One of the black boxes of the crashed China Eastern Airlines flight MU5735 has been retrieved, an official said at a press conference on Wednesday.

There were two black boxes on the plane, recording flight data and cockpit audio respectively and one of them has been found, according to the official. 

He added that the black box was severely damaged, so it is unclear for now what kind of information it carries.

On the day of the air crash, there were no reports of dangerous weather on the route and the crew kept normal communication with air controllers before its sudden descent, Sun Shiying, Chairman of China Eastern Airlines Yunnan Co., Ltd., said at the press conference.

The three pilots of the aircrew were all in good health with qualified licenses and experience, Sun said, also noting that they performed well at work and had harmonious families. 

Two black boxes

Unlike the nickname of black boxes, the recorders are in high-visibility orange. These crash-survivable devices can withstand high shock, high temperatures and seawater immersion.

Boeing 737-800 aircraft are equipped with two flight recorders produced by the U.S. company Honeywell, the official said.

One of them is a flight data recorder (FDR), which is installed at the rear of the cabin and begins recording when the engine is started, and stops when the engine is stopped. With a total recording time of around 25 hours, FDR records about 1,000 parameters, including altitude, speed, angles and crew operations.

The other is a cockpit voice recorder (CVR), which is installed at the rear of the cargo compartment and starts recording when the aircraft is powered on and stops when it is powered off.

The CVR is capable of two to three hours of digital audio.

Experts arrive at plane crash site

About 20 civil aviation technical experts on Wednesday arrived at the site of Monday's plane crash to carry out an investigation into the tragedy, Xinhua reported. 

"We'll conduct the on-site investigation today to calculate the distribution of the wreckage, possibly expanding the investigation area based on the aircraft's trajectory," said one of the experts.

"As the site may have to be excavated soon, we'll try to complete as much of the investigation as possible today," the expert added.

Affected by continuous rainfall, the site is slippery and muddy, and the search and rescue work has been hindered to some extent.

A road leading to the core site of the accident has been built and weaved bamboo has been laid along the road to reduce the impact of the rain.

(With input from Xinhua)