Air traffic control (ATC) uses an aircrafts transponder to identify it and jets with defective transponders are not allowed to enter the European airspace, prompting the Air India flight to return.
The incident, airport sources said, was reported at 2.15 am when the flight AI 127 was scheduled for take off to Chicago. Sources said that there were 313 passengers and 16 crew members on board.
The flight took off at 2.15 am but when it entered Afghanistan airspace, it developed a snag in its transponder whereby the signal transmitted by the ATC to the aircraft was not visible in the aircraft. The transponder assists ATC in identifying aircraft within its radar and can give collision warning to the pilot if it gets dangerously close to another aircraft in air. Though pilots are in constant touch with the ATC, the transponder helps give distress signals in case of an emergency, a senior airport official said.
The Air India aircraft had reached Afghanistan but since it had to go through Europe, which does not permit entry of such an aircraft in its airspace, it returned to IGIA at 5 am.
The Air India aircraft landed safely at IGIA. Technically, the aircraft was fit to fly and there was no danger. Since there was no emergency, we asked the pilot to come back and not land anywhere else as passengers could be sent again from Delhi easily, the official said.
Air India arranged for another flight, which took off from Delhi at 4 pm on Monday. The flight was scheduled to take off at 2 pm but since the duty hours of the crew had ended by that time, passengers were asked to board another aircraft.
What is a transponder
An aircraft that is not equipped with a transponder will still show up on the Air Traffic Controller (ATC)s radar screen, but he/she wont be able to know which aircraft it is, if at all it is an aircraft or some other object, or the altitude at which it is flying. An aircraft equipped with a transponder is able to give an Air Traffic Controller additional information, such as the planes identity, altitude, type of aircraft and destination, etc.