* German Airbus destroyed in remote area of ravines
* First black box damaged but viable
* Terror attack not seen likely
* Leaders of France, Germany, Spain to visit crash area
* Victims include German school party, opera singers

Reuters/Seyne-Les-Alpes, France

French President Francois Hollande and German Chancellor Angela Merkel arrived together by helicopter on Wednesday in the remote Alpine region where an Airbus plane smashed into a mountain, ahead of an international tribute to the 150 victims.

The two leaders were flown over the nearby ravine where the Germanwings airliner came down on Tuesday, Hollande's office said. Spain's Mariano Rajoy later joined them in the village of Seyne-les-Alpes, headquarters of search operations.

Germanwings said 72 Germans were killed in Tuesday's crash, the first major air passenger disaster on French soil since the 2000 Concorde accident just outside Paris. Spanish officials said 49 Spaniards were among the victims.

Hollande, Rajoy and a visibly moved Merkel thanked search teams and were due to meet families of victims of the still-unexplained crash, which followed a sharp descent of the Airbus A320. A simple tribute was planned later in the day.

The ceremony will take place on a site with a view in the distance of the mountain against which the Airbus crashed. French officials arranged it to give the families a mental image of the area in which their loved ones died.

Earlier Lufthansa said it could not explain why the Airbus run by its low-cost Germanwings unit crashed. Investigators said the remoteness of the crash site meant it could be days before a clear picture of the tragedy emerged.

However they said the fact that debris was restricted to a small area showed the A320 was not likely to have exploded in mid-air, suggesting a terrorist attack was not to blame.

"It is inexplicable this could happen to a plane free of technical problems and with an experienced, Lufthansa-trained pilot," Lufthansa chief executive Carsten Spohr told reporters in Frankfurt.

Lufthansa said the 24-year-old plane had on Monday had repairs to the hatch through which the nose wheel descends for landing. A spokeswoman said that was not a safety issue but that repairs had been done to reduce noise.

Police and forensic teams on foot and in helicopters investigated the site about 100 km (65 miles) north of Nice where the airliner came down en route to Duesseldorf from Barcelona.

"When we go to a crash site we expect to find part of the fuselage. But here we see nothing at all," said pilot Xavier Roy, coordinating air operations.

Roy said teams of investigators had been dropped by helicopter onto the site and were working roped together at altitudes of around 2,000 meters (6,000 feet).

It would take at least a week to recover all the remains of the victims, he said.

No distress call was received before the crash, but French authorities said one of the two "black box" flight recorders, the cockpit voice recorder, had been recovered, albeit in need of repair.

"The black box has been damaged. We will have to put it back together in the next few hours to be able to get to the bottom of this tragedy," Interior Minister Bernard Cazeneuve told RTL radio.

Beyond Germans and Spaniards, victims included an American, a Moroccan and citizens of Britain, Argentina, Australia, Belgium, Colombia, Denmark, Israel, Japan, Mexico and the Netherlands, French officials said. However DNA checks to identify them could take weeks, the French government said.

FLIGHTS CANCELLED

Germanwings said it cancelled one flight on Wednesday and was using 11 planes from other carriers for about 40 flights after some of its crew members had refused to fly.

Employees laid candles and flowers by Germanwings headquarters at Cologne/Bonn airport, while Lufthansa and Germanwings staff worldwide held a moment of silence at 10:53 a.m. local (0953 GMT) - the moment the plane went missing.

Among the victims were 16 teenagers and two teachers from the Joseph-Koenig-Gymnasium high school in the town of Haltern am See in northwest Germany. They were on their way home after a Spanish exchange visit near Barcelona.

The school held a day of mourning on Wednesday. A hand-painted sign said simply: "Yesterday we were many, today we are alone."

"On Tuesday last week we sent off 16 happy young people, with two happy young colleagues, on a journey and what was meant to be a happy trip ... has ended in tragedy," headmaster Ulrich Wessel told reporters.

Barcelona's Liceu opera house said two singers, Kazakhstan-born Oleg Bryjak and German Maria Radner, died while returning to Duesseldorf after performing in Wagner's Siegfried.

Germanwings said on Tuesday the plane started descending a minute after reaching cruising height and lost altitude for eight minutes. Experts said that while the Airbus had descended rapidly, it did not seem to have simply fallen out of the sky.

The A320 is one of the world's most used passenger jets and has a good safety record.

At 24 years old, the plane was older than many other planes at Lufthansa, where the average age of its fleet is 11.5 years. The plane was delivered to Lufthansa in 1991 and had clocked up around 58,000 flight hours over the course of over 46,000 flights, Airbus said.