RANKFURT, July 8 - Information from the flight recorders aboard the two planes that crashed into each other one week ago over southern Germany show that a Swiss air traffic controller in effect put the planes on a collision course by ordering the Russian pilot to descend at the same time that the plane's own collision-avoidance system was urging him to climb.
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The new data, released today by German investigators, show that the two planes' automated systems communicated with each other exactly as they were meant to do and that the accident would probably not have occurred if the Russian pilot had simply ignored the Swiss controller in Zurich. The collision killed 52 Russian schoolchildren and 19 adults.
In addition, German investigators said German controllers saw danger nearly two minutes before the crash and desperately tried to warn the controllers in Zurich, who had responsibility for both planes.
Air controllers in the German city of Karlsruhe said they had received only busy signals every time they tried to call Zurich, even when they used a special priority line.
The disclosures by the German Federal Agency for the Investigation of Air Accidents strongly suggested that Zurich's air traffic control center was understaffed, overwhelmed and befuddled with technical glitches just before the crash. Only one traffic controller was on duty for that sector at the time of the crash.
Last weekend, Swiss officials announced that they were putting into effect a 20 percent reduction in air traffic over Switzerland, which, given the country's pivotal role in directing flights throughout Europe, is likely to disrupt service across the continent. The announcement was made by the privatized Swiss air traffic company, Skyguide.
The midair collision, near Lake Constance in Germany and just north of the Swiss border, was between a passenger plane operated by Bashkirian Airlines and a Boeing 757 cargo plane operated by DHL Worldwide, an overnight courier service.
The Russian plane was carrying a large group of schoolchildren to a vacation in Spain.
Swiss traffic control officials had initially and bluntly blamed the Russian pilot, saying he was very slow to respond to their repeated warnings. Though the planes were converging at the same altitude, the crash occurred after both began diving at the same time to avoid each other.
But investigators studying radio transmissions discovered last week that Zurich controllers had been much slower to recognize the danger than they originally claimed and did not send their first warning until 44 seconds before the crash occurred.
Swiss officials have also admitted that only one controller was on duty in the crucial minutes before the crash, that the air traffic center's alarm system was switched off for maintenance, and that the telephone lines were not working properly.
German investigators said today that tapes from the flight data recorders showed that both planes' collision avoidance systems worked exactly as they should have and would have prevented the crash if the traffic controller had not intervened.
Known as a Traffic Alert and Collision Avoidance System, or TCAS, the on-board systems communicate with each other and plot out evasion maneuvers for each airplane.
Slightly less than one minute before the crash, the systems aboard both planes barked out a preliminary warning of "traffic, traffic."
About 15 seconds after the first warning, the system aboard the DHL cargo plane issued an order of "descend, descend." The system aboard the Russian Tu-154 issued an order of "climb, climb."
But in a moment of confusion, the traffic controller in Zurich ordered the Russian plane to "descend level 350, expedite descent" - a direct and urgent order to drop from an altitude of 36,000 feet to 35,000 feet.
The Russian pilot appears to have hesitated briefly, but he began his descent about 14 seconds after the controller's first warning and 30 seconds before the crash.
The tape disclosed today makes it clear that the Russian plane was grappling with conflicting instructions that came almost simultaneously, and aviation experts said this was probably the reason the pilot hesitated before reacting.
"He knows the air traffic controller is telling him to descend, and he knows the TCAS system is telling him to climb, and that is a difficult spot to be in," said Peter Quaint-Mere, a former air traffic controller who is now the director of Global Aviation Audit, a consulting firm that specializes in airport safety. "Pilots tend to listen to the air traffic controller because they trust a human being and know that a person wants to keep them safe."
But Georg Fongern of Cockpit, the German pilots union, said that if the TCAS does come on during a flight, the pilot should give that top priority. "We train for that," he said. "Ultimately, the safety of the aircraft is the pilot's responsibility."
New details about the last minutes before the crash also provide hints about why the Swiss control center had a number of problems and distractions. Swiss officials acknowledged last week that an automated alarm system in the control center, known as a Short-Term Conflict Alert, was switched off for maintenance work at the time of the crash.
In a possible breach of its procedures, the control center allowed one of the two controllers on duty to take a break at the time. The remaining controller was supervising five planes, and communicating on two radio frequencies at the same time - his own and his colleague's. The Swiss have not disclosed the controller's identity.
German investigators also discovered that the Swiss controller had telephone problems. The control center's main line was out of order. A backup line was available to controllers, for use in talking with other air control centers. The controller made repeated and unsuccessful efforts to call a neighboring control center in Germany until less than two minutes before the crash, though it was unclear where he was calling or why.
Meanwhile, controllers in Karlsruhe were desperately trying to reach the Zurich controller to warn of the planes' collision course. The Karlsruhe controllers said they spotted the potential problem at 11:33 p.m., nearly two minutes before the crash.
The German controllers "tried vainly" to reach the Zurich controller, but they received only busy signals every time they called, the German investigators said, adding, "The controller tried repeatedly to establish a connection via the priority button, but it failed."
European aviation experts say the air crash was a result of an increasingly acute shortage of qualified air traffic controllers. The West European air traffic authorities say they need about 1,800 to 2,000 controllers in addition to the 18,000 already in service. In Germany alone airports need 100 more controllers, experts say, and the country has already begun importing Irish controllers.
Air traffic has been increasing steadily by about 6 percent a year for the last decade. In January, to accommodate more airplanes, the European authorities reduced the minimum vertical distance between planes from 2,000 to 1,000 feet.
Air Traffic Controllers-European Union Coordination, a group representing controllers, warned today that attempts to do more with fewer people had increased risks.
"The first pieces of information we have gathered so far lead us to believe that similar accidents could happen again, anytime and anywhere in Europe," the group said. Some airports, it continued, have been "forced to give priority to productivity at all costs and to short-term capacity results."