French officials said today there were no signs of problems with Air France Flight 447 before takeoff and it was unclear whether the chief pilot was at the controls when the plane plunged into the Atlantic Ocean.

The head of France's accident investigation agency, Paul-Louis Arslanian, also said he was "not optimistic" that rescuers would recover the plane's black boxes miles underwater.

Pilots on long-haul flights often take turns at the controls to remain alert. Asked whether the chief pilot was in the cockpit when the plane went down, Arslanian told a news conference in France that there was no information either way.

"We don't even know the exact time of the accident," he said.

But Arslanian said there was no indication of problems with the plane before it left Rio de Janeiro on Sunday night en route to Paris.

At the search scene Brazilian navy divers rushed to reach the wreckage and start retrieving debris from the Atlantic.

Four navy ships with recovery equipment and a tanker were heading to a three mile (5km) strip of water strewn with plane seats, an orange buoy, wiring, metal pieces and jet fuel stains about 745 miles north-east of the coastal city of Recife.

As the navy battled rough weather, officials warned of the difficulty in finding the flight data and voice recorders – the black boxes – that hold clues to why the plane crashed during a severe storm in the middle of the night.

Officials said the recorders needed to identify the causes of the mysterious crash could be on the ocean floor at a depth of 6,600 to 9,800 feet (2,000 to 3,000 metres).

The recorders are designed to send homing signals for up to 30 days when they hit water.

One expert said it could be among the hardest recoveries since the decades-long search to find the Titanic.

"If you think how long it took to find the Titanic and that the debris would be smaller, you are looking for a needle in haystack," said Derek Clarke, joint managing director of Aberdeen-based Divex, which designs and builds military and commercial diving equipment.

However, the Brazilian president, Luiz Inacio Lula da Silva, said he was confident that the black boxes – two separate devices containing cockpit voice recordings and instrument data – offer the best chance of finding out why the Airbus jetliner vanished. .

"I think a country that can find oil 6,000 metres under the ocean can find a plane 2,000 metres down," he told reporters yesterday in Guatemala, referring to recent oil finds by Brazil's state energy company in ultra-deep waters.

Officials from the US National Transportation Safety Board and from Eurocontrol say that investigators have many ways to begin investigating the accident even before they recover any wreckage or the black boxes.

"Investigators will have to do a forensic analysis, by piecing together all available information as best they can," said Jim Hall, a former chairman of the NTSB.

They will review the maintenance records of the aircraft, interview the crews who flew the plane in the last few weeks and go to the locations where recent maintenance was done to interview mechanics.

They will also study the personal histories of the crew members and reconstruct what they did in the last 36 hours before the crash.

"In other words, they'll be compiling as much background information as they can to compensate for the lack of other data," Hall said.

法国官员今天表示没有迹象表明法航447航班（Air France Flight 447）在起飞之前有问题，目前尚不清楚当飞机坠入大西洋的时候，机长是否在驾驶飞机。

法方事故调查局负责人保罗·路易斯·阿斯朗尼亚（Paul-Louis Arslanian）也表示，对于搜救人员能否找回在海面之下数英里的黑匣子“不乐观”。

飞行员往往在长途飞行中轮班以保持清醒。在法国召开的一个新闻发布会上，当被问及飞机坠落时，机长是否在飞机座舱里，阿斯朗尼亚表示对此没有任何信息。

“我们甚至不知道事故发生的确切时间”，他说道。

但是阿斯朗尼亚说在周六晚离开里约热内卢飞往巴黎之前，没有迹象显示飞机有任何问题。

在搜索现场，巴西海军潜水员正迅速赶往残骸发现地点，并开始打捞大西洋中的飞机残骸。

4艘携带搜索设备的海军舰艇和1艘油轮正赶往3英里（5公里）长的出事海域，地点位于海滨城市累西腓（Recife）东北745英里处，在那里发现了飞机座椅、一个橙色救生圈、电线、金属部件、以及飞机燃油油污带。

在海军同恶劣天气搏斗的同时，官员们提醒说，飞行数据和语音记录器（即黑匣子）将很难找到，而它记录了飞机在暴风雨午夜失事的线索。

官员表示，这只记录了神秘坠机原因的黑匣子，可能在深达6600到9800英尺的海底（2000到3000米）。

记录器在入水之后，还能在长达30天的时间里发回信号。

某专家表示，这可能是自数十年前搜索泰坦尼克号之后最困难的残骸回收任务之一。

“想想吧，寻找泰坦尼克号花了多长时间，现在呢，残骸更加小，如同海底捞针”，德里克·克拉克（Derek Clarke）说道，克拉克是一家位于阿伯丁（Aberdeen）的潜水服务公司的联合总经理，该公司设计制造军用或者商用潜水设备。

然而，巴西总统卢拉（Luiz Inacio Lula da Silva）表示对找到黑匣子有信心。黑匣子实际上是两个分别记录机舱通话和仪表数据的独立装置，能够给出空中客车消失的答案。

“我认为一个能够在海底6000米发现石油的国家能够在2000米找到飞机”，昨天在危地马拉，他以巴西国营能源公司发现深海油田为例对记者如是说。

来自美国国家运输安全委员会（US National Transportation Safety Board）和欧洲空中航行安全组织（Eurocontrol）的官员说调查人员有很多方法展开事故调查，甚至在找到任何遗骸或者黑匣子之前就能展开。

前NTSB主席吉姆·豪尔（Jim Hall）说道：“如同展开一场法医调查，调查人员需要尽全力将所有信息组织起来。”

他们将核查飞机维护记录，采访最近几周驾驶过该飞机的机组人员，并且前往最近维护的地点采访那里的机械师。

他们还会研究机组人员的个人历史，并重现在事故36个小时前他们的所作所为。

豪尔说：“换句话说，他们将尽可能把背景信息组织起来，以弥补其他信息的缺失。”