During the night, a trip to London Heathrow Airport from JFK had to return because of a transponder issue two hours into the flight. Apparently, the main control head of the two transponders compelled both units to enter the test mode, which rendered the entire system inoperative. It left ATC without a discrete radar target to track. In this post-9/11 world, that makes controllers nervous. So before the airplane was allowed home, the flight was instructed to complete a series of turns to confirm it was under the crew’s control.