As owners and operators of aircraft engines we are (or should be) quite familiar with recommended TBO values for the engine(s) we fly behind or between. In my case, my pair of IO470's have a factory TBO of 1500 hours. It should be noted that Part 91 operated aircraft are under no obligation to overhaul their engines at the manufacturer's stated TBO hours. The annual inspection by your A&P/IA will determine if your engine is airworthy for continued service.

To quote the Continental SIL98-9C:

"TBO periods were established on most CMI engines beginning in the 1960s. Since that time, CMI has made significant engineering improvements to virtually all major engine components. CMI has refined manufacturing processes and implemented computer numerical controlled (CNC) machining tools enabling CMI factory engines to meet higher standards than possible when CMI engines were originally granted FAA Type Certificates. These improvements have enabled CMI to increase TBO limits for many of our new and rebuilt engines."

Indeed, many component manufacturing process, diagnostic and analytical improvements have been made at the component levels in all areas from metallurgy to equipment manufacturing tolerances and as well the lubricant folks have not been sitting idle either. I seriously doubt that they are making the exact same product they made in the 1960Õs. Suffice it to say that we have a considerably different level of sophistication going into the components and lubricants we experience in our engines today.

There is another key quote from SIL98-9C that IÕd like to draw upon:

"An engine's published TBO DOES NOT mean that every engine will operate the number of hours or years listed without requiring component replacements and/or unscheduled maintenance events. Noncompliance with CMI instructions for continued airworthiness, operational and/or environmental factors may necessitate repair or replacement of the engine, engine components and accessories earlier than the published TBO."

I have supplied the bold emphasis on a particular part of the text which many of us know to be true due to the wide variability of factors affecting engine TBO. Just as there are outliers on the left side of the TBO bell curve, it stands to reason in my mind that there must be outliers on the right side of that bell curve.

A36 owner and ABS Member, Marc Charron, is sporting a FRMN TN IO520 from 1994 that has 1941 hours on the bottom end (2541 on the case). Here are Marc's engine experiences:

Marc reports using Aeroshell multi viscosity oil until about 500 hours of use when the starter adapter began slipping. He then moved to Phillips multi for a while and for his last 300 hours has been using Aeroshell 100W and oil and filter changes between 45-50 hour intervals. For quite a few years Marc operated in the deep cold climate of Canada and made extensive use of engine pre-heating below 40F for multi viscosity oil and at 50F and below for straight weight oil. Along the way there was a top overhaul at 800 hours on the case and some cylinder replacement and cylinder repairs to date. Marc reports operating this TN engine lean of peak throughout his ownership. Oh, and it's sporting a sweet but modestly priced paint job from Hawk Aircraft Painting in Tampa, FL.

Baron E55 owner & ABS Member, Stuart Spindel, has operated two IO550 engines on his aircraft lean of peak with current total time of 2625 hours. Over the years he has replaced both starter drives. After about 500 hours Stuart reports that one of the adapters began slipping. He switched from Aeroshell 15W50 to Phillips 20W50 and the slipping stopped. He reports no further issues for the past 1000 hours and began using Cam Guard a couple years ago.

Bonanza V35A owner and ABS Member, Bo Harper (gone West December 2022), currently has 2000 hours total time on his Ultimate Engines IO-520BA since 1995. Bo has owned and flown the airplane 700 hours since 2009 from his base in the Chicago area. The aircraft and engine has been operated on Aeroshell 100W itÕs entire life with engine sump pre-heat pad and an engine pre-oiler. The pre-oiler is run prior to every start to pressurize the engine with oil prior to start. Being located in the engine bay the pre-oiler adds a smidge of help with CG. Bo reports a couple of cylinder replacements have been done since the 1995 overhaul.

Bo's Bonanza was sold in June 2023 with the same engine that had about 2200 hours total time and was still airworthy and as of this date (June 5, 2024) is still flying based at KDTO. It should also be noted that this engine had only one cylinder replaced in all this time due to a crack. The other 5 originally configured cylinders by PAP were still producing excellent compressions. PAP is where my cylinders are going for overhaul!

