TBO considerations

From Bondline

All discussions about TBO should start by referring to Lycoming's SI 1009AR, the factory definitions and table of recommended TBO hours for each engine model.

As Lycoming is quick to point out, there is no guarantee that a particular engine will make it to TBO. These are just recommendations that they have made based on the extensive experience with the engines that come back for OH. Part 91 operators are not required to adhere to the TBO, unlike Part 121 Scheduled Airlines and Part 135 Air Taxi operators, that are required to pull engines from service at TBO unless special permission is granted by their FSDO.

This is good news for the private owners. We can take good care of our engines and try to extend the life past an arbitrary "average" that Lycoming has published. This doesn't come without some possible problems. The first one is that most of us don't fly enough to reach the TBO in hours in service before the Lycoming recommended TBO in calendar years.

The means to determine the condition of the engine, and whether it is time for an OH are usually based on maintenance checks like Compression Tests, or upon pilot experience with items like oil consumption or making rated static RPM during a run-up.

Lycoming says that the engine has a TBO of 2000 hours on the models we use on our Grummans, except the O-235 in the AA-1x, if it has been modified with a different part number piston, and if only genuine Lycoming parts are used. They also state that there is a CALENDAR limit of 12 years since new or the most recent Major OH.

Another important test of the engine's readiness to fly past the TBO is the Valve Wobble Test as explained in SB388C. We know what causes stuck valves, and we know what causes valve breakage, so there is no point in ignoring the test that will tell you if either condition is occurring in the engine.