Aircraft engines must be maintained according to a manufacturer's published TBO (pronounced Tee-Bee-Owe) or Time Before Overhaul. An airplane engine is typically considered reliable for about the same duration as a car engine, perhaps two thousand hours of use, but when a car's engine wears out, most owners discard the car and engine.1 With an airplane, you overhaul the engine: returning it to factory specifications, replacing whatever parts are necessary to achieve that. It's expensive and time consuming, interrupting normal use for the airplane and , like any maintenance procedure, sometimes introducing new problems. Engine life is a bell curve, of course, so a few engines break down and require overhaul before their TBO and many are in fine condition after reaching TBO. For this reason, regulatory provisions are available to allow an operator to run an engine beyond the TBO, subject to various conditions.
I was amused by this line at the beginning of the Transport Canada description of the process: "The program was not being used as envisioned, for an extension to a useful life, but has been extended, beyond its intended purpose, to an overhaul avoidance program." Well duh! What would the purpose of extending an engine's useful life be if not avoiding overhauling it? It's not a humanitarian gesture to allow old engines to feel useful, or an environmental statement. Every decision made in an aviation environment is balancing safety against economics, and if you're cynical even the safety decisions are based on economic criteria: accidents are expensive.
As I read it, the on-condition program is a bet the operator makes that his pilots, AMEs and operating conditions are better than the manufacturer's assumptions and that therefore his engines don't have to be overhauled as soon as the manufacturer says. You have to make that bet for your whole fleet, and stick to it. There's nothing that specifically says you can't hedge your bets and decide to go on condition a few hundred hours from the end of the engine life, but once one aircraft is officially on condition, the others of the same type in the fleet must be too, regardless of the relative time on their engines. Once the "approved maintenance schedule" specifies that the fleet operates on condition, then all the relevant extra checks must be done.
The price of the bet the operator makes is:
a) the specified inspections, which I assume are slightly more thorough than the regular inspections of an engine, and
b) the risk that the engine fails an on condition inspection before TBO.
I think b) is not a great worry, because if an engine component were in a condition that it would fail such a check, then you'd be glad to have caught it in a routine inspection rather than through a catastrophic failure causing further damage and downtime.
The payoff of the bet is that you may be able to extend the engine life past the usual TBO. Experience with our fleet suggest that it's not necessarily a safe bet, because the engines seem to need major attention pretty close to TBO anyway. If the on condition inspections cost any appreciable time or money over the regular ones, it might not be a good bet for the long term. But it always seems like a good idea just as an overhaul is coming due on both engines of a reliable airplane that is needed online.
This afternoon I went out for a run with the new AME. He's fast. His pace quickly sends my heart rate out of my training zone, but just switch off my heart rate monitor and keep up. A fast run now and again is probably good for me. After that it's Pasta Tuesday at Boston Pizza, which we admit sadly has become the highlight of our week.
1. There are people who do overhaul automotive engines, and keep automobiles running rather than discarding them. This is much more common in the US than Canada, and evident as soon as you cross the border, because the mix of automobiles on the road immediately skews to older and more American makes. I'm guessing it's because the American cars manufactured twenty or thirty years ago were a lot more amenable to user-maintenance than the European and Japanese models that were already more common in Canada. American movies always show guys fixing up old cars, sometimes as a rite of passage. I think it's part of the culture.