What happened
On November 10, 2016, at approximately 04:02 UTC, two Air India Boeing 787-800 aircraft, registrations VT-ANI and VT-AND, experienced a serious air proximity incident within the Delhi Area Control Centre (ACC) airspace. The aircraft were operating scheduled international flights, with VT-ANI arriving from Vienna and VT-AND arriving from Paris, both destined for Delhi.
Initially, the aircraft were separated by nine minutes. However, to manage arrival spacing, the controller instructed VT-ANI to perform an orbit, which resulted in VT-AND moving ahead of VT-ANI by two nautical miles. During the descent phase, a series of communication and clerical errors led to a breakdown in vertical separation. The aircraft's separation reduced to just 700 feet vertically and 0 nautical miles laterally, with radar symbols for both flights overlapping. The situation was only resolved when both flight crews received Traffic Advisory (TA) and subsequent Resolution Advisory (RA) alerts from their TCAS units, prompting evasive maneuvers.
The investigation
The investigation by the AAIB focused on the sequence of instructions issued by the ACC-West radar controller. Investigators examined radar data, ATC tape transcripts, and flight recorder information. The inquiry looked into the high traffic density in the Delhi sector, which involved 18 aircraft under the controller's responsibility, and the impact of congestion in the adjacent ACC-South sector.
Key elements examined included the accuracy of the controller's data block entries and the clarity of radio transmissions. The investigation specifically scrutinized a series of descent clearances where instructions intended for one aircraft were inadvertently applied to others, and the failure of the controller to acknowledge automated conflict warnings generated by the ATM system.
Findings
- The primary cause of the incident was incorrect descent clearances issued by the radar controller, specifically directing VT-AND to descend to FL210 instead of the intended aircraft.
- The controller inadvertently assigned a descent clearance for FL210 to aircraft AIC156, which was intended for VT-ANI.
- A Predicted Conflict Warning (PCW) was generated between the two aircraft but was not acknowledged by the controller.
- High traffic density and complexity in the Delhi ACC-West sector contributed to frequency congestion and increased workload.
- The technique used to manage arrival spacing via an orbit was inappropriate for the traffic situation.
- A descent instruction to VT-ANI to descend to FL190 was not read back by the crew, leading the aircraft to maintain FL210 while the following aircraft continued its descent.
- Standard separation was eventually re-established only after the crews performed TCAS-mandated maneuvers.