Officials say the recording device is damaged as rescuers are forced to pause search efforts because of heavy rain.
One of the two black box recorders from the crashed China Eastern airliner has been recovered as investigators try to piece together what made the jet carrying 132 passengers crash in southern China.
The device is so damaged that investigators were not able to tell whether it is the flight data recorder or the cockpit voice recorder, Mao Yanfeng, director of the accident investigation division of the Civil Aviation Authority of China, said on Wednesday.
Mao said an all-out effort is being made to find the other data recorder. It was not clear if the damage to the recovered one would limit its usefulness.
Recovering the so-called black boxes, which are usually painted orange for visibility, is considered key to figuring out what caused plane crashes.
The cause of the recent disaster has mystified aviation authorities who have scoured rugged terrain for clues, finding no survivors from the incident.
Officials have still refrained from declaring all of the passengers dead despite the pulverised mass of twisted metal and charred belongings found on the mountainside where it went down.
The Boeing 737-800 plane crashed near Wuzhou in southern China on Monday after losing contact with air traffic control.
Flight tracking website FlightRadar24 showed the jet sharply dropped from an altitude of 8,900 to 2,400 metres (29,100 to 7,850 feet) in just a minute.
After a brief upswing, it dropped again to 983 metres (3,225 feet), the tracker said.
‘My heart sank’
Relatives of passengers began arriving on Wednesday at the gate to Lu village just outside the crash zone, where they, along with reporters on the scene, were stopped by police and officials who used opened umbrellas to block the view beyond.
One woman was overheard saying her husband, the father of their two children, had been on board the flight.
“I’m just going in there to take a look. Am I breaking the law?” she said. The woman and a companion were then escorted away and reporters were told to stop filming.
Another man, who gave his surname Ding, said his sister-in-law had been on the plane. He said he hoped to visit the site but had been told little by the authorities.
“We’re just coming here to have a look,” said Ding. “My heart sank all of a sudden” upon hearing about the crash, he added. He too was escorted away.
On Wednesday, rescuers were forced to pause the search as rains raised risks to teams working in a zone where a large pit has been bored out by the impact of the aircraft.
A reporter for state broadcaster CCTV, given access to the crash area, said there were risks of “small-scale landslides” as rain destabilised the steep slopes.
Chinese President Xi Jinping was swift to order an investigation into the crash, dispatching senior Communist Party officials to the scene, including close aide Vice Premier Liu He.
The Civil Aviation Administration of China has said it will conduct a two-week safety inspection across the industry.
China Eastern said the crashed plane, which was nearly seven years old, had met all airworthiness requirements pre-flight. The company has headquarters in Shanghai and is one of China’s three largest carriers with more than 600 planes.
In August 2010, an Embraer ERJ 190-100 operated by China’s Henan Airlines hit the ground short of the runway in the northeastern city of Yichun and caught fire. It carried 96 people and 44 of them died. Investigators blamed pilot error.