What happened
On April 16, 2017, at approximately 08:06 UTC, a serious air proximity incident occurred within the Varanasi Area Control Center (ACC) involving two Airbus A320 aircraft. The first aircraft, an Air Asia flight (VT-JRT) traveling from Bagdogra to New Delhi, was initially cleared by the Radar Controller to descend from FL360 to FL340. While the flight crew correctly acknowledged this instruction, the aircraft continued its descent beyond the authorized altitude, reaching FL320 without permission.
Simultaneously, an IndiGo flight (VT-IEW) was cruising at FL330 on a reciprocal route from New Delhi to Bagdogra. Because the Air Asia crew descended through the altitude of the IndiGo aircraft, the vertical separation was lost, leaving only 5 nautical miles of lateral separation and zero feet of vertical separation. The aircraft passed each other with a high relative closure rate of approximately 850 knots. Both crews received Traffic Advisories (TA) from their onboard systems. During the encounter, the Radar Controller instructed the IndiGo aircraft to turn left, but the crew opted for a right turn to avoid the conflict. Following the event, a heated argument occurred over the radio between the Air Asia crew and the Radar Controller.
The investigation
The investigation conducted by the AAIB examined the flight paths, air traffic control (ATC) communications, and the operational status of the Varanasi ATC unit. Investigators analyzed the ATC tape transcripts and radar replays, which confirmed the unauthorized descent and the subsequent loss of separation. The inquiry also scrutinized the professional credentials and duty records of the air traffic controllers involved.
Findings
- The primary cause of the incident was the unauthorized descent by the crew of VT-JRT below the flight level cleared by ATC.
- The Air Asia crew likely experienced expectation bias, as they had previously requested FL320 and descended to that level despite being cleared only to FL340.
- The Radar Controller failed to maintain adequate surveillance of the aircraft and was distracted by non-priority communications.
- The investigation revealed significant systemic issues at the Varanasi ATC unit, including several controllers holding invalid ratings because they had not performed sufficient duties in specific units (Tower, Approach, or Area) within the required 180-day window.
- Administrative failures were noted, as the ATS-in-charge and Training-in-charge failed to assign duties necessary to keep controller ratings current. Additionally, improper handovers and a lack of personal duty logs among controllers were identified.
Safety action
- Recommendations were made for the Radar Controller to undergo training in conflict resolution, radar techniques, and radio telephony (RT) discipline.
- The Air Asia crew was recommended for Crew Resource Management (CRM) and RT discipline training.
- The investigation called for the Airports Authority of India (AAI) to implement procedural changes regarding controller ratings, including stricter requirements for duty hours to maintain currency and a standardized logbook system to track duties performed in various ATC units.