A Flight Over China in Clear Skies, Followed by a Nosedive

China Eastern Airlines Flight 5735 was at 29,100 feet in clear afternoon skies over the hills of southern China, seemingly ready for a smooth landing in Guangzhou. The plane was seven years old. One of China's most experienced pilots was in the cockpit.

The aircraft plummeted at 2:20 p.m. on March 21.

The air traffic controllers made frantic calls. The pilots did not send mayday messages. The plane fell in less than a minute. After gaining 1,200 feet of altitude in 15 seconds, it descended into a hillside covered in bamboo groves and banana trees. The plane was almost vertical and approaching the speed of sound as it tore into the soft earth, with pieces of the aircraft driving as deep as 60 feet into the mud. Seven miles away are two pieces of a wingtip.

A seven-member team from the United States arrived Saturday and are trying to understand what happened in the final minutes before the plane crashed, killing all 132 people on board.

Every air crash investigation has its own challenges. The plane was obliterated by the high speed of its impact at a 90-degree angle, making this the worst air disaster in more than a decade. Rescuers recovered 49,117 pieces from the wreck at the end of the search last week.

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China has a tight grip on information, censoring discussion and speculation online, and allowing only limited coverage of the disaster. Much of the information about the crash has been tightly guarded by Chinese officials. The families of the victims have been monitored to prevent them from speaking with the media. The names of the pilots were published in a state-owned newspaper in Hong Kong.

The cockpit voice recorder and the data recorder are the most important parts of the plane. The recorders of the black boxes are designed to survive crashes, but experts say the impact of this one may have damaged them. The flight data recorder ended up five feet underground after firefighters removed a large tree root above it.

Peter Knudson, a spokesman for the N.T.S.B., said that the board was helping China download information from the cockpit voice recorder.

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The tragedy has raised questions about the country's flight safety record in an important year for China's top leader, who will be seeking a third term at a Communist Party congress later this year. China Eastern and its subsidiaries have grounded a third of their fleet for safety checks. The Politburo Standing Committee, the country's top ruling body, said last week that officials should determine the cause of the crash as soon as possible.

Some key plane components, including parts of the engines and landing gear, have been recovered by officials overseeing the search. They were trying to recreate the plane's flight path using data from air traffic control radars and also looking for clues about what luggage and mail were put on the plane.

The Convention on International Civil Aviation, a global agreement, states that preliminary conclusions should be released within 30 days of the crash. The agreement calls for the participation of representatives from the aircraft manufacturer and the transportation safety regulator from the manufacturer's home country. A team of investigators from the N.T.S.B. and Boeing arrived in China on Saturday, though it was not clear if they had to complete 14 days or more of coronaviruses sterilization in the country before starting work.

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State media reports about the crash have been limited. A division of Jinan Daily, a state-owned publication from a city 1000 miles from the crash site, published interviews with two farmers who said they saw black smoke and white smoke coming from the aircraft. The final seconds of the plane were recorded by a camera at a distant mine. There was no smoke in those images.

Peter Marosszeky, a semiretired airline executive engineer and Boeing technical adviser who is now the managing director of Aerospace Developments, said that nothing really makes sense with this one. Black smoke would suggest burning fuel, while white vapors would suggest leaking fuel, but they are unreliable in crash investigations.

Air crash experts study the details of a crash investigation for signs of a midair collision or explosion, mechanical problems aboard the aircraft or pilot suicide. Experts said that the odds of a terrorist bombing or other high-altitude accident were reduced by the fact that most of the plane crashed in one place. There were no traces of explosives on the debris of the crash, according to the fire chief.

The discovery by a farmer of a piece of the plane seven miles west of the crash site had initially raised speculation that the plane suffered a midair break up. Chinese authorities later confirmed that the fragment and a much smaller fragment found a mile away were pieces of one of the aircraft's winglets. Air crash experts said it was not surprising that lightweight parts could land far from the site.

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It is a winglet, so it will not come down like a piece of aluminum.

It wouldn't cause a nosedive if a winglet was damaged or lost, Mr. Goglia said.

Aviation Partners Boeing, a Boeing joint venture with a wingtips manufacturer that makes winglets, refused to comment on the discovery of winglet fragments.

Mr. Goglia has focused on the aircraft's vertical position at impact. The natural tendency of commercial aircraft is to level off in flight. The horizontal stabilizers on either side of the aircraft need constant, extreme force to achieve a nosedive.

The stabilizers control whether the plane goes up or down. The question is whether the nose of the plane was pushed down because of a technical malfunction or because of a pilot's decision.

In the China Eastern crash, the ability of the plane to fly almost perfectly straight down, without fluttering or gliding, helps show that, according to Martin Craigs, the chairman of the Aerospace Forum Asia, a Hong Kong-based trade group for aviation equipment suppliers.

The Germanwings crash was an example of a pilot suicide.

Amy Chang Chien and Li You were involved in research.