I believe the problem started on Wednesday, when AA728 PHL-LHR was cancelled at short notice. It operated as AA9605 at 1835 the following day, displacing that evening's scheduled flight, which was cancelled. It sounds like AA might still be down an airframe.

I was just about to board AA728 (pre-boarders were already on board) when the captain announced a "short delay" due to mechanical issues. The departure time was repeatedly put back by 20 minutes until the flight was eventually cancelled at about 2330. I was put up in a grim Holiday Inn about 5 miles from the airport (at least the hotel voucher and shuttle turned up promptly) and given a $12 food voucher, which didn't even cover breakfast.