Avionics IV. Flight data communication and registrars (6)

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Aircraft navigation is the science about the means and tools of controlling aircrafts while flying from one point on earth’s surface to another according to the trajectory selected in time and space.  Flight data are pilots‘conversations with each other, communication with satellites, the ground services and the cockpit noise (background). Aircraft engine speed, engine gas temperature, vibration level, the aerodynamic surfaces and chassis bearings, fuel supply, fuel, oil and hydraulic fluid pressure, and many dozens of other aircraft systems parameters are called the flight data. (End, Begining  N0.20 )

Communications and flight data are recorded (stored) in flight data recorders.

The need to have data about aircraft crashes and accidents have emerged since the beginning of aeronautics. About 1940 it was started to collect them during the flight, but then technical level did not allow creating registrars that keep stored data after aircraft crash, on impact to the ground or fire. Only in 1958 ICAO has released documents about the basic requirements to registrars that industry was able to produce. The data registrars (recorder) had planes as Boeing 707, Douglas DC -8, Caravelle. They were able to record the most necessary information – the magnetic course, the flight altitude and speed, vertical acceleration time during a short period of time. Data were recorded into copper or steel foil, but it has not always survived after the crash.
There were produced other non-compliant with ICAO registers, for example, altitude or vertical overload was recorded by needle scratches on rotating drum. Before the flight the drum had to be covered with soot over a flame. These first-generation recorders still can be found in aviation museums.

The invention of new methods of recording of communications and flight parameters on magnetic tapes and films increased the number of recorded data and recording duration.
The key was to keep the recorded information from accidents and crashes in order to explore the causes. In order to keep information safe, magnetic tape of photo film was put into container, resistant to shock, heat and water. Such registers were called emergency. Tapes were winded by sophisticated shock- resistant traction equipment (Figure 25). As it was difficult to put and remove films to such containers, most widely used was magnetic recording method.
Photographic films were able to accumulate less information, were not able to recorded speeches. It was not possible to erase or overwrite information.
Magnetic tape is resistant to temperature (keeps information up to 400° C), it is possible to delete and overwrite information. For example, the information can be recorded on one edge of the tape, and then, pulling in different direction – on the other.

Tape is pulled at a speed of few centimeters per second. As size of pulling mechanism is limited, such registrar can only record the last 30 minutes of flight information. Such recording duration is almost enough to investigate disasters.

25 pav. Juostos traukimo mechanizmas konteineryje
Figure 25. Tape traction mechanism in the container

Very tightly closed containers for emergency registrars are opened only during maintenance work. As a result, they got the unofficial title “black box“, even though the containers are painted in bright red or bright orange. As the need to examine the equipment and operation of the pilots‘ actions after each flight have emerged, production of flight registers was launched. They are sensitive to shock, flame and similar factors. They have cartridge with tape, so the information can be continuously registered for at least 24 hours. The cartridges can be removed after each flight and stored information analyzed: verify the mode of operation of the aircraft engines and systems, how pilots of aircraft operated and how they keep communication. This helps to find faults in the aircraft and clarify incorrect actions of pilots.

Conversations and performance data on magnetic tape have to be recorded separately, as their signal structure is different. It is well established tradition, approved by ICAO, to produce two kinds of information the registrars – communications data CVR (Cockpit Voice Recorder) and FDR (Flight Data Recorder). Produced are universal FDR too, to record both communication and performance data. Larger aircrafts have emergency CVR and universal FDR. These second- generation recorders are used in older aircrafts.

In 1965, industry began to produce emergency registrars, which accumulated on the magnetic strip at least 30 minutes of the flight information, it remained undestroyed after 1,000 g shock force or tens of minutes exposed to the flame. However, these requirements were not sufficient because blast often blow magnetic heads from their holding screws and damaged magnetic tape in the most important places where the last seconds of information flight was recorded. To restore the last words or last parameters, experts had to glue torn pieces and use very complex signal analysis equipment. There are only a few such signal analysis laboratories around the world.

