An Airbus with 136 passengers on board and an inbound cargo jet that had dropped to the same altitude were so close to each other two weeks ago that the two blips representing the aircraft on the radar actually merged.

Aviation officials who did a “radar replay” on Tuesday wondered how the pilots of the planes avoided a repeat of the 1996 mid-air collision off Delhi that killed 349 people.

The Air India Airbus A-320 was flying to Agartala and the Boeing 737 cargo carrier, owned by Alliance Air, was coming from there to Calcutta. The planes came within sniffing distance of each other 60 nautical miles from the city airport, after the pilot of the cargo jet had descended 1,000 feet more than what Air Traffic Control (ATC) recommended.

The cargo jet was at 24,000 feet when ATC personnel asked its pilot, said to be a veteran, to descend to 18,000 feet and await further communication. “He would have received permission to land after the ATC gave instructions to the pilot of the aircraft headed for Agartala,” the official said.

The directorate-general of civil aviation has started a probe to ascertain what made the pilot descend more than he had been instructed to.

The other aircraft had maintained the specified altitude of 17,000 feet.

“It is unbelievable that two planes came so close to one another and did not collide,” an ATC official said.

Blips representing planes merge on the radar when the horizontal distance between them is 10 km or less. A distance of 10 km might seem reasonable, but that could have been covered in a minute by jets travelling at approximately 600 kmph.

“The inquiry will show whether it was the fault of the cargo jet pilot or a case of miscommunication between the two aircraft and the ATC,” the official said.

Aviation experts suspect the two planes were a fraction of a second away from colliding when the pilots changed course.

The aircraft were between Comilla, in Bangladesh, and Calcutta when the incident occurred. The Air India aircraft had taken off from Calcutta at 11.10am and the cargo plane from Agartala at 10.30am. Flights between Calcutta and Agartala fly over Bangladesh.

The ATC official said the planes would certainly have collided had the Traffic Collision Avoidance System that is installed in all modern aircraft malfunctioned.

“The systems warned the pilots of both aircraft in the nick of time about how dangerously close to disaster they were. Whenever two flights are on collision course, a computer-generated advice tells the pilots to either descend, ascend or change direction. In this case, the A-320 ascended and the cargo flight descended, avoiding the collision.”

Air India officials were unavailable for comment.

The only mid-air collision in the country — near Delhi on November 12, 1996 — is also the worst in the history of civil aviation. That incident involved a Saudi Arabia Boeing 747-168B and an Air Kazakhstan flight to Delhi. The collision occurred about 60 miles west of the capital. All 23 crew members and 289 passengers on the 747 and the 10 crew members and 27 passengers on the Air Kazakhstan plane died.