Communication gap caused Charkhi Dadri mishap: ATC Guild
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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.
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