The sequence of events leading to the mid-air collision

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The Boeing 747, one of eight Series 168Bs (ie, the -100B) operated by Saudi Arabian, departed New Delhi's Indira Gandhi International Airport at around 18.33 local, and headed on a westerly course (270¹) from the Delhi VOR navigation beacon (DPN).

The 747, which seems to have been operating a "PARVI" westerly standard instrument departure (SID), was cleared to climb to flight level 140 (14,000ft) although it had requested clearance to continue its climb through that altitude.

The PARVI SID involves a westerly (270¹) track through two reporting points - PARVI and LUNKA - which are 131km (71nm) and 326km (176nm) respectively from the DPN VOR (see illustration).

Meanwhile, the Ilyushin Il-76, which was travelling inbound from the LUNKA way point on the reciprocal course (ie, 090¹), was descending through FL230 (23,000ft) to FL180 (18,000ft) and reported that it was "74 miles [137km] from the DPN VOR" (ie roughly at the PARVI waypoint).

The Kazakh aircraft was then instructed to descend to, and to maintain, FL150 (15,000ft), with the intention to vector the aircraft to the south of the airport for a landing from the east.

The Saudi Arabian 747 pilot reported approaching FL140, and requested further climb clearance, but was instructed: "Maintain FL140, standby for higher". The Kazakh pilot informed Delhi ATC that his aircraft had levelled at FL150, that it was "46 miles" (85km) from the Delhi DPN VOR beacon, and that it was on the 270 radial (ie tracking 090¹).

ATC instructed the Ilyushin crew: "Maintain 150 [15,000ft]. Identified traffic 12 o'clock reciprocal. Saudi Boeing 747, report when in sight." The Kazakh crew then asked ATC: "repeat, how many miles?" to which ATC replied: "14 miles now." Moments later ATC reported "...traffic is 13 miles now, level 140 [14,000ft]."

At this stage, both aircraft, although travelling directly towards each other, were thought to be separated by 1,000ft vertically.

At 18.40, around 1 min after the last instruction to the Kazakh crew, the two aircraft collided near the town of Charkhi Dadri.The accident sites were around 74km from the airport, and the wreckage suggested the Il-76 had collided with the top of the 747.