NEW DELHI: A Chicago-bound Boeing 777 of Air India on Monday returned to Delhi six hours after taking off from IGI airport as the aircraft suffered a transponder failure just when it was about to cross Afghanistan. The flight, AI 127 with 313 passengers and 16 crew members, had flown for about three hours out of India when the transponder failed.The pilot then had to return to Delhi as an aircraft cannot enter Europe without a working transponder. The aircraft was a Boeing 777-300 ER (extended range)."The plane returned to Delhi safely. Passengers will be sent to Chicago after a while as the crew duty time limitation kicked in. We are trying to make alternate arrangements at the earliest," said an AI official.Transponder plays several crucial roles: it gives a collision warning to the pilot if another aircraft gets too close for comfort; pilots can use it to send distress signal discreetly to ATC and finally, it gives all details of the aircraft on the blips that appear on ATC radars. The emergency situations that transponders are used to warn ground controllers about are communication failure, hijack and any other emergency.Transponder is the primary means for ground radar to identify an aircraft so that radar controller knows the position, altitude and speed of an aircraft. Apart from safety issues, failure of transponder means that in a busy airport an unidentified blip causes confusion and ATCs have to contact the aircraft.In the past few months, Boeing's Dreamliners have been suffering transponder failures and AI has taken this issue up with the US aircraft major.Before Monday's transponder failure, an AI Dreamliner too had suffered the same problem over Afghanistan about a fortnight back.