Q. Do the airlines fly "repositioning flights" (for example, there is a 777 on the ground at Los Angeles, but that airplane is needed at New York JFK) that are not listed on their web sites, but that do have seats available for sale through their call centers only?
A. According to airline expert Ramsey Qubein, when an aircraft is repositioned, it is either flown as a ferry flight (no passengers) or publicly sold as an extra section. The former happens all the time: a plane taking a sports team somewhere and not returning with them; a plane going for maintenance at another station, where it is flown empty to and from the airport (Tulsa and Greensboro, for example, see all kinds of large international planes coming and going, but they’re empty because they were flown in for maintenance). The latter happens only during an IROP (irregular operation). For example, if a plane “goes mechanical” and they have to send another plane to pick people up in the event they couldn’t be accommodated on other flights, the new plane would fly over empty.
Also, airlines add extra sections (mostly between hub cities) when planes are being repositioned and they will sell seats on those. However, these flights are publicly listed in the reservation systems and on airline web sites. They usually can be spotted because they have unusual flight numbers (often four digit numbers beginning with 8 or 9) and only operate once, but most people would have no idea they’re repositioning flights.