9 July 2002
Crash jet 'got conflicting instructions'
By Paul Marston, Transport Correspondent
The Russian flight crew involved in last week's mid-air collision were given conflicting instructions by Swiss air traffic control and their own cockpit instruments, air investigators said yesterday.
The German air accident inquiry branch released details of the flight deck voice recorders, which showed that the Bashkirian Airlines pilots were told to descend by ground control one second after their on-board anti-collision computer had ordered them to climb.
The Zurich controller restated his instruction after 14 seconds and the crew of the Tupolev 154 obeyed. Thirty seconds later they flew into a Boeing 757 cargo plane, which had complied with its computer's command to lose height. Seventy-one people, including 45 Russian children, died.
Under international aviation regulations pilots are supposed to abide by the advice of the cockpit Traffic Collision Avoidance System (TCAS) even if it contradicts information from the ground. If two flight paths come into potential conflict, the TCAS systems on the aircraft involved communicate directly with each other and prevent a collision by issuing complementary instructions to the respective crews.
The recorders revealed that the Tupolev and Boeing pilots received a preliminary audible warning of "traffic, traffic" from their TCAS systems just under a minute before the crash as they cruised at 36,000ft. About 15 seconds later, the Boeing TCAS gave the order "descend, descend" while its counterpart on the passenger jet called "climb, climb".
But within a second, the Russian crew received their first communication from the Zurich controller. "Descend Flight Level 350 [35,000 ft]. Expedite. I have crossing traffic," he said.
Fourteen seconds later, he radioed again: "Descend Level 350. Expedite descent." Aviation professionals said the delay before the Tupolev began to descend probably reflected the crew's hesitation over which instruction to follow.
It was thought that the Russian commander may have been dissuaded from complying with TCAS because of the virtually simultaneous nature of the controller's first intervention and his rapid reaffirmation of the necessary avoiding action.
It also emerged that German air traffic controllers tried to warn Swiss colleagues of the collision danger, but could not make contact because the Zurich telephone system was undergoing maintenance.