What happened
On 5 October 2021, an Air India A320-251N, registration VT-EXQ, was operating a scheduled flight from Patna to Mumbai. While descending through the Mumbai FIR, the flight crew encountered heavy weather and significant traffic complexity. During the descent, the First Officer (Pilot Flying) mistakenly accepted a descent clearance to FL150 that was actually intended for a different aircraft, AIC644.
As the A300-251N descended, it entered the flight path of a King Air 65-C90B, registration VT-NKF, which was maintaining FL170. The separation between the two aircraft dropped to approximately 1 NM horizontally and 400 feet vertically. The incident triggered a TCAS Resolution Advisory (RA) in the cockpit of VT-EXQ, prompting the crew to level off and avoid the intruder. The King Air, VT-NKF, was not equipped with TCAS.
The investigation
AAIB India examined radar recordings, cockpit voice and flight data recorders, and communications between the aircraft and Mumbai Area Control. The investigation focused on why the descent clearance was misapplied and why the air traffic controller's conflict warnings did not prevent the loss of separation. The inquiry also looked into the reliability of the VHF communication equipment and the crew's adherence to standard operating procedures during the descent.
Findings
- The primary cause was a human error by the First Officer, who accepted a descent clearance intended for another aircraft due to expectation bias.
- The Pilot in Command failed to monitor the Flight Control Unit (FCU) altitude selection due to distractions and reduced situational awareness.
- Non-essential cockpit conversations during the descent diverted the crew's attention from critical tasks.
- The air traffic controller did not receive the pilot's readback of the incorrect clearance, and the controller did not verify that the correct aircraft had acknowledged the instruction.
- Adverse weather conditions and heavy communication congestion contributed to the complexity of the airspace.
- Technical issues, including the unreliable performance of the VHF 133.425 MHz receiver and the delayed generation of the controller's altitude non-conformance warning, hindered effective intervention.
Safety action
- It is recommended that Air India Ltd. implement modules on managing distractions in CRM training and review procedures for headset use during high-workload phases.
- Recommendations were made for the Airports Authority of India to ensure the reliability of VHF communications and to sensitize controllers regarding the importance of verifying ATC readbacks.
- The investigation also highlighted the need for the operator to implement formal Fatigue Report Management systems.