I was booked on UA916 (a 747, the first of three daily IAD-FRA flights operated by UA), and it was showing a delay. As luck would have it UA916 was cancelled the night before as well, so there were plenty of passengers enduring a 24+ hour delay at Dulles. All of a sudden, at around 3PM, UA adds a “phantom flight,” a 747 which would operate as UA9792, meant mostly for the passengers that were stranded from the night before and didn’t get reacommodated. Oddly enough this flight wasn’t added the night before but only shortly before UA916 got delayed, and since it was showing an earlier departure time I got rebooked on it. At the time it was still totally empty, but as the delay for UA916 kept getting worse and worse more and more people were rebooked on this flight, to the point that this new phantom flight was totally packed while the original UA916 was almost totally empty, something they didn’t anticipate.

As a sidenote UA9792, the phantom flight, ended up being delayed by about six hours too due to a mechanical, and a part was being flown in from DEN. So in the end both flights were delayed by around six hours and the original flight was almost empty while the new phantom flight totally filled up.

My guess is that something similar happened at AA. They probably cancelled the flight due to a mechanical or something else, rebooked everyone, and then either realized that they had another plane or that the mechanical was easier to fix than they anticipated, so instead of just parking it at ORD they decided to fly it to LHR so the flight the next day ex-LHR wouldn’t have to be cancelled as well.

“However, this would have left a plane load of west-bound passengers stranded in London Heathrow who were due to fly back to the US on the same aircraft.”