<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by Vulcan:
Gaucho:
Airlines swap engines in and out constantly. When one needs to be pulled for a check, they don't usually have the luxury of picking one to replace it with similar time on it as the one they leave on the plane. While it might be unusual to have such a disparity, from what I have read its not uncommon.</font>
I would almost think of it as a safety advantage. It seems better to have one engine with 9,500 hours and a second with 1,000 hours, rather than both of them with 9,000+.
I'm sure they have pretty big safety tolerances built into the flying hours between overhauls, but nonetheless, its got to be a lot less likely they would both have problems on the same flight if they have different service lives.