How Black Boxes Work After the crash

               After the crash, they didn't find a single body for five days. Even with military and civilian personnel frantically scouring the seas, it was as if Air France Flight 447 had simply vanished over a remote area of ocean 600 miles from Brazil -- with 228 people onboard. It didn't happen in the early days of the airline industry; it occurred in 2009, on a fancy modern aircraft controlled by a competent company.

                Airplane accidents are statistical rarities. But when they happen, they're often fatal, and people want answers as to why their loved ones died.

There are usually many unanswered questions as to what brought the plane down.                                Investigators turn to the airplane's flight data recorder (FDR) and cockpit voice recorder (CVR), also known as "black boxes," for answers. Following any airplane accident in the U.S., safety investigators from the National Transportation Safety Board (NTSB) immediately begin searching for the aircraft's black boxes.

               It took investigators nearly two years to find the FDR from Flight 447. The box had not only survived impact, but also being submerged under nearly 13,000 feet of salty, corrosive seawater. In the end, the data proved that pilot error had contributed to a stall that eventually caused the crash.

             These recording devices, which cost between $10,000 and $15,000 each, reveal details of the events immediately preceding the accident. In this article, we will look at the two types of black boxes, how they survive crashes, and how they are retrieved and analyzed.

The widespread use of aviation recorders didn't begin until the post-World War II era. Since then, the recording medium of black boxes has evolved in order to log much more information about an aircraft's operation.

                     Older black boxes used magnetic tape, a technology that was first introduced in the 1960s. Magnetic tape works like any tape recorder. The Mylar tape is pulled across an electromagnetic head, which leaves a bit of data on the tape. These days, black boxes use solid-state memory boards, which came along in the 1990s.

                Solid-state recorders are considered much more reliable than their magnetic-tape counterparts. Solid state uses stacked arrays of memory chips, so they don't have moving parts. With no moving parts, there are fewer maintenance issues and a decreased chance of something breaking during a crash.

                 Data from both the CVR and FDR is stored on stacked memory boards inside the crash-survivable memory unit (CSMU). The memory boards have enough digital storage space to accommodate two hours of audio data for CVRs and 25 hours of flight data for FDRs.

Airplanes are equipped with sensors that gather data such as acceleration, airspeed, and altitude, flap settings, outside temperature, engine performance, and cabin temperature and pressure. Magnetic-tape recorders can track about 100 parameters, while solid-state recorders can track a lot more.
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                 For instance, in the Boeing 787, the units can log a whopping 146,000 parameters, resulting in several terabytes of data for every single flight. That incredible load of data is a double-edge sword; it's great for monitoring the aircraft, but it can overwhelm engineers and maintenance personnel. To manage all of that data, they need sophisticated data management software.

              Whether the system is an older version or fully modern, all of the data collected by the airplane's sensors is sent to the flight-data acquisition unit (FDAU) at the front of the aircraft. This device often is found in the electronic equipment bay under the cockpit. The flight-data acquisition unit is the middle manager of the entire data-recording process. It takes the information from the sensors and sends it on to the black boxes.

Cockpit Voice Recorders

                    In almost every commercial aircraft, there are several microphones built into the cockpit that listen to flight crew conversation. These microphones also track any ambient noise in the cockpit, such as switches being thrown or any knocks or thuds. There may be up to four microphones in the plane's cockpit, each connected to the cockpit voice recorder (CVR).

                    Microphones send audio to the CVR, which digitizes and stores the signals. In the cockpit, there is also a device called the associated control unit, which provides pre-amplification for audio going to the CVR. The four microphones are place in the pilot's headset, co-pilot's headset, headset of a third crew member (if there is a third crew member) and near the center of the cockpit, to pick up audio alerts and other sounds.

                Most magnetic-tape CVRs store the last 30 minutes of sound. They use a continuous loop of tape that completes a cycle every 30 minutes. As new material is recorded, the oldest material is replaced. CVRs that use solid-state storage can record two hours of audio. Similar to the magnetic-tape recorders, solid-state recorders also record over old material.