Data recorders separately (in different magnetic tape track) record conversations of aircraft commander, second pilot, aircraft cockpit sounds (conversations with other persons and noises). It is equipped with a microphone and timer to generate time signals. Flight data recorders record data of the major aircraft systems, parameters OF equipment and aircraft engine and time signals. Parameters are recorded approximately every second, quickly changing parameters – more frequently. According to the recorded time signals one can compare recorded data of different equipment. The magnetic tape can record almost 100 parameters. The signals from the aircraft equipment and systems are collected, processed by the electronic recording unit FDAU (Flight Data Acquisition Unit), which is in the Registrar ( Figure 26). Recorded parameters are of two types – regular and one-time commands. Regular signals from the aircraft equipment and instruments are recorded at all times until the registrar is turned on. One-time commands are e.g. autopilot turning on and off, chassis is extracted, and signals that occur only in the abnormal operation of equipment: overloads, parameters, dangerous for pilots to control the aircraft.
Emergency Registrars aircraft is usually located in the tail (Fig. 26), because there’s a greater chance for them to remain undamaged after an accident or crash. Larger aircraft may have several identical registers, and then there is a greater chance that one of them will remain after the crash.

26 pav. Registratoriai lėktuve
Figure 26. Plane‘s registrars

Registrars in the aircraft turns on and off automatically, for example, after pushing engine start lever from the “Stop” position, or after increase the oil pressure in the engine. Before the flight, in the control panel it is necessary to set up the flight number, date and exact time. In case of emergency registrar is automatically turned off by inertia switch which activates in case of large impact, when the aircraft struck the ground.
When the aircraft parking brakes are activated, it is possible to delete recorded information by pressing the button on the control panel. If there were remarks on communications during the flight, deletion of recorded content is forbidden.
Registrar tape traction mechanisms of large and heavy, they need constant maintenance. Magnetic tape might be destroyed after big aircraft impact to the ground or fire. Since 1990 production of third-generation recorders (Figure 27) was started. In these recorders communications and flight data is recorded in chips. They do not need constant maintenance, and the accumulated information can be read or transferred to other devices quickly and conveniently, because there is no need to remove the tape from the storage device.

27 pav. Ryšio ir skrydžio duomenų registratorius
Figure 27. Communications and flight data recorder

Since 1995 production of high -capacity memory circuits was launched. Emergency information recording time was increased from 30 min to up to 2 hours. Such recorders can record all (several thousand) flight data and communications parameters of large aircraft without increasing the dimensions of the container. Container are shock resistant up to 3400 g, they withstand pressure up to 2 tons of weight from either side. Containers withstand temperature to 1100° C for 1 hour, and then another 10 hours – 260 °C. This is close to the real conditions of the aircraft burning on the ground. The container is made of titanium and covered with temperature-resistant ceramics (Figure 28). . On the ceramic protective overlay layer is applied, which turns into a foam during container burning, and protects container from overheating. Container has underwater alarm signal transmitter: if the aircraft falls into the water, transmitter batteries are filled with water and start operating, it sends 37.5 kHz acoustic signals that helps determine location of the container. Alarm can operate even at a depth of about 6 kilometer for one month; a signal is heard several kilometers away.

28 pav. Skrydžio duomenų registratoriaus konteineris
Figure 28. Flight data recorder container

If aircraft hits the ground at a high speed, only container remains intact. It can be pressed deep into the ground or thrown out of the aircraft (Figure 29).

29 pav. Konteineris po avarijos
Figure 29. Container after crash

After the flight the cassette with tape is removed be the ground service with special equipment and stored information is overwritten to the computer. Information recorded on a chip is directly transferred into temporary storage, and from there – into computer. Then, the information is reviewed (decrypted) using special software. Flight data (engine speed , vibration , pressure systems , control surface positions and few dozen of other parameters) are shown on computer screen in different colors and shapes of the lines , also shown is the time line ( Figure 30). Recorded data values can be measured to respect of a base line. According to the recorded time signals it is possible to determine what and when pilots said, when parameters changed and time when one-time commands occurred.

30 pav. Įregistruoti įrenginių parametrai
Figure 30. The recorded equipment parameters

Data recorders are improved in two directions. Registrars for large aircrafts can record on-board video information and access to the cockpit. Some registrars designed for smaller aircrafts can record only a few flight data and they are not subject to major requirements for the protection of information against shocks and temperature, so they are small, lightweight and cheaper.