Airprox Serious incident Airbus A320-251N VT-TQL, Friday 10 November 2023

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Date:	Friday 10 November 2023
Time:	04:47 UTC
Type:	Airbus A320-251N
Owner/operator:	Vistara
Registration:	VT-TQL
MSN:	10748
Year of manufacture:	2022
Engine model:	CFMI CFM LEAP-1A26
Fatalities:	Fatalities: 0 / Occupants: 178
Other fatalities:	0
Aircraft damage:	None
Category:	Serious incident
Location:	near Delhi Airport (DEL) - India
Phase:	Approach
Nature:	Passenger - Scheduled
Departure airport:	Ahmedabad-Sardar Vallabhbhai Patel International Airport (AMD/VAAH)
Destination airport:	Delhi-Indira Gandhi International Airport (DEL/VIDP)
Investigating agency:	AAIB India
Confidence Rating:	Accident investigation report completed and information captured

Narrative:
Vistara flight UK946, an Airbus A320-251N (VT-TQL), and Ethiopian Airlines flight ET689, a Boeing 787-8 (ET-ATL), were involved in an airprox incident near Delhi Airport (DEL), India.

Flight ET689, departed from runway 29R at DEL. At about the same time, flight UK946 was expected to land on Runway 29L.
On the day of incident, Segregated Dependent Westerly Mode of operations were in progress in which parallel runways 29L and Runway 29R were being used exclusively for arrivals and departures respectively.
The incident occurred when ET689 was allowed for take-off from Runway 29R while UK946 was approaching Runway 29L with a Time-To-Threshold (TTT) below the SOP minimum of 90 seconds. UK946 initiated a missed approach due to wind shear and came into conflict with departing aircraft ET689. Since the missed approach track of UK946 and SID track followed by ET689 intersected, this led to a breach of separation between the two at 04:45:48 UTC.
TCAS RA was generated for both aircraft which were followed by respective flight crews.
Upon contacting the Approach Departure Controller, ET689 received instructions intended to de-conflict the traffic, but these were not suitable for the prevailing traffic scenario. Lateral spacing between ET689 and UK946 again started reducing and a second TCAS RA event occurred. At 04:47:53 UTC, the minimum separation between the two aircraft was reduced to approximately 0.2 NM horizontally (laterally) and 400 feet vertically.
However, the required separation was re-established and ET689 continued to its destination without any further event. UK946 was re-vectored for an ILS approach to runway 29L and landed at Delhi Airport at 05:04.

Probable Cause(s)
The serious incident of airprox occurred due to a loss of situational awareness of the Aerodrome Controller (ADC-S2), wherein an aircraft was allowed to take off from Runway 29R even though the arriving aircraft’s Time-to-Threshold (TTT) had already breached the minimum set value of 90 seconds while approaching dependent parallel Runway 29L.
Subsequently, the arriving aircraft, due to an unstabilised approach on finals, initiated a Go-around from Runway 29L and simultaneously the other aircraft got airborne from the parallel Runway 29R which consequently led to intersection of their projected flight paths.

Contributory Factors
The following are identified as contributory factors to the serious incident:
a) Operational Interdependency and Complexity:
The ADC-S2 controller’s dependency, while issuing take-off clearance to aircraft under their jurisdiction, on the position of arriving aircraft on Runway 29L and concurrent operations on Runway 27 under adverse weather conditions created a complex and high-stress environment, increasing susceptibility to human error and compromising decision-making.
b) Cognitive Overload on the ADC-S2 Controller:
The ADC-S2 controller was required to manage multiple simultaneous safety-critical tasks at a very busy airport resulted in cognitive overload. This impaired the controller’s ability to effectively prioritize actions and respond to critical situations in a timely manner.
c) Delayed Transfer of Communication During Missed Approach:
The ADC-S1 controller’s delayed transfer of communication to the Approach Departure Controller regarding execution of missed approach, impeded efficient conflict resolution and limited the controller’s ability to manage the situation effectively.
d) Ineffective Conflict Resolution Instructions:
The Departure Controller issued inappropriate resolution instructions to the aircraft involved in the conflict potentially resulted into less separation between both aircraft and escalated the risk of airprox.
e) Inadequate Separation Assurance in SID Design:
The design of the Standard Instrument Departure (Dudum6C) did not adequately ensure positive separation between departing aircraft from Runway 29R and aircraft executing a missed approach from Runway 29L, increasing the risk of loss of separation

Accident investigation: