What happened
On February 29, 2020, a serious incident occurred in the Delhi Approach sector involving two Airbus A320 aircraft: VT-CCU, operated by Air Asia (India) Ltd, and VT-IAR, operated by IndiGo. At the time, the Delhi airspace was experiencing complex traffic conditions due to heavy thunderstorms, which forced several aircraft to deviate from their assigned flight paths.
VT-CCU was arriving from Goa and was instructed by the Air Traffic Controller to descend to flight level 110. Simultaneously, VT-IAR had departed Delhi for Chennai and was climbing to flight level 100. Because of the weather-related deviations, the two aircraft ended up on reciprocal tracks. Although the flight crew of VT-CCU correctly read back the instruction to maintain flight level 110, the aircraft subsequently descended below this assigned altitude. This unauthorized descent brought the two aircraft into extremely close proximity, reducing lateral separation to just 0.5 NM and vertical separation to 600 feet.
The investigation
The investigation by the AAIB examined cockpit voice recordings, air traffic control transcripts, and automation system logs. The inquiry established that while the flight crew of VT-CCU had acknowledged the correct altitude, the altitude selector on the flight control unit was actually set to flight level 100.
Investigators also reviewed the actions of the Air Traffic Controller. The automation system had generated a "level burst" alert, indicating a discrepancy between the altitude entered by the crew and the altitude assigned by the controller, but this alert was not acted upon by the controller. The investigation further noted that the crew did not utilize standard inter-cockpit phraseology to confirm the altitude set on the flight control unit, nor did they cross-verify the setting before beginning the descent.
Findings
- Severe weather conditions in the Delhi area caused aircraft to deviate from standard arrival and departure routes, creating complex traffic patterns.
- The flight crew of VT-CCU performed an unauthorized descent below the assigned flight level 110.
- Although the crew read back the correct altitude, the physical altitude setting on the aircraft was incorrectly set to flight level 100.
- The Air Traffic Controller failed to respond to a system-generated level burst alert.
- The conflict was eventually resolved by the Traffic Collision Avoidance System (TCAS), as both aircraft received Resolution Advisories (RA) and performed evasive maneuvers.
- Increased cockpit workload due to the ongoing weather conditions contributed to the lack of monitoring.
Safety action
- It is recommended that the flight crew of VT-CCU undergo corrective training with a specific focus on situational awareness.
- The Airports Authority of India has been advised to instruct controllers to exercise greater caution regarding system-generated alerts.