making important decisions about their aircraft. If you have an engine with a 2000 hour TBO that needs a full top end at 1800 hours, it may or may not make sense to complete a full engine overhaul based on other information. If there is no reason to believe that the bottom end is going to fail in the next 400 to 500 hours, it may be worth doing only a top-end overhaul. But if you are starting to notice wear-and-tear, it is in your best interest to "do it while it's apart." Do not, however, just go for a full overhaul just because someone says that "you minus well."
As always, we here at Gemco are available for your questions. Feel free to contact us at any time, and we'd be happy to help you.