Chinese authorities on Sunday recovered the second black box from a China Eastern Airlines jet that plunged into a mountainside last Monday with 132 people on board, state media reported.
The recovery of the flight data recorder (FDR) from the crash site follows the retrieval of the cockpit voice recorder (CVR) on Wednesday.
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 CVR for pilot voices and cockpit sounds, and a 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?
Both of the China Eastern jet's black boxes were manufactured by Honeywell, Chinese accident investigator Mao Yanfeng told reporters on Wednesday, without naming the models.
The CVR installed at the rear of the passenger cabin had a recording time of two to three hours, with four channels including the captain and first officer, a backup channel and one for ambient noise in the cockpit, he said.
The China Eastern plane crashed 66 minutes after take-off.
The FDR at the back of the cargo bay had a recording time of 25 hours including around 1,000 data parameters, Mao said.
Where will the data be read?
The CVR, discovered first, was sent to an institute in Beijing for decoding.
The recording material from the CVR appeared to have survived the crash in relatively good shape, Mao said.
It could take 10 to 15 days to arrive at a preliminary analysis, and longer before a final conclusion can be presented in a report, according to Chinese state media.
The FDR will be sent to Beijing for checks on Sunday, according to state media.
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 usually result from a combination of factors.
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