China Eastern Airlines flight MU5735 sharply dropped from an altitude of 29,100 feet to 9,075 feet in just two minutes and 15 seconds, crashing into a mountainous area in south China's Guangxi Zhuang Autonomous Region on Monday, and two days later one of its two black boxes was retrieved.

What happened to the plane? Will the black box provide answers? CGTN asked experts to explain the finding of the black box. 

One retrieved black box believed to be the cockpit voice recorder

The black box recovered from the China Eastern Airlines flight MU5735 is believed to be the cockpit voice recorder, an aviation official told a press briefing on Wednesday night.

The exterior of the black box has been severely damaged, but its data storage unit is relatively complete despite the damage, head of the aviation safety office of the Civil Aviation Administration of China (CAAC), Zhu Tao told reporters. The device is being sent to Beijing for decoding.

Mark Weiss, a retired American Airlines captain and President of Weiss Consulting Group, told CGTN that the Boeing 737-800 aircraft is one of the most popular models in the Boeing family which is different from the banned Boeing 737 MAX.

Boeing 737-800 aircraft are equipped with two flight recorders in high-visibility orange color produced by the U.S. company Honeywell. The two can record flight data and cockpit audio respectively, withstanding high shock and high temperatures as well as seawater immersion.

Weiss said one of them is a cockpit voice recorder (CVR), installed at the rear of the cargo compartment, and it starts recording when the aircraft is powered on and stops when it is powered off. The CVR is capable of recording two to three hours of digital audio.

The other one is a flight data recorder (FDR), installed at the rear of the cabin, and it 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.

Vertical crash unusual, black boxes crucial to find out why

Professor Hu Xiaobing of the School of Safety Science and Engineering of the Civil Aviation University of China told CGTN that this crash was very unusual as it was vertical. It seems to him, for some reason unknown yet, that the pilots may have become unconscious during the steep descent.

According to historical experience, there are many possible causes that could lead an airplane to descend steeply, such as technical failures like engine malfunction, airplane body structure damage, or even a bird strike. Sometimes human factors such as pilot error or a terrorist attack could also cause an airplane crash.

However, Hu pointed out that it's too early to comment on the cause of this tragedy, and the public should wait for the official investigation result which will largely rely on the black boxes.

If there is any technical failure causing the crash, the FDR could usually provide some details because it records important data about on-board equipment, said the professor.

If there is any human factor causing the crash, the CVR could give some clues because it records cockpit conversations. Therefore, black boxes are crucial for the investigation.

This is why, Hu stressed, the black boxes are strongly built to survive airplane crashes. For example, they can survive a 500 kilometers per hour airplane crash, tolerate temperatures up to 1,100 degrees Celsius, as well as half an hour of water pressure about 6,000 meters underwater.

Weiss echoed that the CVR retrieved should have recorded every single sound of the area as there were numerous microphones turned on once the engine started. The CVR is going to show a lot of information, and once we put information about the two black boxes together it's going to give the whole picture of what had happened in the airplane.

Investigation underway with experts on-site

So far, no survivors have been found among 123 passengers and nine crew members aboard the ill-fated plane. 

About 20 civil aviation technical experts on Wednesday arrived at the plane crash site to carry out an investigation into the crash.  

Mao Yanfeng, head of the aviation accident investigation center of the CAAC, told at a news briefing on Wednesday that the weather was normal and there were no hazardous weather conditions when the jetliner crashed.

According to the air/ground communication records between the aircrew and air traffic controllers, they had maintained normal communication since the plane took off from Kunming until it suddenly dived to crash en route to its destination, Mao added.

The Boeing 737 aircraft was delivered to China Eastern Airlines on June 22, 2015. Before its take-off on Monday, it met maintenance release standards and airworthiness requirements, according to Sun Shiying, the chairman of China Eastern Airlines' Yunnan branch. 

All three pilots, including one captain and two co-pilots, had flight licenses and health certificates within the validity period, with good health and complete flight experience, also said Sun, adding that "from what we know at present, their families were in harmony."

Before the crash, China's airlines had marked a safety flight record of over 100 million continuous hours over 4,000 days, according to the CAAC.

This was the best safety performance in the history of China's civil aviation industry, one of the world's best over that period, CAAC's Zhu Tao said, noting that the quality of safety operation of China's airlines had been continuously improved.

(CGTN reporters Xia Cheng and Gasthoori Manickam, and video editors Zhang Chunnan, Li Yahui, Mu Xile and Wu Chutian contributed to this story.)

Read More:

Explainer: How will flight MU5735 crash investigation be carried out?

Expert: 10 to 15 days needed to decode black box of crashed plane

CGTN liveblog on China plane crash