Explained | Why did the China Eastern Airlines plane nosedive before crash? 

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The story so far: On March 21, a China Eastern Airlines Corporation flight (MU5735) on a domestic flight in China, from Kunming to Guangzhou, abruptly impacted the ground west of Guangzhou in a mountainous area. The aircraft had 123 passengers and nine crew on board. At a press briefing on March 23, the Civil Aviation Administration of China  (CAAC) said that one of the black boxes had been recovered but was “substantially damaged”. Suspected debris (a large metal strip) has also been found 10 km away from the crash spot, pointing to possible mid-air disintegration.

A CAAC briefing has said that based on a preliminary assessment, the black box device recovered is the cockpit voice recorder and that the “recording material appeared to have survived the impact in relatively good shape”. It is being decoded in Beijing, according to a report.

A black box must be able to withstand many accident scenarios. A document by the German Federal Bureau of Aircraft Accident Investigation says that ‘these devices are tested for extreme conditions such as an impact with a concrete wall at 750 kilometres per hour; a static load of 2.25 tons for at least five minutes, a maximum temperature of 1,100 degree Celsius (2,012 degree Fahrenheit) for one hour and water pressure of depths of up to 6,000 metres or over 19,500 feet’.

An aviation expert says the data files must be decoded first, as graphs, to get a sense of what happened. There is even the use of “spectral analysis” in some cases allowing experts and investigators to pinpoint even faint alarms or the hint of an explosion.

The U.S. National Transportation Safety Board (NTSB) — invited by China — is to take part in the investigation, as a U.S.-made aircraft is involved.

An aviation expert says that public and media pressure can be intense given that this is an age where people want instant answers. He cautions that getting an idea of what might have happened could take days, preliminary reports could appear after some months, while a detailed investigation could take even a year or more.

Flight MU5735 had left Kunming at around 1.10 p.m. local time and was scheduled to land at Guangzhou a little after 3 p.m. Introduced into service in June 2015, the Boeing 737-800 aircraft with CFM56 engines had completed a little over 18,000 hours of flight. A report has quoted a CAAC official as saying that the air controllers had been in contact with the crew throughout the flight — the implication is that there was no emergency/distress call. Later, the crew did not respond to controller attempts to contact them, which could point to a catastrophic event having occurred.