Mumbai, Jul 13 (PTI) : A Jet Airways flight from Chennai came dangerously close to an Air India aircraft over Mumbai after it was asked to descend to 11,000 feet by the ATC but a last minute alert averted a mid-air collision.

The Mumbai-bound Jet Airways Konnect flight 9W 2119, with 142 passengers on-board, was instructed by the Air Traffic Control to descend to 11,000 feet on Saturday.

When the flight was on descent, the Traffic Collision Avoidance System (TCAS), a hi-tech system designed to reduce the incidence of mid-air collisions between flights, alerted the aircraft about Air India flight IC 174 being in its way, a source said.

The Air India flight had 70 passengers on-board and was also on way to Mumbai from Chennai.TCAS monitors the airspace around an aircraft for other aircraft equipped with a corresponding active transponder, independent of air-traffic control, and warns pilots of the presence of other transponder-equipped aircraft which may present a threat of mid-air collision (MAC).

As a rule, two aircraft are required to maintain a vertical separation of 1,000 feet and and lateral separation of five-miles.

If any aircraft breaches these limits, TCAS sends out an alert message.
When contacted, an Air India spokesperson said the report filed by the pilot of its flight suggested that an aircraft was noted to be on its way."Our commander of flight IC 174 on July 10 has made an observation in his report regarding another aircraft while approaching Mumbai," the spokesperson said.

Jet Airways also said its pilot had received an alert message, advising of another aircraft in the vicinity."The flight 9W 2119 (Chennai-Mumbai) had a Resolution Advisory (RA), an alert message advising of another aircraft in the vicinity. This incident took place while holding over Mumbai under ATC instructions," Jet Airways said.The Directorate General of Civil Aviation (DGCA) is investigating the matter.