The "black box" is a generic term for two recording devices carried aboard commercial airliners. The Flight Data Recorder (FDR) records a variety of parameters related to the operation and flight characteristics of the plane. The Cockpit Voice Recorder (CVR) records the voices of the flight crew, engine noise, and any other sounds in the cockpit. All large commercial airliners and certain varieties of smaller commercial, corporate, and private aircraft are required by law to carry one or both of these boxes, which generally cost between $10,000 and $15,000 apiece. The data these devices provide is often invaluable to experts investigating the events leading up to an accident. The recovery of the boxes is one of the highest priorities in any mishap investigation, second only to locating survivors or recovering the remains of victims. FDR information is also often used to study other aviation safety issues, engine performance, and to identify potential maintenance issues.
Example of a Cockpit Voice Recorder (CVR)
Black boxes first began to appear in the 1950s and became mandatory during the 1960s. These early devices used magnetic tape for data storage, much like that used in a tape recorder. As the tape is pulled over an electromagnetic head, sound or numerical data is recorded on the medium. Analog black boxes using magnetic tape are still present aboard many planes, but these recording devices are no longer manufactured. Newer recorders instead use solid-state memory boards, called a Crash Survivable Memory Unit (CSMU), that record data in a digital format. Instead of the moving parts present in older recorders, solid-state devices use stacked arrays of memory chips similar to a USB memory stick. The lack of moving parts eases maintenance while reducing the chance of a critical component breaking in a crash. Solid-state recorders can also save considerably more data than older magnetic tape devices and are more resistant to shock, vibration, and moisture.
Magnetic tape from within the FDR of EgyptAir 990 that crashed in 1999
Analog magnetic tape recorders are required to store four audio channels for at least 30 minutes while digital solid-state devices are required to record for two hours. Both types use continuous recording such that older information is written over as new data is collected beyond the maximum time limit.
Sample data recovered from a Flight Data Recorder
The data measured by the different sensors is collected by the Flight Data Acquisition Unit (FDAU). This device is typically located in an equipment bay at the front of the aircraft beneath the flight deck. The FDAU assembles the desired information in the proper format and passes it on to the FDR at the rear of the plane for recording. The Federal Aviation Administration (FAA) required the FDR to record between 11 and 29 parameters, depending on aircraft size, up to 2002 but now requires saving a minimum of 88 sets of data. Analog FDRs can save a maximum of around 100 variables while digital recorders are often capable of collecting over 1,000 parameters over the course of 25 hours.
Diagram of data flow to aircraft black boxes
A common misconception states that the black boxes are "indestructible." No manmade device is indestructible, and no material has ever been developed that cannot be destroyed under severe enough conditions. The black boxes are instead designed to be highly survivable in a crash. In many of the worst aviation accidents, the only devices to survive in working order are the Crash Survivable Memory Units (CSMUs) in the black boxes. The remainder of the recorders, including the external case and other internal components, are often heavily damaged.
Interior cut-away of a black box design
Current regulations require the black boxes to survive an impact of 3,400 g's for up to 6.5 milliseconds. This rapid deceleration is equivalent to slowing from a speed of 310 miles per hour (500 km/h) to a complete stop in a distance of just 18 inches (45 cm). This requirement is tested by firing the CSMU from an air cannon to demonstrate the device can withstand an impact force at least 3,400 times its own weight. The black boxes must also survive a penetration test during which a steel pin dropped from a height of 10 ft (3 m) impacts the CSMU at its most vulnerable point with a force of 500 pounds (2,225 N). In addition, a static crush test is conducted to demonstrate that all sides of the CSMU can withstand a pressure of 5,000 pounds per square inch (350 kg/cmï¿½) for five minutes. The fire resistance of the CSMU is further tested by exposing it to a temperature of 2,000ï¿½F (1,100ï¿½C) for up to an hour. The device is also required to survive after lying in smoldering wreckage for ten hours at a temperature of 500ï¿½F (260ï¿½C).
Underwater Locator Beacon on a black box
Upon completion of the testing, the black boxes are disassembled and the CSMU boards are extracted. The boards are then reassembled in a new case and attached to a readout system to verify that the pre-recorded data written to the device can still be read and processed.
Another factor important to the survivability of the black boxes is their installation in the tail of the aircraft. The exact location often varies depending on the plane, but the FDR and CVR are usually placed near the galley, in the aft cargo hold, or in the tail cone. The recorders are stored in the tail since this is usually the last part of the aircraft to impact in an accident. The entire front portion of the plane acts like a crush zone that helps to decelerate the tail more slowly. This effect reduces the shock experienced by the recorders and helps to cushion the devices to improve their chances of surviving the crash.
Flight Data Recorder recovered from United Airlines 93 in 2001
Upon receipt of the recorders, the NTSB uses a series of computer and audio equipment to process and analyze any information that can be recovered. The data is translated into formats readily usable by investigators and is usually critical in identifying the probable cause(s) of the accident. This process may take many weeks or months depending on the condition of the black boxes and the level of processing required to make sense of the data. Outside experts are also often consulted to help analyze and interpret the data.
Animation image created using FDR data from American Airlines 587 that crashed in 2001
Flight recorder design has improved considerably since the devices were first introduced in the 1950s. However, no recording device is perfect. Black boxes are sometimes never found or too badly damaged to recover some or all of the data from a crash. To reduce the likelihood of damage or loss, some more recent designs are self-ejecting and use the energy of impact to separate themselves from the aircraft. Loss of electrical power is also a common event in aviation accicents, such as Swissair Flight 111 when the black boxes were inoperative for the last six minutes of flight due to aircraft power failure. Several safety organizations have recommended providing the recorders with a backup battery to operate the devices for up to ten minutes if power is interrupted.
Cockpit Voice Recorder recovered from United Airlines 93 in 2001