How can TBO increase by 33%

Cessna 182s have been burdened by a 1,500 hour TBO for many years. However, the O-470-U engine is now available with a 2,000 hour TBO. What possibly could they have done to the engine to extend the TBO by 33%?

MARC COAN

Taos, N.M.

 

While my background is primarily in Lycoming engines, I think I may be able to shed some light on your question. As usual, this will be from my viewpoint and should not be taken as an official explanation, but I think it will provide you with the general concepts engine manufacturers use to establish their recommended TBO times.

I detect a serious question in your mind about how our friends at Continental could increase the TBO on the O-470-U from 1,500 to 2,000 hours, or a 33% increase. I can’t speak for Continental, but I can provide a parallel on several models of Lycoming engines, which, I think, should show you how these increases are arrived at.

My first example is the Lycoming IO-360-A and C series, which are rated at 200 horsepower. When these engines were first introduced into the marketplace, they had a recommended TBO of just 1,200 hours. After several years in service and by closely observing the engines that were returned to the factory as exchange cores, it was becoming obvious that the weak link was the retention of the main bearings. They originally were designed using a stepped dowel to retain the main bearings and it was noted that, for some reason, some of these dowels were found broken, which allowed the bearing shell to begin to rotate. Obviously this isn’t good and could lead to complete engine failure. The engineering staff noted this situation and promptly redesigned the main bearing dowel retention method. The redesign incorporated a heavier straight main bearing dowel, which increased the integrity of the retention of the bearing shell considerably.

Shortly thereafter, the FAA issued an AD Note calling for the installation to the new configuration. It was at this time that Lycoming recognized the fact that the serviceability and reliability of this design improvement was something that would permit the company to increase the recommended TBO from 1,200 hours to 1,400 hours.

Another benefit from closely observing the engines returned to the factory on exchange is that the engineering staff has the opportunity to observe all aspects of the engine condition when it has reached TBO time. As a result of this, again on the IO-360 series, they noticed some clues that there could be some improvement in the camshaft. This led to a redesigned camshaft, which allowed another TBO increase. This time the TBO was increased from 1,400 hours up to 2,000 hours, where it remains today.

This is an example of taking the same model engine and incorporating improvements, allowing an increase in TBO, providing compliance with the Service Bulletins was accomplished.

There are some other Lycoming engine models that also have had their TBOs increased because of redesigned components, such as crankcases, connecting rods and heavier pistons. Just plain product improvements over time also lend themselves to increased TBO times.

I think it’s safe to say TBO times are for the most part established using a conservative number of hours. With more experience gained as engines are returned to the factory and through service records, TBO times may be increased as the overall data are reviewed by the engine manufacturer.

I’d be remiss if I didn’t mention the fact that the recommended TBO by the manufacturer is really just a number and is strictly determined by the way the engine is maintained and operated. Routine maintenance, such as the recommended intervals for oil and filter changes, are vital to move you towards the TBO time. TBO times are impacted by many variables, most under the control of the owner/operator and his choices of maintenance and operation.

One last thing Marc: If you are really curious how Continental increased the TBO on the O-470-U, I’d contact the company directly for specifics.

 

Paul McBride, recognized worldwide as an expert on engines, retired after almost 40 years with Lycoming. Send your questions to: AskPaul@GeneralAviationNews.com.

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