Max kingsley-Jones/LONDON David Learmount/DUBAI

The mid-air collision near New Delhi, India on 12 November between a Saudi Arabian Airlines Boeing 747-100B and an Ilyushin Il-76 of Air Kazakhstan has raised concerns over the safety of Delhi's air-traffic-control (ATC) routeing system. It has also heightened concerns over language problems and the operation of CIS aircraft in international airspace, particularly the potential for altitude confusion, because CIS aircraft show altitude in metres, while the rest of the world calibrates its altimeters in feet .

The accident occurred when the inbound Il-76 (UN-76435), operating a freight charter into Delhi from Chimkent, Kazakhstan, collided with the 747 (HZ-AIH), which was heading outbound from Delhi on the same route, but in the opposite direction. The 747 was flying a scheduled Saudi Arabian flight (SV763) to Dhahran and Jeddah in Saudi Arabia, with 312 passengers and crew on board. Thirty-eight people were on board the Il-76. All occupants of both aircraft were killed.

Initially, there were suggestions that the collision may have occurred after an instruction had been misunderstood. After listening to ATC recordings, however, the Indian authorities announced: "It is quite clear that both crews had understood and confirmed their instructions."

The Il-76, built in 1992, may have been equipped only with a metric altimeter. Since at least one of the two aircraft had transgressed its cleared flight level, there have been suggestions that a metric/ imperial altitude conversion error could have been made by the Il-76 crew.

At the moment, New Delhi's ATC centre is equipped only with primary airport-surveillance radar (ASR), which provides position information but not altitude. Details of each aircraft's exact altitude at impact will not be known until their flight-data recorders have been examined. Neither aircraft was equipped with a traffic-alert and collision-avoidance system.

ATC authorities accept that ideal practice in terminal areas is to separate arrival and departure tracks laterally, and most of them undertake that policy. The Indian Air Traffic Controllers' Guild, in the aftermath of the collision, alleges that it has been pressing for more than three years to separate Delhi's western departures and arrivals. It says that the Indian air force had refused airspace allowing a widening of the corridor. The air force denies this.

Raytheon has been working on a $120 million contract to install secondary-surveillance radar (SSR) which was due to have been completed by mid-1996. The latest estimate for entry into operation is early 1997. SSR provides flight identification, altitude and speed information.

The last time a mid-air collision between commercial airliners occurred was on 11 August 1979, involving two Aeroflot Tupolev Tu-134s near Dneprodzerzinsk, Ukraine, killing 165 people.


Source: Flight International