THE pilot of the doomed Russian passenger jet in last week’s mid-air collision over Germany had 14 seconds to choose between contradictory instructions immediately before the crash which killed 71 people, it emerged last night.
A flight-voice recorder obtained from the wreckage of the Tupolev-154 aircraft has revealed the cockpit proximity warning system initially ordered him to pull the aircraft up to avoid a collision.
Just one second later, he received a contradictory radio instruction from a Swiss air-traffic controller telling him go into a dive.
There was silence for 14 seconds, as the pilot faced an agonising dilemma, before the air-traffic controller sent another radio message repeating the instruction to take the plane into a descent.
The pilot obeyed the control instruction and over-rode his cockpit warning to begin his descent from 36,000ft. Just 30 seconds later, his aircraft slammed into a Boeing 757 cargo plane crossing his path.
A separate examination of the Boeing 757 voice recorder has shown that its pilot had also received an automatic proximity warning at exactly the same time as the Tupolev-154 pilot. The computer system had correctly ordered the Tupolev-154 to climb and the Boeing 757 to dive so the collision would be avoided.
The discovery of the contents of the flight voice recorders will place further pressure on the Swiss air traffic controllers who had responsibility for the two aircraft at the time of the collision on 1 July.
Although the collision took place over southern Germany, it was on an approach to Zurich airport and the flightpaths were controlled by air-traffic controllers in Zurich.
Swiss air-traffic control said the Zurich tower would have had no way of knowing the pilot was receiving a contradictory instruction from his cockpit warning system - or Traffic Control Advisory System (TCAS).
"He only finds out about it if the pilot tells him," spokesman Markus Luginbuehl said. "If the pilot reacts to a TCAS alarm, he is supposed to advise the controller. And the pilot assumes responsibility for the manoeuvre."
Russian aviation officials, Bashkirian Airlines and Moscow’s Domodedovo Airport, where the Russian flight started, said in the case of a contradiction between the on-board, anti-collision system and air-traffic control instructions, ground command had priority.
"The air-traffic controller gets the last word," said Sergei Rybanov, of Bashkirian Airlines.
But Herbert Schmell, a spokesman for the national airline, Swiss, said the cockpit-warning system should have been obeyed over ground control. "A TCAS system makes no sense if it is overruled, especially in a phase when there isn’t much leeway any more," Mr Schmell said.
Skyguide, the private company that operates air-traffic control in Switzerland, initially claimed the pilot had been warned up to two minutes before the crash and had "repeatedly" ignored the instruction. It later revised the margin down to 50 seconds.
The voice recorder reveals the time of the first Skyguide instruction to dive was actually given 44 seconds before impact. It also offers a possible explanation for the pilot’s 14-second hesitation as he deliberated which of the contradictory orders to follow.
Other factors being examined include the fact that an automatic collision-alert system at Zurich air-traffic control tower was switched off at the time of the crash and only one air-traffic controller was on duty. It also emerged yesterday that German air-traffic controllers had tried to warn Skyguide the planes were on a collision course but the telephone network was down.
The head of the Swiss Air Accident Investigation Bureau still suggested the Russian pilot may have been at fault when asked about the findings yesterday. Jean Overney said he didn’t know what rules Russian pilots have to follow in the case of a conflict between instructions from air-traffic controllers and the plane’s own collision avoidance system.
Mr Overney said: "In the West, the pilot must follow the collision avoidance system."
All 69 people, including 45 schoolchildren, aboard the Russian Tupolev-154, and two crew members on the Boeing 757 were killed when the aircraft collided.