Synopsis
On 4 November 2018, Turkmenistan Airlines Flight TUA562, a Boeing 752 (registration EZA014), operating from Amritsar to Ashgabat, and Indigo Airlines Flight IGO048, an Airbus A320 (registration VTIEP), operating from Dubai to Amritsar, experienced a Traffic Alert and Collision Avoidance System Resolution Advisory (TCAS-RA) event. The incident occurred in Pakistan airspace and was investigated by the Aircraft Accident Investigation Board (AAIB) of Pakistan under ICAO Annex 13. No injuries or aircraft damage were reported. The minimum vertical separation between the two aircraft was approximately 800 feet, with a lateral separation of 4.38 nautical miles at its closest point. The AAIB determined the incident resulted from a lapse by the IGO048 aircrew, with a possible contributing lapse by Lahore Air Traffic Control.
History of Flight
At 03:57:40 UTC, descent coordination for IGO048 was made between Lahore ATC and Amritsar ATC for flight level (FL) 110. At 04:08:06, IGO048 contacted Lahore ACC Radar East and was instructed to report ready for descent. TUA562 departed Amritsar, and at 04:16:13, coordination between Lahore and Amritsar ATC instructed TUA562 to maintain FL100 by position RABAN. At 04:20:45, IGO048 requested descent and was cleared to FL250. Traffic information about a Qatari B777 was provided. At 04:23:48, IGO048 was re-cleared to FL170. At 04:28:11, TUA562 was cleared to climb to FL140 and to cross RABAN at FL120 or above. At 04:31:05, IGO048 requested further descent and was told to stand by. At 04:31:16, IGO048 was cleared to FL150, but the clearance was not acknowledged. At 04:31:50, TUA562 reported approaching RABAN at FL120 and was cleared to FL140. At the same time, IGO048 was instructed to maintain FL150 with traffic information about the reciprocal traffic climbing to FL140. IGO048 acknowledged. Radar data showed IGO048 descending and selecting FL140 in its Mode S, while TUA562 climbed. At 04:33:23, IGO048 reported a Resolution Advisory. Lahore ATC instructed IGO048 to maintain FL150, but IGO048 was at FL148. TUA562 later reported descending to FL130 due to traffic. At 04:34:11, the aircraft crossed with TUA562 at FL135 and IGO048 at FL148. Post-incident, both aircraft continued to their destinations. IGO048 later stated they believed they were cleared to FL140, but ATC reiterated the clearance was FL150.
Aircraft and Crew Information
Turkmenistan Airlines TUA562: Boeing 752, registration EZA014, departing Amritsar for Ashgabat in IMC, maintaining FL140. Indigo Airlines IGO048: Airbus A320, registration VTIEP, departing Dubai for Amritsar in IMC, descending from FL250 to FL150. No significant weather was reported at the operating altitudes.
Injuries and Damage
No injuries were reported on either aircraft, and no damage occurred as a result of the incident.
Analysis and Findings
The investigation found that at the time of closest approach, the vertical separation was approximately 800 feet and lateral separation was 4.38 nautical miles. Both aircraft received TCAS Resolution Advisories. The AAIB concluded the incident was caused by a lapse on the part of the aircrew of IGO048, with a possible contributing lapse by Lahore ATC.
Safety Recommendations
The AAIB issued appropriate safety recommendations to both operators and the involved air navigation service provider.