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Calendar TBO Limits

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Nov 29, 2001
A mechanic recently told me that he'd heard that the FAA is now going to require piston aircraft operated under FAR Part 135, to have their engines overhauled at TBO (or) 5 Calendar Years, which ever comes first. This is ludicrous! Has anyone heard, or know of anything about this, or is it a hoax? Apparently there was some concern over rust and internal corrosion build up with operators who aren't flying much.
This isn't a new requirement, it's an existing one. It isn't five years, however. TBO isn't simply spelled out in terms of operating hours or cycles. Time Between Overhaul is also spelled out in years, and many powerplants have year limitations which must be respected under Part 135. A 12 year limit on many piston powerplants, for example, means that if the engine hasn't been flown in the past 12 years it's still due for an overhaul. Many propellers have similiar TBO limits on them, usually at shorter intervals of three or five years.

Think about it this way: piston engines need regular oil changes. Typically 25 or 50 hours are used as oil change intervals. However, oil should also be changed every 3-4 months, even if the airplane doesn't fly at all. Acids wash off case walls, acids sit in the oil around bearings and races and parts subject to corrosion, and the oil in your airplane and in your car needs to be changed regularly...even if the engine is never run. Both time limits (in hours, or for your car usually in miles or kilometers) and calendar limits apply to many aspects of maintenance.

Many inspections, airworthiness directives, and required maintenance is done on both a calendar and on a time or cycle basis, and this includes overhaul of piston engines. This is not a new requirement...it's in effect now.

This doesn't apply to aircraft engines operated under part 91...where one can fly beyond TBO and operate "on-condition."
ah they must have changed something then. Years ago we were required to maintain tbo on rentals.

For rentals, only hourly TBO limits ususally have to be adhered to. But for 135 operators, the calendar limits (such as the 12 year requirement in Lycoming SI 1009AT) are ususally addressed in a 135 operators Operations Specifications. My FSDO started enforcing this requirement back in the early 90's.

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