Originally Posted by planemechanic

To schedule a revenue trip, and to sell tickets to fill the airplane, you need months of advanced notice. Maintenance ferry flights can move +/- several days to a week. In addition you need a properly trained and qualified crew available. As UA does not fly the 767 to HKG on any other flights we do not have crews who can legally make that flight. HKG is also an airport that requires special authority as a pilot to land there, and again, as we don't operate 767's to HKG as part of our 121 certificate we have no pilots who fly that airplane who can fly the odd 767 to HKG on a revenue trip. Our Engineering Test Pilots operate under Part 91, and are not under that specific requirement from HKG authorities.

Airlines are not a simple business to run, and this is just one example. Hope this helps.