Communication gap caused Charkhi Dadri mishap: ATC guild

The Air Traffic Controllers Guild submitted before the Justice R C Lahoti court of inquiry that the mid-air collision between the Kazakh IL-76 and the Saudia aircraft on November 12, 1996 was caused because the IL-76 crew did not understand English well.

ATC Guild general secretary Brijender Shekhar Shukla said the resulting confusion may have caused the mishap.

As per the cockpit voice recorder transcript of the two aircraft and the ATC tape, it is evident that the Kazakh aircraft crew had 'difficulty' understanding instructions, he said, adding that the presence of an interpreter (the Kazakh radio operator) made the aircraft commander relax without having things under control.

Shukla said the commander knew he was told to fly at 15,000 feet, but just seconds before the accident inquired about his height. when he found the aircraft was still descending below the assigned level. He said the CVR transcript would show why the aircraft descended below 15,000 feet.

The transcript shows that the radioman was initially not able to understand the conversation about the traffic. That is why he stated that ''Will report ... How many miles,'' to which the ATC replied, ''Eight miles now''. Later when radioman said, ''Report eight miles,'' the controller corrected him, saying, ''Traffic is at eight miles, level one four zero,'' meaning there was another aircraft at level one four zero (14,000 feet). This is the transmission which assumes maximum relevance, Shukla said.

Although the radioman seemingly understood the implication, the co-pilot, who was at the controls, was busy with other work and wrongly concluded that it meant he was being cleared to fly at 14,000 feet, the ATC guild representative contended.

About 38 seconds before the collision, the Kazakh aircraft was at 14,800 feet and descended to 14,100 feet 25 seconds before the collision and maintained that level, resulting in the accident.

An aircraft accident investigating branch, UK, report also mentioned that there was not sufficient turbulence to cause the rapid change in altitude recorded, and put it down to faulty equipment.

The ATC Guild submitted that although there were weather equipment in both the Kazakh and Saudi aircraft capable of detecting any turbulence, but none was reported. The meteorological department too did not report any turbulence over Charkhi Dadri on the day of the mishap. The prevailing high pressure also did not support the existence of any such turbulence, Shukla said, adding that evasive action was impossible since the aircraft crashed within a cloud.

Shukla said it had been hinted that ATCs could opt for greater distance between aircraft - more than 1,000 feet. The 1,000 feet vertical separation has been approved by the International Civil Aviation Organisation after taking into account all possible errors and tolerances. This standard is being implemented all over the world, he added.

Shukla contended that since both aircraft were instructed to maintain flight levels that separated them by 1,000 feet and since there was no essential traffic flying, no collision was expected. Traffic information was provided to the Kazakh aircraft only as additional information, to let them know why the aircraft was being asked to stay at 15,000 feet, he said.

Meanwhile, the government has given the Justice Lahoti court of inquiry till July 15 to submit its report on the mishap. Justice Lahoti had sought more time to complete his investigations. Officials at the Indira Gandhi International Airport and the air traffic controllers are yet to be questioned in this regard.

Related story:ATCs blameless in Charkhi Dadri tragedy, claims counsel