The black box is popular name for the flight data recording devices. Such instruments have been around for a relatively long time in the aviation industry. Initially, their purpose was to record the vibrations of the fuselage and wings, as well as the forces acting on the aircraft structure during flight. The information carrier of the first recorders was a sooted cylinder on which a stylus scratched a recording line. This idea is still used in glider barographs (the information carrier is a coated aluminium foil). Over time, recording techniques developed and the range of parameters recorded expanded. Records were made on chalked, blackened tapes or exposed film; burned or punched on paper tapes; and finally, sets of on-board instruments were photographed at intervals of a few seconds.
[001] Soviet spherical flight recording device, inside magnetic tapes and recording unit.
The development of magnetic recording technology has led to huge advances in the design of recorders. Initially, magnetic wire and tape were used for recording. The metal wire was more resistant to high temperatures. Today, due to the use of increasingly effective insulating materials, magnetic tape has become widely adopted. The transition to magnetic recording has made it possible to significantly increase the number of parameters recorded, from a few or a dozen to several hundred. At the same time, it has allowed to reduce the size and weight of the recorders. The analysis of the situation on the basis of the recording reconstructed from magnetic tape is only a part of the body of evidence when investigating the causes of an air accident. Conversations of the aeroplane crew with the airspace control station, sound recordings in the aeroplane cabin, testimonies of witnesses, examination of the aeroplane or its wreckage, the placement of wreckage and meteorological messages are also analysed. Despite the use of increasingly accurate investigation methods, the causes of about 10% of air crashes remain unknown. 
[002] Midas - British flight data recorder. Then what is this mysterious recorder? This device records dozens or even hundreds of data on magnetic tape. It is specially protected during an accident. The information is usually recorded for 25 hours of the plane's operation, counting from the moment the engines start. According to international agreements, data such as flight speed, altitude, magnetic course, vertical overloads are collected. Also recorded are pitch and roll, glide angle, position or forces exerted on the controls, position of flaps, airbrakes, engine thrust, angle of attack, fuel consumption, engine shaft moments, ambient temperature, atmospheric and cabin pressure, autopilot on and off. Individual data are recorded at a frequency of one to eight times per second. Commonly accepted is cipher recording in Harvard binary code at a rate of 64 twelve-bit words per second. The information carrier is a metal tape with a metal or terylene backing, 30 µm thick, usually 1/2 inch wide and several tens of metres long. The life of the tape is several hundred working hours. The number of tracks varies from four to sixteen. The tape is rewound sequentially from one spool to another, back and forth. A change of recording track occurs automatically when changing the tape rewinding direction. A recording from 25 hours ago is erased continuously. Playback of the recording can be carried out at a speed several times greater than the recording (4-6 times). Data to be recorded are taken directly from on-board instruments transmitting information to the crew, or from special transducers serving only the recorder. The signal from the converters, usually in analogue voltage form, is converted into digital form. Electronic circuits are made using printed circuit technology, with extensive use of integrated circuits. The latest designs are equipped with circuits for automatic checking of the correct operation of the device. The device's electronics, which used to occupy a separate box, have now been considerably miniaturised and are housed in a common rectangular casing, together with a container for the drive and tape recording mechanism. Dimensions of a typical black box are 497x194x124 mm, weight up to 15kg. As mentioned, the recording and drive mechanism is placed in a special container, resistant to the destructive factors accompanying a disaster. Initially, these enclosures were made in the shape of spheres (smallest area), now they are short cylinders. Only the units necessary for drive, recording, playback and erasing are housed in the casing. The outer shell of the protective casing is made of titanium, due to its heat resistance. The inner surfaces are lined with a thermally insulating plastic layer. The materials used here are those used in rocket and spacecraft construction. The next casing layer is a material with a high heat capacity, often water. Then comes another plastic insulation layer and an inner casing made of titanium or alloy steel. The recording mechanism is attached to the inner casing. 
[003] Flight data recorder in common box with visible inner casing.
The suitability of the case for aeronautical purposes is assessed by a very stringent test of the entire recorder. The test starts with an impact test with an acceleration of 1000 g, acting sequentially along the three main axes of the recorder. This is followed by a crushing test with a static force of 5000 lbs also in three axes. This is followed by a puncture test with a sharpened rod. A fire test is then performed. The enclosure should withstand a 1000 °C flame for 30 minutes. It should also protect the interior against the ingress of seawater, jet fuel and other liquids for at least 36 hours. During the tests, the electronic block placed in a normal box is destroyed, and damage to the protective casing is also acceptable. However, it is important that the recording is fully readable. The presented tests fully simulate the conditions created during a disaster. To make it easier to find the recording container, the casing of the recorder and the container itself are covered with heat-resistant orange or yellow paint. Some designs incorporate location radio transmitters with their own power source, allowing signals to be emitted for 30 days at a range of 6 kilometres. 
[004] Black box recovered from the aircraft wreckage. Magnetic tape cover and radio transmitter undamaged.
The expected further development of recorders is to reduce weight and size, increase reliability and durability, and make them easier to use. Perhaps the reading of the box will be remote, without the need to remove it from the aircraft. Let us hope that all improvements will have a positive effect on the safety of every aircraft. • • •
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