Flight-data recorders: Orange is good, black is bad

As the tragic events in New York, Washington, and Pennsylvania remind us, there are seldom survivors of airplane accidents. Once again, investigators began a search for the "black boxes," in hopes that their contents would help shed more light on the events that led to such a terrible loss of life—both on board and on the ground. The boxes, in fact, are not black but orange; they got their name because by the time they reached the NTSB (National Transportation Safety Board) laboratory, they had usually been blackened by fire. The events of September 11 provided four cases of extreme conditions for the survival of flight records. Air safety is different from air-travel security. The latter deals with ensuring that the occupants and the cargo of the plane do not pose a threat to normal operations. The former deals with the ability of the crew and of the aircraft to perform as expected. Today, all commercial airplanes and many military and civilian planes carry flight recorders. The primary purpose of the flight recorders is to help establish the cause of an air disaster. The Federal Aviation Administration divides air disasters into six categories. "Controlled flight into terrain" is the leading cause of fatal commercial-aviation accidents worldwide, and "loss of control" is the number one cause of fatal accidents involving US airlines. There is no specific category for the willful destruction of a plane, and we hope there never will be.

The federal government began requiring commercial aircraft registered in the United States to carry FDRs (flight-data recorders) in 1958 and CVRs (cockpit voice recorders) in 1965. Worldwide regulation of air safety falls under the authority of the ICAO (International Civil Aviation Organization) based in Montreal. Of course, technology has changed quite a bit since the late 1950s, although some of the commercial aircraft based in Asia and Africa are still using the original technology. CVRs and FDRs are housed in the part of the plane that is least likely to experience damage in an accident. So, they must be away from the bulkheads, fuel lines, and fuel tanks, and also from major structural areas, such as wing spars, which could damage the recorders on impact. The best location is above the ceiling at the rear of the passenger cabin, underneath the tail fairing. First-class passengers may find the flight quieter and smoother, but the passengers in the back of the cabin have a better chance of surviving a crash.

The purpose of a CVR is to record conversations in the cockpit, the dialogue with air-traffic controllers, and ambient noises. Separate channels record the voices of the pilot and copilot, voices on the intercom, and miscellaneous cockpit sounds, and the device saves the last 30 minutes recorded at all times. The frequency response of the cockpit-area microphone is broad enough to make it possible to determine whether an explosion or a catastrophic engine malfunction causes a disaster.

The mission of an FDR is to record a number of in-flight parameters that can provide examiners with an idea of the environmental- and mechanical-parameter values at the time of the accident. Initially, the equipment recorded airspeed, altitude, engine-speed, and temperature information on 400-ft rolls of metal foil that you needed to replace after every flight. Scribe arms attached to coils and a mechanical sensor scratched data on the foil. In 1965, the ICAO mandated the installation of CVRs in commercial aircraft, which use magnetic tape as their recording device. This mandate provided the impetus for engineers to equip FDRs with ¼-in. magnetized Mylar tape. The number of parameters recorded has increased over the years—to 11 in older 737s and to more than 100 in the newest commercial-aircraft models. The latest FDRs can record more than 300 parameters, but the law presently requires newer airplanes to record only 28. The length of time that the recordings cover has also increased; it is now 25 hours. All CVRs and FDRs come with an underwater-locator beacon to help search crews find it in the event of an accident over water. The beacon, called a "pinger," activates when the recorder is immersed in water. It transmits over a 37.5-kHz frequency an acoustical signal that a special receiver can detect. The beacon can transmit from depths of 14,000 ft. Both CVRs and FDRs must be able to withstand an impact force of 3400Gs, have a fire resistance of 1100°C for 30 minutes, sustain a water pressure equivalent to being submerged 20,000 ft, and have an operational battery life of 30 days.

The main reason for recording more data than is mandated by law is that the information is valuable to maintenance personnel and even to designers of new airplanes. Inspecting data collected over a number of flights allows maintenance personnel to validate and modify schedules and procedures. Designers and systems engineers are interested in the way particular components wear with use and how they might modify electrical and mechanical configurations to improve performance, safety, and durability.

Teledyne Controls has responded to the need to collect data not mandated by law by designing and manufacturing a digital FDAU (Flight Data Acquisition Unit) that comprises two sections. One section provides the inputs for the CVR and the FDR, and another section provides inputs for a number of other parameters. These parameters can be stored in an auxiliary storage device and processed according to the requirements of the airline, the airplane builder, or both. The nonmandated recording device is called a QAR (Quick Access Recorder), and it is generally not crash-resistant. A number of analog and discrete signals, as well as the ARINC 429 Broadcast Bus, provide inputs to the CVR and the FDR. The ARINC 429 Broadcast Bus carries inputs for the QAR. The bus carries digital data and operates in simplex fashion in a single source, multisink arrangement. One transmitter can communicate with as many as 20 receivers. The system can operate at two transmission rates—a high rate of 100 kbps and a low rate of 12 to 14.5 kbps—and transmits serially in 32-bit words. The Aeronautical Radio Incorporation publishes the standard and holds its copyright; the current release is known as ARINC 429-15.

In the late 1970s, in part under the auspices of the US Air Force F-16 program, Smiths Industries Aerospace began to implement CPM (Crash Protected Memory) in FDRs. CPM units use memory chips rather than magnetic tape as their recording medium. The chips increase reliability, because units using chips have fewer moving parts. Since the mid-1980s, all new recorders have used CPM as their storage media and combine both CVR and FDR in one crash-resistant unit. Replacing tape with CPM in CVRs increases recording times from 30 minutes to two hours before rewriting occurs. An example of the latest technology is Smiths Industries Aerospace's IDARS (Integrated Data Acquisition and Recording System, Figure 1). The IDARS initially targeted the military market, but it is now also used in commercial aviation. This integrated architecture offers obvious benefits. The IDARS provides both required data recording (CVR and FDR) and an FDAU (the equivalent of the QAR) in one unit. It provides separate power to the CVR and FDR to maintain electrical isolation for the recorders, as required by law.

As fly-by-wire technology becomes more pervasive in commercial aviation, the ability to monitor large amounts of various kinds of data will improve safety. Presently available microprocessor and DSP technology can provide real-time data analysis. This information will provide not only better records for deferred analysis but also real-time feedback to the cockpit crew, enabling them to make better informed decisions. The Data Acquisition and Processing Unit from Smiths Industries Aerospace is an example of the next generation of black boxes. In addition to the functions described above, it also provides a vibration-monitoring system. It provides feedback to the crew by displaying relevant exceptions, alerts, and data on the cockpit-control unit. Black boxes will still not contribute to improved flight security; however, pilots will be better informed during flights, and when things go terribly wrong, the NTSB is likely to have a better idea of what caused the accident.

Author Information

You can reach Technical Editor Gabe Moretti at gabe@eda.org.