What aircraft was involved and where did it happen?

The accident on 2022-09-14 involved a Airprox between M/s Air India, B787, registration VT-ANM, at IN.

What was the probable cause of the accident?

The incident was primarily caused by the flight crew of G-VBZZ mistaking a fragmented VHF radio transmission for an authorized descent clearance, a mistake reinforced by confirmation bias regarding their flight profile.

Near-Collision of Two Boeing 787s in Mumbai Oceanic Airspace

A serious airprox incident occurred in the Mumbai Oceanic region when a Virgin Atlantic Boeing 787 descended into the flight path of an Air India Boeing 787, reducing vertical separation to zero.

What happened

On 14 September 2022, two Boeing 787 aircraft experienced a significant loss of separation while cruising through the Mumbai Oceanic region. The first aircraft, an Air India B787 registered VT-ANM, was flying from Dubai to Mumbai at flight level 390. Trailing behind on the same airway was a Virgin Atlantic B787, registration G-VBZZ, maintaining flight level 410.

During the cruise phase, the flight crew of G-VBZZ initiated a descent. At approximately 02:39 UTC, the aircraft's altitude selector was changed from 41,000 feet to 34,000 feet, causing the aircraft to descend through the altitude of VT-ANM. This maneuver resulted in the vertical separation between the two aircraft being reduced to zero, while lateral separation dropped to just 3 nautical miles. The descent continued until the aircraft reached 38,900 feet before the crew corrected the altitude and climbed back to the assigned level.

The investigation

AAIB India examined the cockpit voice recordings, flight data, and air traffic control transcripts to determine the cause of the breach. The investigation focused on the communication environment in the Mumbai Oceanic sector and the cockpit procedures of the Virgin Atlantic crew.

Investigators found that at the time of the incident, the relief pilot of G-VBZZ had left the flight deck, leaving the co-pilot as the sole pilot flying. The investigation also scrutinized the air traffic control workload and the technical state of the communication systems. It was noted that the U-5 controller had not yet acquired the aircraft on their workstation, which prevented the generation of an automated conflict warning. Furthermore, the investigation identified discrepancies in the reporting of the incident, noting that the DGCA had released the aircraft without ensuring the preservation of flight recorder data.

Findings

The pilot flying on G-VBZZ believed they had received a descent clearance, though no such instruction was issued by ATC via the primary CPDLC data link.
A broken VHF communication transmission was mistaken for a descent clearance.
The crew likely experienced confirmation bias, expecting a descent due to the aircraft's position behind traffic and a recent request to increase speed.
Poor VHF radio coverage in the Mumbai Oceanic region contributed to confusing communications and the inability of the crew to hear repeated calls from the controller.
The controller's attempt to rectify the communication issue involved a mistyped frequency, which further complicated the interaction.
No TCAS RA (Resolution Advisory) alerts were triggered, and no automated conflict warnings were generated by the controller's workstation because the aircraft were not yet under the specific sector's active surveillance.

Safety action

It is recommended that airlines review the risks associated with poor VHF coverage in the Mumbai Oceanic region within their briefing materials.
The Airport Authority of India should investigate ways to improve VHF signal effectiveness in this area.
The DGCA is urged to ensure that all operators, including foreign airlines, strictly adhere to the requirement of notifying the AAIB of all incidents.
The DGCA should enforce protocols to ensure aircraft are not released for subsequent flights until flight recorder data is preserved and the investigation authority is notified.