SYDNEY (REUTERS) - Chinese authorities said on Wednesday (March 23) they had recovered one of the black boxes of a China Eastern Airlines jet that plunged into a mountainside on Monday with 132 people on board.

The device recovered is the plane's cockpit voice recorder, based on an early assessment, an official from China's aviation regulator said, adding that the recording material appeared to have survived impact in relatively good shape.

Both of the jet's black boxes were manufactured by Honeywell , the regulator said, without naming the models.

This is how the black box readout process works.

What are black boxes?

They are not actually black but high-visibility orange. Experts disagree how the nickname originated but it has become synonymous with the quest for answers when planes crash.

Many historians attribute their invention to Australian scientist David Warren in the 1950s. They are mandatory.

The aim is not to establish legal liability but to identify causes and help prevent accidents.

How have they evolved?

The earliest devices recorded limited data on wire or foil. Modern ones use solid state memory.

The recordings are housed inside crash-survivable containers able to withstand 3,400 times the force of gravity on impact.

Both black box recordings were recovered from a Ethiopian Airlines 737 Max crash in March 2019 that, like the China Eastern plane, dove into the ground at a very high velocity.

How big are they?

They weigh about 10 pounds (4.5 kg) and contain four main parts:

- A chassis or interface designed to secure the device and facilitate recording and playback

- An underwater locator beacon

- The core housing or "Crash-Survivable Memory Unit" made of stainless steel or titanium

- Inside there, the recordings on chips or older formats.

There are two recorders: a cockpit voice recorder (CVR) for pilot voices and cockpit sounds, and a flight data recorder (FDR) that captures information on parameters including altitude, airspeed, heading and engine thrust.

How will the recorders be handled?

Technicians peel away protective material and carefully clean connections to make sure they do not accidentally erase data. The audio or data file must be downloaded and copied.

The data itself means nothing at first. It must be decoded from raw files before being turned into graphs.

Investigators sometimes use "spectral analysis" - a way of examining sounds that allows scientists to pick out barely audible alarms or the first fleeting crack of an explosion.

How much information is available?

It will depend on the exact models of the Honeywell recorders, which has not been disclosed.

An Air India Express 737-800 with two Honeywell black boxes that crashed in 2020 after overshooting a runway in heavy rain had a CVR capable of 120 minutes of digital audio, according to a final report into the crash.

That would be more than enough to cover the 66-minute China Eastern flight.

The FDR on the Air India Express plane recorded about 25 hours of flight data.

Where will the data be read?

The first black box discovered is being sent to an institute in Beijing for decoding, although how long that takes would depend on the extent of the damage, said Mr Zhu Tao, an official at China's aviation regulator.

When will results be made public?

Interim reports into a crash are published after a month but are often sparse. Deeper investigations take a year or more to complete. Experts say air accidents are usually caused by a cocktail of factors.