Flight data and cockpit voice recorders on the Jeju Air jet that crashed on Dec 29, killing 179 people, stopped recording about four minutes before it crashed, South Korea's transport ministry said on Saturday (Jan 11).

Here are some details on black boxes and moves to improve them.

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 have evolved from early devices using wire, foil or magnetic tape to digital chips inside bright metallic casings.

They are mandatory and the aim is to preserve clues from cockpit sounds and data to help prevent future accidents, but not to determine any civil or criminal liability.

There are two recorders: a Cockpit Voice Recorder (CVR) for pilot voices or cockpit sounds and a separate Flight Data Recorder (FDR).

In broad terms, investigators say the FDR helps them analyse what happened and the CVR can - but not always - start to explain why. But experts caution that no two probes are the same and virtually all accidents involve multiple factors.

HOW BIG ARE THEY?

They weigh about 4.5kg and contain four main parts: 

* a chassis or interface designed to fix 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 and able to withstand forces equivalent to 3,400 times the feeling of gravity

* this housing contains the recording media which nowadays are finger-nail sized chips on circuit boards.

HOW ARE RECORDERS 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 must be decoded from raw files before being turned into graphs.

HOW MUCH INFORMATION IS AVAILABLE?

The capacity of flight recorders has been debated for years as authorities weigh improvements against the cost and the risk of inadvertently creating other problems, such as drawing power from other systems needed in an emergency. Cockpit monitoring has also been a sensitive topic with pilot unions.

FDRs must record at least 88 essential parameters but modern systems can typically track 1,000 or more additional signals.

The CVR usually contains two hours of recordings on a loop but this is being extended to 25 hours.

Implementing such regulatory changes can take years. 

A spate of accidents in which recorders stopped working when onboard electrical power was lost, including an Egyptair flight from New York to Cairo in 1999, led the US National Transportation Safety Board to recommend enough backup power to provide 10 minutes of extra recording.

The Federal Aviation Administration proposed the change in 2005 and it was adopted for new planes delivered from 2010, eight months after the 737-800 involved in the Jeju crash left the Boeing factory, according to data from FlightRadar24.

Pressure to lengthen the loop of voice data to 25 hours to reflect trans-oceanic flights began with French recommendations following the crash of Air France 447 in 2009, and accelerated after the disappearance of Malaysia's MH370 in 2014.

Last year, the US Federal Aviation Administration Reauthorization Act included the 25-hour requirement for cockpit voice recorders, echoing previous decisions in Europe.