Close call: Two IndiGo aircraft approached perilously near Delhi Airport in November; investigation into

On November 17, when they departed from Delhi’s Indira Gandhi International (IGI) Airport, two aircraft belonging to IndiGo, the biggest airline in India, flew dangerously close to one another, with their vertical and lateral separation falling short of the legally allowed minimums.

The two airplanes registered VT-IUO and VT-ISO, respectively, are the Airbus A321 and A320 that were involved in the incident. The A320 was traveling from the capital to Raipur, while the A321 was doing a route from Delhi to Hyderabad. It is being investigated by the Aircraft Accident Investigation Bureau (AAIB), which has designated it as a “serious incident.”

“The lateral separation was 1.2 nautical miles (NM) at the moment of the closest vertical separation, which was 400 feet. The vertical separation was 800 feet at the moment of the closest lateral separation (0.2 NM). No one was hurt among the passengers in either aircraft. In its first incident report, the AAIB said that there was no damage. It should be noted that when in the air, aircraft are required by standards to maintain a minimum vertical separation of 1,000 feet and a minimum lateral separation of 5 nautical miles. Regarding the incident and the AAIB inquiry, IndiGo has not yet made any comments.

The A321 took off on November 17 at 12:31 PM from Runway 27 at IGI Airport, and the air traffic controller gave the go-ahead for the aircraft to ascend to 8,000 feet, according to the report from the AAIB. Instead, the aircraft “was observed turning left toward the RWY 29R (Runway 29 Right) takeoff path.” Simultaneously, the A320 that was piloting the Delhi-Raipur aircraft was cleared to leave and departed from Runway 29 Right.

“A breach of separation occurred during this sequence…triggering a Current Conflict alert,” the AAIB said, noting that TCAS-RAs (Traffic warning and Collision Avoidance System-Resolution Advisory) were sent for both aircraft. TCAS-RAs are detailed instructions that pilots get from the Traffic Alert and Collision Avoidance System (TACAS) on how to avoid causing trouble with other aircraft. These instructions may include commands to descend, ascend, or modify the aircraft’s vertical speed. TCAS is a vital safety feature in airplanes that helps prevent collisions in midair.

A timetable for the current inquiry was not provided in the preliminary investigative report, however. The AAIB collected the data for the investigation from a variety of sources, including the aircraft’s voice recorders and flight data recorders, airline and airport operating manuals, recordings of radar data, and tape recordings of Air Traffic Control. The first testimonies from the concerned aircraft crew and air traffic controllers have also been recorded by the aviation accidents investigate agency.

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