Two cargo planes involved in a mid-air close call in late February at the Hong Kong International Airport were cleared for takeoff “only 13 seconds apart” instead of the minimum three minutes, according to a preliminary investigation report.

The Air Accident Investigation Authority (AAIA) released on Monday its preliminary report on the Feb 27 incident involving an MNG Airlines Airbus A330 and a Silk Way West Airlines Boeing 777 that led to the issuance of a collision alert.

At their closest, the two planes were 100 feet apart vertically, according to the report.

It noted that since both aircraft, which were taxiing on adjacent runways, were on a “convergent track", Air Traffic Control (ATC) should have provided a “3-minute (180-second) minimum separation” between their departures.

“The data indicated the two aircraft concerned were cleared for takeoff only 13 seconds apart,” the report reads.

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According to the report, the Airbus plane, on the center runway and heading for Turkmenistan, was cleared for takeoff at 06:59:09 by the air movement controller in the North Control Tower.

At 06:59:22, the air movement controller in the South Control Tower gave the Boeing aircraft the green light to take off from the south runway on its way to Azerbaijan.

“At that moment, (the Airbus plane) was commencing takeoff from RWY 07C,” the report said.

Noticing the incident at 06:59:57, the aerodrome supervisor of the South Control Tower called his counterpart in the North Control Tower to coordinate remedial actions.

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The Airbus plane was told to expedite its climb to 5,000 feet while the Boeing aircraft was instructed to restrict its climb to 3,000 feet. Both aircraft reported visual contact with each other, the report said.

Based on preliminary data, the closest spacing between the two aircraft was approximately 0.4 nautical miles horizontally and 100 feet vertically, with both planes on converging headings.

At 07:02:07, the Boeing aircraft advised ATC that its onboard Airborne Collision Avoidance System issued a Resolution Advisory to descend from 3,000 ft, which it followed. 

Thirty seconds later, “a vertical separation of 1,000 ft between the two aircraft was re-established” as the Airbus plane climbed through 3,500 ft while the Boeing aircraft descended through 2,500 ft. They then proceeded to their destinations. 

A spokesperson for the AAIA said that the report has been published to provide factual information established in the initial phase of the investigation and should be regarded as tentative.

"The investigation team is conducting a detailed analysis of the data and information collected in order to determine the circumstances and causes of this occurrence with a view to preventing a recurrence in conjunction with identifying areas for further investigation or lines of inquiry to follow up," the spokesperson said.

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The AAIA said it will conduct detailed analysis that will focus on, among others, ATC operational procedures, including coordination among ATC units; recovery actions taken by the ATC and flight crews during the incident; and “human factors that may have influenced the event”.

“The detailed analysis of the data and information collected will enable the investigation team to determine the circumstances, causes and contributing factors of this serious incident,” the report added.