og, there are two factors that affect the maintenance schedule for aircraft, hours operated and cycles. The different types of maintenance checks have to be performed after a certain number of hours operation or a certain number of cycles, whichever comes first.

For this reason, an aircraft that has operated a lot of very long haul services (like the -ER's flying MEL-LAX-MEL) will run out of hours long before hitting the cycle limit. A prudent operating plan will have such an aircraft moved onto a few short-haul operations in order to extend the time between major maintenance by using up some cycles before it hits the hours limit.

This also affect the life of the aircraft to some extent, as the cost of a D-check can sometimes be more than the value of the aircraft. This is one reason why QF moved the 767-238ER's onto the domestic runs. They spent their early years flying lots of trips into Asia and the Pacific, clocking up lots of hours but not many cycles. If they maintained that schedule of operations, they would have run out of hours (for a D-check) well before the cycle limit. By putting them onto the monorail for the last part of their life, the cycles came back into line with the hours. Now that they are approaching D-check time, its off to Mojave or other deserts for storage or parting.

It is common to see the ugly sisters on the Trans Pacific routes as well. The GE CF6-80 engines are ever so slightly more efficient than the RB211's. Also note that the early model 747-338 (reg OHA-OJJ or so) were less effieicnet than the newer ones with the Trent upgrade to the final (hot) stage (known as the RB211-524G/T). However, I believe they were all supposed to be upgraded to the G/T version - at RR's expense since the original engines never met the performance specification quoted by RR.