What happened
On October 7, 2017, at approximately 18:24 UTC, a serious air proximity incident occurred at Delhi's Indira Gandhi International Airport involving two large wide-body aircraft. A British Airways B787-9, registration G-ZBKF, was on an ILS approach to Runway 29 after arriving from London. Simultaneously, a Thai Airways B777-300, registration HSTKC, was cleared for an intersection takeoff from CE2.
As the British Airways flight approached the threshold, the crew initiated a missed approach at less than one nautical mile from touchdown. During this period, the departing Thai Airways aircraft was already on the runway. The simultaneous execution of the go-around and the departure resulted in a breach of standard separation, with lateral and vertical distances between the two aircraft dropping to less and 200 feet, respectively. Following the event, air traffic controllers instructed the departing aircraft to limit its climb to 2,600 feet and directed the arriving aircraft to expedite its climb to 3,600 feet.
The investigation
AAIB India examined radar replays, ATC transcripts, and flight data monitoring (FDM) from the G-ZBKF aircraft. The investigation focused on the sequence of communications between the Approach Radar Controller, the Tower South Controller, and the flight crews. Investigators analyzed the speed profiles of both aircraft, noting that the British Airways flight was maintaining a high speed of approximately 200 knots while the preceding arrival was at 140 knots. The investigation also reviewed the aerodrome configuration, specifically the displaced threshold of Runway 29 and the intersection takeoff point used by the Thai Airways flight.
Findings
- The Approach Radar Controller did not instruct the British Airways flight to reduce speed to the published approach speed, which would have helped maintain the required 7 NM inter-arrival spacing.
- The Aerodrome Controller cleared the Thai Airways flight for takeoff without accounting for the high speed of the approaching B787-9, the reduced spacing, or the impact of the displaced runway threshold.
- The simultaneous execution of a missed approach and an intersection departure led to the loss of standard separation.
- The inter-arrival spacing between the arriving and departing aircraft was approximately 5.27 NM when considering the specific runway geometry and aircraft speeds.