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Chinese searchers found on Wednesday one of two black boxes from a China Eastern Airlines (600115.SS) plane that crashed this week in forest-clad mountains with 132 people onboard, the aviation regulator said.

The device was severely damaged, and it was not immediately clear if it was the flight data recorder or the cockpit voice recorder, an official of the Civil Aviation Administration of China (CAAC) told a media briefing.

Flight MU5735 was en route from the southwestern city of Kunming to Guangzhou on the coast on Monday when the Boeing 737-800 jet suddenly plunged from cruising altitude at about the time when it should have started its descent ahead of landing.

Most of the jet appears to have disintegrated upon impact and no survivors have been found.

The cause of the crash has yet to be determined, with aviation authorities warning that their investigation would be very difficult because of the severe damage to the aircraft.

"We found a black box on the scene but this black box has been severely damaged on the outside, so the investigation team on the front-line is still ascertaining whether it is the flight data recorder or the cockpit voice recorder," Mao Yanfeng, head of aircraft investigation at CAAC, told the briefing.

Weather along the flight path on Monday did not pose any danger to the aircraft, and air controllers had maintained communication with it after its take-off and prior to its rapid descent, Mao told reporters.

During its descent, the plane did not respond to repeated calls from the air controllers, CAAC said previously.

The jet had met airworthiness standards before take-off, and the three pilots - one more than normally required on a 737 - were in good health, authorities have said.

The captain was hired in January 2018 and had 6,709 hours of total flying experience, while the first and second officers had 31,769 hours and 556 hours, respectively, a China Eastern official told the briefing.

"From what we know, the performance of the three pilots had been good and their family life relatively harmonious," said the official.

In response to the crash, China Eastern has tightened precautions, requiring two senior captains and a senior co-pilot on a three-person crew on some aircraft types, the state-backed The Paper reported.

China Eastern and two subsidiaries have grounded their fleet of more than 200 Boeing (BA.N) 737-800 jets.

China had made great strides in improving air safety standards over the past two decades, and Monday's disaster was the first major crash in a dozen years.

SAFETY

Heavy rain in southern China on Wednesday hampered the search for victims and flight information black boxes, with wet weather forecast to last for the rest of the week.

Grief-stricken relatives of those on board visited the crash site, among them was a retiree surnamed Zhang from Shenzhen whose eyes filled with tears as he told Reuters that his nephew was on the flight.

"I hope the country can thoroughly investigate this matter and find out whether it was the manufacturer's fault or it was a maintenance problem," Zhang said.

The CAAC has launched a two-week inspection of the sector in response to the crash involving checks at all regional air traffic control centres, airline companies and flight training institutes to ensure "absolute" safety.

FlightRadar24 data showed the aircraft plunged at a rate of 31,000 feet per minute - the height of a 50-storey building every second.

The China Eastern official told the briefing the grounding of the 737-800 fleet was not necessarily in response to any safety issue but was more of an emergency reaction to the crash.

The disaster comes as Boeing seeks to rebound from several crises, notably the impact of the coronavirus pandemic on air travel and safety concerns over its 737 MAX model following two deadly crashes.

The last commercial jetliner to crash in mainland China was in 2010, when an Embraer E-190 regional jet flown by Henan Airlines went down. (reuters)