What happened
On 2 2, at approximately 1552 UTC, two Airbus A320 aircraft experienced a serious loss of separation within Mumbai airspace. The first aircraft, an IndiGo flight VT-IAY, was descending from FL380 toward Mumbai, while an Air Asia India flight VT-HYD was cruising at FL360 en route to Goa.
The incident began when the pilot of VT-IAY requested a descent. The Air Traffic Control (ATC) controller cleared the aircraft to descend to FL370. However, the flight crew incorrectly read back the clearance as FL310, and the controller mistakenly confirmed this erroneous level. Following this error, the aircraft began a rapid descent.
As the aircraft descended, a Predicted Conflict Warning (PCW) was triggered in the radar system. The controller attempted to correct the altitude via radio, but the crew of VT-IAY failed to respond. During this period, the pilot flying left the cockpit, and the co-pilot was in the process of re-entering. By the time the crew regained communication, the aircraft had already descended through the cleared level and reached FL360, the same altitude as VT-HYD. This triggered a Current Conflict Warning (CCW), with vertical separation dropping to zero feet and lateral separation narrowing to 3.8 NM.
The investigation
The AAIB investigation focused on the breakdown in communication and cockpit procedures. Investigators established that the crew of VT-IAY was not maintaining a proper listening watch during the critical descent phase. The investigation also scrutinized the controller's actions, noting that the controller continued to attempt communication with the non-responsive VT-IAY rather than immediately issuing evasive instructions to the other aircraft, VT-HYD, once the conflict was predicted.
Findings
- The primary cause was the non-adherence to Standard Operating Procedures (SOP) by the VT-IAY crew, specifically the failure to maintain a listening watch during a critical phase of flight.
- A critical readback error occurred where the pilot requested FL310 instead of FL 370, which was erroneously validated by the controller.
- The crew of VT-IAY was distracted by a cockpit handover and the pilot leaving the flight deck to use the lavatory, which interrupted monitoring.
- The controller experienced a loss of situational awareness by focusing on the non-responsive aircraft instead of proactively managing the potential conflict with VT-HYD.
- The aircraft VT-IAY utilized a descent rate significantly higher than prescribed for the intended level due to the incorrect altitude setting in the Flight Management System (FMS).
Safety action
- The investigation recommended that IndiGo issue advisories to flight crews regarding the necessity of maintaining a continuous listening watch and monitoring all resources to prevent clearance discrepancies.
- A recommendation was made to the DGCA to consider procedures that prevent both pilots from being out of the cockpit or in a handover state simultaneously, ensuring cockpit continuity.