Here is Norm H.'s over TBO story:

Bert Weinstein's 2500 Hour Pirep

I have a 1968 36 Bonanza (serial number E-44) and installed a factory new normally-aspirated IO-550B engine in it back in 2004. The engine was rated at 1700 hours TBO.

I currently have just shy of 2000 hours on the engine (actual current time is 1950.2). I just replaced both mags about 3 months ago because they were beginning to have issues. I’ve also replaced 2 cylinders during the life of the engine as needed. We inspect and replace hoses and seals at every annual as needed. I use AeroShell 15W-50 multi-grade oil, do oil changes every 50 hours, and do an engine oil analysis every oil change. I haven’t run LOP operations with my engine until recently when I began experimenting with it due to installation of a new JPI 930 engine monitoring system. I have normally run 50-degrees ROP which keeps the engine cool with Beryl D’Shannon baffling. GAMI’s are next on my list of engine improvements.

The most major repair was a top overhaul at 1300 hours due to a landing-gear extension failure where the gear locked approximately 10% into their extension while preparing to land. Additionally, the manual crank also locked up making any gear extension impossible. So, after about 2.5 hours circling the airport trying to break the gear free, I ultimately had to perform an engine-off landing. The prop stopped in the worst possible position and drug the ground after landing which required a new prop and an insurance-required engine inspection and top overhaul. My incident caused a Service Bulletin to be issued because it was the first of its kind (where a washer had dislodged in the manual crank case and lodged in the gear mechanism causing complete lock-up of the gear-box).

Cam Gully

Eastland, TX

ABS Member #52952

Having two ongoing experiments in TBO with my IO470-L engines on my B55 that I have personally run and maintained for just over 1600 hours and 10 years now, I can report a right engine FRMN from 1999 that now has nearly 2000 hours on it and a left engine from 2004 that has about 1600 hours SMOH on it. My engines histories include a promotional install of Superior Millennium standard cast cylinders in 2007 at 800 and in 2006 at 400 hours, respectively. Aeroshell 100W oil (sans additives) and spin on filters with 50-hour oil & filter changes.

My cold weather operations in Wisconsin (and to a lesser degree Dallas, Texas) are always accompanied by generous Tannis full engine and cylinder preheats. The onset of pitting lifters was nipped in the bud on the right engine during a top overhaul when we examined the engine internals in 2007. Last year's 2013 annual included new left engine hoses and an overhauled fuel pump. This year's 2014 annual plan calls for new right engine hoses and an overhaul of the fuel system. See fuel system components overhauled HERE.

In my opinion, going long service hours on the bottom end makes it more important to pay attention to the external items that can cause an engine to go quiet at a very inopportune time. I consider engine hoses and the fuel pump/system to be tops in this critical category. See the fuel system overhaul page HERE. Additionally, you folks with Barons and un-feathering accumulators, give a special watch to that decades old hard line and hose that runs from the prop governor to the accumulator ball.

They are single points of failure and as such, command a great deal of my attention as I climb past TBO and toward the 2500 Hour Club. Accessories like the starter, alternator, magnetos, vacuum pump and cylinders can be overhauled/changed on condition or inspected at specified hours with much less economic fanfare than a case splitting overhaul event.

Key factors that seem to put the odds in our favor for greatly exceeding TBO on our engines are:

1. Regular and frequent flight operations (infrequent ops consider the use of a corrosion preventing oil additive)
2. Quality lubricating oils and filters changed at or below recommended intervals
3. Cold WX ops utilizing engine preheating
4. Lifter inspection diligence
5. Proper cylinder replacement and inspection procedures
6. Proper front-mount alternator inspections and replacement procedures
7. Careful monitoring and replacement of high time engine hoses (time or age limits may apply) and fuel pumps

With the cost of engine overhauls continuing to increase, getting the most out of our engines safely can afford us a much more pleasing financial experience with which to enjoy flying. If you are over TBO and approaching the 2500 Hour Club, please contact me with some of your details so that your experiences can be added to this page.

Happy Skies!