What Is a Black Box? The Two Flight Recorders That Help Investigators Understand What Happened
Daily Explainer: one everyday bit of aviation, in plain English.
When there is a serious aircraft accident, the news often says that investigators are searching for the "black boxes". The name is misleading. They are not black, and there are normally two of them rather than one. They are among the most important tools that accident investigators have, and the way they work is easier to understand than the name suggests.
Not black, and not one box
The devices are usually painted bright orange or another high-visibility colour so that they can be found in wreckage. The term "black box" is an old nickname whose origin is debated, and it stuck.
An airliner typically carries two recorders:
- The flight data recorder (FDR) records technical information about how the aircraft was flying.
- The cockpit voice recorder (CVR) records sound in the cockpit.
Sometimes the two functions are combined in a single unit, and some aircraft carry more than one set.
What the flight data recorder captures
The FDR collects a stream of data from the aircraft's systems. This includes basic flight parameters such as altitude, airspeed, heading and vertical speed, as well as control inputs and the positions of surfaces like flaps and rudder, engine settings and many other measurements. Modern recorders can capture hundreds or even thousands of parameters.
Together, the data allows investigators to reconstruct, second by second, what the aircraft was doing and how the systems were responding.
What the cockpit voice recorder captures
The CVR records audio in the cockpit: conversations between the pilots, radio calls with air traffic control, and background sounds such as alarms, switch clicks and engine noise. These sounds can help investigators understand what the crew knew and said at the time, and can reveal the timing of events that data alone may not show.
Newer standards require recorders to keep a much longer recording than the older two-hour loops, and many modern units keep around 25 hours of audio on a continuous loop, with the oldest recording overwritten by the newest.
Where are they on the aircraft?
On many airliners the recorders are mounted in the rear of the aircraft, in the tail section. This is generally the part of the aircraft that tends to experience the least severe impact forces in many accidents, which gives the units a better chance of surviving. The exact location varies by aircraft type.
How do they survive an accident?
The recorders are built to withstand extreme conditions. They sit inside a protective housing designed to survive high impact forces, fire, and deep-water pressure. Standards require them to survive tests that simulate crashes and prolonged exposure to heat and immersion.
How are they found?
Each recorder carries an underwater locator beacon. If the unit is submerged, the beacon sends out a regular acoustic ping that search teams can detect with specialist equipment. The beacon battery lasts for a limited time, which is why search teams are often in a race against the clock after an accident at sea.
What happens after they are recovered?
Investigators take the units to a specialist laboratory. The memory modules are carefully removed, cleaned and read out under controlled conditions. The data is then analysed, and the audio is transcribed. Investigators combine this with wreckage evidence, air traffic recordings, maintenance records and witness accounts. The results of an investigation, including conclusions, are published by the responsible authority, usually after a lengthy process.
Can they be read in real time?
Normally no. Standard recorders store their data onboard, and investigators read it after recovery. There has been wide discussion in the industry about additional methods of transmitting data, but the physical recorders remain the core source.
Why do they matter to passengers?
Recorder data does not only help with accidents. It supports safety work more broadly. Airlines routinely analyse flight data from normal flights to spot trends, such as unstable approaches, and to improve procedures. Lessons from investigations feed back into design, training and regulation, which is one reason aviation has become safer over the decades.
Quick answers
- Is the black box really black? No. They are usually bright orange.
- Are there two of them? Usually yes: a flight data recorder and a cockpit voice recorder.
- Do they always survive? They are designed to, and often do, but a recovery is never guaranteed.
- Who gets to hear the cockpit recording? Access to the recordings and transcripts is controlled by the investigating authority and the law, and details are generally not released in full to the public.
The takeaway
The "black box" is really a pair of tough, bright-orange recorders that keep a record of what the aircraft and the crew were doing. They give investigators the evidence they need to understand an event and to make flying safer for everyone.