What happened
On January 25, 2020, at 0721 UTC, a serious airprox incident occurred within the Delhi airspace involving two Airbus A320 aircraft. The first aircraft, UK32022 operated by Uzbekistan Airways, was descending toward Runway 29. The second aircraft, VT-IHQ operated by IndiGo, was also in its descent phase toward Runway 27.
While the Uzbekistan Airways flight was being managed by Approach (Final) Control, the crew was instructed to descend through various flight levels, eventually receiving a clearance to descend to 2600 feet. However, instead of following the descent profile, the aircraft began an unauthorized climb. This deviation caused the lateral separation between the two aircraft to shrink to 1.7 NM, while vertical separation was completely lost. The conflict triggered a Traffic Collision Avoidance System Resolution Advisory (TCAS-RA) for both crews, necessitating immediate evasive maneuvers.
The investigation
The investigation by AAIB India examined flight data recorder (DFDR) analysis, air traffic control (ATC) recordings, and crew statements. Investigators reviewed the actions of both the Approach (Area) and Approach (Final) controllers, noting that both units reacted promptly to automated Short Term Conflict Alerts (STCA) and the TCAS-RA.
Data from the DFDR of UK32022 revealed that the aircraft performed two unauthorized climbs during the approach. The investigation also scrutinized the flight path of VT-IHQ, which was maintaining FL80 under the jurisdiction of Approach (Area) Control at the time of the breach.
Findings
- The primary cause of the incident was the unintentional movement of the Engine #2 thrust lever to the TOGA position by the pilot monitoring during a descent from FL70 to FL60.
- This action activated the "SRS" (Speed Reference System) mode, which initiated an automated climb instead of the requested descent.
- Although the crew attempted to disconnect the autopilot to arrest the climb, the TCAS-RA had already been triggered.
- The aircraft was also noted to be operating at high speeds, requiring multiple speed reduction instructions from ATC.
- The separation between the two aircraft was restored within 23 seconds through the execution of TCAS-RA maneuvers.
- No injuries to passengers or crew were reported, and no damage was sustained by either aircraft.