BERLIN -- A Russian pilot received contradictory instructions before crashing into a cargo plane over Germany last week, German investigators said Monday after reviewing voice recorders from the two jets.
The voice recorders show that a Swiss air traffic controller's order for a Russian pilot to descend contradicted the cockpit warning system's command for the Tu-154 to climb, the investigators said.
The automatic cockpit warning systems issued simultaneous instructions for the Russian passenger jet to climb and a cargo jet to descend about 45 seconds before they ultimately collided over southern Germany, killing all 71 people on board.
But one second after the on-board system warnings, the Zurich tower, which was in charge of directing the planes even though they were flying over Germany, told the Russian plane to descend, German investigators said, citing voice recorders from both planes recovered at the crash site.
The Russian pilot did not immediately respond to the tower's command, and the Swiss air controllers repeated the order 14 seconds later, the voice recorders indicate.
The Russian pilot responded, and 30 seconds later the planes crashed at 35,000 feet.
German investigators did not release a transcript of the voice recorders, or further details of any exchanges.
The head of the Swiss Air Accident Investigation Bureau, Jean Overney, did not know what rules Russian pilots have to follow if there is a conflict between instructions from air traffic controllers and the plane's own collision avoidance system. But Overney said, "in the West, the pilot must follow the plane's collision avoidance system."
Earlier Monday, German air traffic controllers said they tried to warn the Swiss control tower by phone two minutes before two planes collided, but the only available line was busy.
Investigators are focusing on the actions of Swiss air traffic controllers and whether they gave the Bashkirian Airlines Tu-154 enough time to avoid collision with the DHL International Boeing 757.
The planes were being directed by controllers in Zurich, Switzerland. Swiss prosecutors have opened a criminal investigation to determined whether anyone can be charged with negligent homicide.