In aircraft, the flight data recorder (FDR) and the cockpit voice recorder (CVR) are used to record aircraft and pilot behavior in order to analyze accidents, and are usually called "black boxes" by the news media.

The main purpose of the device is to reduce the manufacturer's liability by correctly assigning blame to the failing assemblies or persons. The famous, secondary use is to analyze accidents to prevent future failures.

Traditional flight data recorders use potentiometers attached to strings. The strings are actually steel wire, bolted to the controls and flight surfaces. As the surfaces move, the resistances of the pots change, changing the frequency of an oscillator. The oscillator's electronic howl is wired to a recorder. The howl is better than just recording a voltage because the howl cannot be affected as easily by stray currents or magnetic fields. Since the tape or wire's recording speed can be erroneous, usually the data is recorded for a fixed time, and then the howls are turned off for a bit. This turns the howls into countable electronic pulses on the tape or wire, with higher howls having more pulses per interval. The pulses, along with the pilot's incoming and outgoing audio, are recorded on a recirculating loop of magnetic tape or wire. In the earliest recorders, wire was preferred because it is more resistant to fire. Later, high-temperature magnetic tape and insulated housings were developed.

Digital FDRs usually eavesdrop on the aircraft's main data bus. They record the most significant 8 bits of (usually) 88 parameters, including the control and actuator positions, engine information and time of day. There are 88 parameters required as a minimum by US Federal legislation, but some systems monitor up to 300 variables. Generally each parameter is stored once per second, though some units store "bursts" of data if it begins to change quickly. Most FDRs record more than a day's worth of data. Since a day has 86,400 seconds, a basic black box only needs about 7.6 megabytes of data storage. Most units use flash memory, which only permits memory to be written about 100,000 times. Although this would be impractical in a general purpose computer, a FDR is only rewritten once per day, so it will last 270 years.

The cockpit voice recorder (mandatory in the USA since 1965) does not store an entire day of data. The system uses a single microphone to collect all 'noise' in the cockpit, not only conversation but also audible warnings, engine noise, and other mechanical sounds. In commercial airliners the microphone is usually mounted above and between the two pilots. CVRs can be analogue or digital. Analogue systems record only thirty minutes of cockpit noise - a direct play onto a continuous loop of magnetic tape. The relative fragility of magnetic tape made the development of robust solid state systems necessary. These systems can store 120 minutes of noise or more. Some sources are pushing for an additional video input to the CVR.

Traditionally, black boxes are placed in the tail of the aircraft. In this position, the entire front of the aircraft acts as a "crush zone" to reduce the shock that reaches the recorder. Also, modern black boxes are double wrapped, in strong corrosion-resistant stainless steel or titanium, with high-temperature insulation inside. Additionally, since they are often crushed into unreachable pieces, or never located, some modern units are self-ejecting, and brightly colored (yellow or orange), with radio and sonar beacons (Underwater locator beacon, ULB, pinging at 37.5 KHz). FAA regulations require both recorders to withstand 1100 ° C for thirty minutes, the water pressure at 20,000 feet deep and 3,400 gravities for 6.5 ms.

An example FDR (The L3 FA2100) is 140x127x320 mm and masses 4.54 kg. The L3 FA2100 CVR is identical in size but 50 g lighter.

Black boxes are often poorly maintained, because they are not critical to flight and have six-year batteries. FAA regulations do not require a regular test. There is a growing sentiment that they should be reliable, solid-state digital systems (a digital flight data recorder, DFDR).

Significant FDR manufacturers include Allied Signal, Fairchild Aviation / L3, Honeywell, and Sundstrand.

See also

• Air safety
• EPIRB Emergency position-indicating rescue beacon.
• Compare Quick access recorder

External links

• The Black Box: An Australian Contribution to Air Safety (http://www.dsto.defence.gov.au/corporate/history/jubilee/blackbox.html)
• Flight Data Recorders: How They Work and What They Reveal (http://abcnews.go.com/sections/scitech/DailyNews/blackbox011114.html)
• How Black Boxes Work (http://www.howstuffworks.com/black-box.htm) (Seven page article with links)
• If aircraft "black boxes" are indestructible, why can't the whole plane be made from the same material? (http://www.straightdope.com/classics/a4_001.html) (from The Straight Dope)
• CVR recordings and transcripts (http://www.airdisaster.com/cvr/)