India, Oct. 6 -- A British Airways flight bound for Chicago from London Heathrow declared an aviation emergency over Ireland on Monday, activating the internationally recognised Squawk 7700 distress code and dropping nearly 28,000 feet before abandoning its transatlantic crossing and returning safely to London, according to flight-tracking data published in real time.

Flight BA295, operated by a 30-year-old Boeing 777-200ER registered as G-VIIA, took off from London Heathrow Airport's Terminal 5 at 11:50 AM local time for its scheduled 8-hour and 40-minute service to Chicago O'Hare International Airport.

The aircraft was well past the British coast and over the Irish landmass when aviation observers began flagging the sudden, steep chan...