The black boxes retrieved from the Boeing 737 that crashed in March, killing all 132 people on board, were so damaged that they failed to reveal any clue that could explain the plane’s deadly nosedive, the Chinese government said.
China Eastern flight MU5735 from Kunming to Guangzhou plunged into a terraced field in China's Guangxi province on 21 March, in what is believed to be the country’s first fatal crash in nearly three decades.
A cockpit voice recorder (CVR) was recovered from the crash site and a second black box was found on a mountain slope about 1.5m underground after a week-long search operation.
The Civil Aviation Administration of China (CAAC) in its preliminary crash report reportedly did not include any information from the CVR, which has been sent to Washington for analysis.
Although Beijing has been leading the investigation, it had invited the United States to take part because the Boeing 737 was designed and manufactured there.
"The two recorders on the plane were severely damaged due to the impact, and the data restoration and analysis work is still in progress," CAAC said in a statement.
Li Yong, deputy director of CAAC's aviation safety office, told Xinhua that "there are many difficulties in investigating this accident, and at the moment the available evidence is limited".
But the regulator found that most of the wreckage was concentrated in one area. It added that the crew were qualified, the jet was properly maintained, the weather was fine and no dangerous goods were on board.
The CAAC said the last normal call from controllers to the plane was made at 2.16pm local time while it was cruising at 29,200ft. Less than six minutes later, the plane disappeared from radar.
The agency said it had completed the preliminary report within the limit of 30 days, although a final report into the causes of the fatal crash could take two years or more to compile.
While Beijing is known to not make reports on accidents accessible to the public, experts say CAAC's statement marks a step toward transparency.
As a precautionary measure, China Eastern - one of China’s four major airlines, and its subsidiaries grounded all their 223 Boeing 737-800 aircrafts before resuming services on Sunday.
The 737-800 is a predecessor to the 737 MAX, which has not resumed commercial service in China for over three years after two crashes in Indonesia and Ethiopia.
"Two questions you'd have to look at: did that piece coming off cause the dive or did the dive cause that piece to go off," Anthony Brickhouse, an air safety expert at Embry-Riddle Aeronautical University, told Reuters.
"These boxes are designed to be really, really robust. I really can't think of an accident in recent history where we found the boxes and we didn't get information from them,” Mr Brickhouse added.