PARIS — For about 10 minutes on Tuesday morning, the Germanwings jetliner with 150 people on board hurtled downward from relatively clear skies above the French Alps.
The rate of descent from its cruising altitude of 38,000 feet was alarming but still gradual enough to indicate that the twin-engine Airbus A320 had not suffered any catastrophic damage and might have remained under the control of the pilots or the autopilot. At no point during the descent was there any communication from the cockpit to air traffic controllers or any other signal of an emergency.
When the plane plowed into craggy mountains northeast of Nice, it was traveling with enough speed that it was all but pulverized, killing the 144 passengers and crew of six and leaving behind almost no apparent clues about what caused the crash.
On Wednesday, French officials struggled to solve that mystery as the leaders of France, Germany and Spain converged on the remote site and families, friends and colleagues of the victims mourned around the world.
Investigators, constrained by the difficult topography of the crash location, the condition of the wreckage and technical difficulties, were unable to provide immediate answers, leaving room for speculative theories.
The French aviation authorities initially struggled to extract useful information from the cockpit voice recorder, which had been recovered from the wreckage on the mountainside six hours after the crash and was badly battered by the impact. They eventually succeeded in recovering an audio file, but did not say whether it was partial or complete or whether it contained any information to address what had happened.
At the crash site, a senior official working on the investigation said, workers found the casing of the plane’s other black box, the flight data recorder, but the memory card containing data on the plane’s altitude, speed, location and condition was not inside, apparently having been thrown loose or destroyed by the impact.
The flight’s trajectory ahead of the crash also left many unanswered questions.
Remi Jouty, the director of the French Bureau of Investigations and Analysis, said at a news conference that the plane took off at around 10 a.m. local time from Barcelona and that the last message sent from the pilot to air traffic controllers had been at 10:30 a.m., which indicated that the plane was proceeding on course.
But minutes later, the plane inexplicably began to descend, Mr. Jouty said. At 10:40 and 47 seconds, the plane reported its last radar position, at an altitude of 6,175 feet. “The radar could follow the plane until the point of impact,” he said.
Mr. Jouty said the plane slammed into a mountainside and disintegrated, scattering debris over a wide area, and making it difficult to analyze what had happened.
It often takes months or even years to determine the causes of plane crashes, but a little more than a year after the disappearance of a Malaysian airlines jetliner that has never been found, the loss of the Germanwings flight from Barcelona to Düsseldorf is shaping up to be particularly perplexing to investigators.
One of the main questions outstanding is why the pilots did not communicate with air traffic controllers as the plane began its unusual descent, suggesting that either the pilots or the plane’s automated systems may have been trying to maintain control of the aircraft as it lost altitude.
Among the theories that have been put forward by air safety analysts not involved in the investigation is the possibility that the pilots could have been incapacitated by a sudden event such as a fire or a drop in cabin pressure.
A senior French official involved in the investigation, who spoke on the condition of anonymity, said that the lack of communication from the pilots during the plane’s descent was disturbing, and that the possibility that their silence was deliberate could not be ruled out.
“I don’t like it,” said the French official, who cautioned that his initial analysis was based on the very limited information currently available. “To me, it seems very weird: this very long descent at normal speed without any communications, though the weather was absolutely clear.”
This official said that the lack of communication suggested that the pilots might have been incapacitated as a result of an onboard failure such as a loss of cabin pressure, which could have deprived the crew members of oxygen.
While all pilots are equipped with emergency oxygen masks, the pilots would first have to be aware that a depressurization had occurred, the official said.
“If for any reason they don’t detect the problem in time, they would black out,” the official said.
“So far, we don’t have any evidence that points clearly to a technical explanation,” the official said. “So we have to consider the possibility of deliberate human responsibility.”
Mr. Jouty said it was far too early in the investigation to speculate about possible causes.
“At this moment I have no beginning of a scenario,” Mr. Jouty said. However, he said there was not yet any evidence available that would support either a theory of a depressurization or of a midair explosion
Speaking on the French radio station RTL, Interior Minister Bernard Cazeneuve said on Wednesday morning that terrorism was “not a privileged hypothesis at the moment,” but that no theories had been definitively excluded. Mr. Cazeneuve said the size of the area over which debris was scattered suggested that the aircraft had not exploded in the air but rather had disintegrated on impact.
Lufthansa, the parent company of Germanwings, has characterized the crash as an accident. The airline has not disclosed the identities of the pilots, except to say that the captain was a 10-year veteran with more than 6,000 hours of flying time in A320s.
The French Bureau of Investigations and Analysis, which is leading the technical inquiry into the crash, sent seven investigators to the crash site on Tuesday. They have been joined by their counterparts from Germany, as well as by technical advisers from Airbus and CFM International, the manufacturer of the plane’s engines.
Speaking on Europe 1 radio, Jean-Paul Troadec, a former director of the French air accident investigation bureau, said one of the big challenges for investigators would be to protect the debris at the crash site from any inadvertent damage.
“We need to ensure that all the evidence is well preserved,” Mr. Troadec said, referring both to the pieces of the plane littered across the steep slopes as well as to the remains of the victims. The identification of the victims will most likely require matching DNA from the remains with samples from relatives.
The recovery effort will be a laborious task, given the state of the wreckage, the difficult terrain and the fact that the crash site is so remote that it could be reached only by helicopter.
Cabin depressurization, one of the possibilities speculated about on Wednesday, has occurred before, perhaps most notably in the crash of a Cypriot passenger plane in 2005 that killed all 121 people on board as it approached Athens. In that case, Helios Airways Flight 522, a slow loss of pressure rendered both pilots and all the passengers on the Boeing 737 jet unconscious for more than three-quarters of an hour before the aircraft ran out of fuel and slammed into a wooded gorge near the Greek capital.
Investigators eventually determined that the primary cause of that crash was a series of human errors, including deficient maintenance checks on the ground and a failure by the pilots to heed emergency warning signals.
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