Soon after an air crash, investigators hunt for the "black boxes” as these may contain the answers to why the crash happened.
One of the black boxes is the "cockpit voice recorder". There are several microphones in the cockpit that pick up conversations of the flight crew and other useful sounds such as alarms or sounds made when switches are moved. There may be up to four microphones in the plane's cockpit. Signals from these are sent to the cockpit voice recorder. Older black boxes (such as those in Flight 401) used magnetic tape to store the last half an hour of voice prior to a crash. Newer designs use electronic memory boards which are much more reliable because they do not have moving parts and record about two hours of pre crash voice.
Aircraft have another type of black box called the flight data recorder (FDR) which records other information that may help solve air crash mysteries. There are a large number of sensors located all over the aircraft that record parameters such as altitude ,speed, engine performance ,hydraulic pressure, electrical systems, fuel , and many more. Modern boxes can record more than 700 parameters.
Black boxes are designed to survive the severe physical conditions that occur during a crash . They are located at the rear end of the aircraft as this is the area most likely to survive a crash (now you know where to sit !). The portion that contains information is well protected (e.g. aluminium and titanium ) and insulated (e.g. thick layer of silica) . The recording can survive an impact force equal to 3400 times its weight (3400 G's) and can remain useable even if a 5000 pounds per square inch crushing force is applied. The data would remain intact even if you placed the boxes in a fire (temperature of 2,000 degrees Fahrenheit or 1,100 Celsius) for one hour. In addition , these boxes must be able to withstand submersion to cope with crashes that end underwater.
The boxes are designed to be found. Although they are called "black boxes," aviation recorders are actually painted bright orange. This distinct colour helps investigators locate the black boxes following an accident. If the box gets submerged underwater (e.g. as in a sea crash) it automatically emits a locator signal once per second for 30 days.
After finding the black boxes, investigators take the recorders to a laboratory where they can unload data from the recorders and attempt to recreate the events of the accident. A team of experts is usually brought in to interpret the recordings stored on the cockpit voice recorder. This group typically includes representatives from the airline, airplane manufacturer, and Government air safety agencies. This group may also include a language specialist. This board attempts to interpret 30 minutes of words and sounds recorded by the cockpit voice recorder.
This website tells the story of Flight 401 via the transcript of the cockpit voice recorder recording.
A black box : Note how it is hardly black at all ! The silver cylindrical object at the front is a special underwater transmitter that helps locating a sunk black box. It is automatically activated on contact with water. (Photo copyright and thanks to Fred Wheeler and L-3 Communications Corp.)
