Statistically, Is engine calendar TBO honored more in the breach or the observance?

Question

Surprisingly to me, Lycoming and Continental engines have a recommended overhaul interval (TBO) of 12 years, while Rotax engines have a 15-year TBO. This suggests that engines should undergo frequent teardowns based on the calendar timeline.

In 30 years of flying, I have never encountered engine overhauls unless prompted by accumulated hours or significant events like prop strikes. However, my perspective is based on anecdotal evidence from recreational flying, and it may not reflect the practices followed in commercial aviation.

Is there any available data regarding the number of engines that operate beyond the recommended calendar TBO versus those that are still within it?

Answer 1

Calendar TBO is mostly about aging of seals and the potential for internal corrosion of steel parts on low utilization engines. It's just a guess, intended to mitigate global risk. It's not something you would observe as literal, unless forced to by regulation.

The seal that really matters in Continentals and Lycomings, the only one that can take you down if it fails, is the propeller crank seal. I might change out a prop seal on an engine that's really old.

A carb and mag overhaul might be appropriate as well. You are supposed to get mags inspected at 500 hrs (IRAN inspection) which takes care of that. Case and cylinders? On condition.

Oil analysis is good for monitoring for detecting things like bearing breakdown early.

Corrosion is mainly a problem for cam lobes and followers, and steel cylinder barrels. If the cam is going bad, there will be metal in the oil filter. Engines will run with bad cams until they misfire. I have a Lycoming cam in my garage that came out of an O-320 and one lobe is half way worn off. It only got that bad because the engine was using a screen instead of a spin-on filter that would have picked up the fine metal splinters that are tell tale signs of cam lobes wearing down. It didn't start to show symptoms while running until that point.

Steel cylinder barrels get rust on the peaks of the honed surface irregularities of the walls from lack of use, which gets rubbed off by the piston rings. The effect is premature wear-down of the honed surface. All that means is oil consumption goes up.

There is very little that justifies overhauling the core engine if the oil consumption is good, the filter has no major metal, oil analysis shows no major problems like high levels of bearing metal, cylinders are being inspected internally, compression is acceptable, and you are replacing age sensitive items like rubber seals, hoses etc, in the field.