Explainer: How black boxes preserve vital clues to air disasters

JejuAir Co

12/01/2025 13:08

By Tim Hepher
       Jan 12 (Reuters) - 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.
    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 10 pounds (4.5 kilos) 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 U.S. 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 U.S. Federal Aviation Administration
Reauthorization Act included the 25-hour requirement for cockpit
voice recorders, echoing previous decisions in Europe.
     

(Reporting by Tim Hepher
Editing by Ros Russell)
((mailto:tim.hepher@thomsonreuters.com; +33 1 49 49 54 52;
Reuters Messaging: rm://tim.hepher.thomsonreuters@reuters.net/))