ARINC 717

More data security for better performance

Known of in the field of aeronautics for some while, the ARINC protocols are a means that can be deployed in many ways. They make it possible to combine the many different components in the system to form an assembly that works effectively, in which it is possible to optimally secure transmitted and received data between two or more devices and as a result, ensure smooth running of the processes. Here, ARINC 717 is a protocol that defines a standard of data communication. This communication occurs between the digital flight data acquisition unit (abbreviated as DFDAU) and the digital flight data recorder (abbreviated as DFDR). Here, the DFDAU assumes the position of the computer and the transmission point, as the different data are collected, combined, and transformed into the format of the ARINC 717 protocol on these system components. This data bundle then reaches the DFDR only.

The basic idea behind the ARINC 717 and its operation

The problem that avionics faced with the use of the ARINC 429 was the relatively low data security. However, this shortcoming can be overcome by use of the refined ARINC 717 protocol, as here the security of the collected data can be increased thanks to a bi-phase signal with a DC-free voltage of +/- 5 V. With this, a procedure is used for coding as a basis. Known as the Harvard II Biphase, it provides a logical ZERO as the voltage value. This value can be obtained over the entire bit width. However, when the voltage value is at logical ONE, this value is converted during the bit width.

  • Starting phase of the ARINC 717 - after installing the digital flight data acquisition unit, this unit receives information from the different components in the aircraft. The collected data includes information concerning the condition of the devices. This data can also provide information regarding the altitude, the current and average velocity and the position of the rudder. These data elements are recorded as so-called parameters. Each individual fraction is verified via an 8-bit identifier and additionally, it also contains a payload between 18 bits and 19 bits. The digital flight data acquisition unit buffers the data, thus converting them to frames and subframes. A recording of four seconds of the life of an aircraft is called a frame. The subframes then are the subdivisions of this frame. A frame has a total of four subframes, which are then assigned each second of the four seconds of recorded life time. As such, the subframe contains all of the collected data gathered by the DFDAU. In former times, when ARINC 717 was not yet on the market, aircraft mostly recorded only up to 64 parameters, but since the introduction of the digital flight recorders, the average is 128 parameters. In comparison, 256 parameters must always be recorded for an aircraft in the United States of America, and in the case of the Airbus A380, the largest commercial aircraft that produced in serial production, one recording is made up of 1024 parameters.
  • Final phase of the ARINC 717 - a set of parameters is only considered as having been transmitted completely when one frame, i.e. all four seconds of the recorded life time of an aircraft has been transmitted. Afterwards this process can start again. As a rule a whole 25 hours of the life time of an aircraft are recorded and stored. Each individual parameter of the set to be transmitted holds 12 bits of stored payload. At this point of the ARINC 717 protocol, the bits which regulate identification and synchronization are no longer needed, as the subframes always have the same construction. This makes it possible to determine an element just by its position in the frame, and the parameters can transmit data in their chronological order without interruptions and separations. However, not every parameter is actually needed in each transmission, so that the parameters considered as less important in the system are only used and transmitted in each second or fourth parameter. The subframes also always differ in their first parameter, which is used for the purpose of identification. The remaining parameters then can be installed according to a scheme defined by a specified pattern. Often, the plug-in location is not sufficient as identification, so that a table must be used for decoding.

Without ARINC 717, there would be no progress!

Since first being marketed, ARINC protocols have provided key advantages and better standards. However, the ARINC 717 is characterised as a significant point in this development, starting from which, the standard has been raised and optimized many times over. As such, with regard to low data security, the deficiencies of the precursor model have been eliminated thanks to the ARINC 717 and data can now be recorded in real-time, so that meaningful results are available to aviation and there is an option for technological development to a higher level.