Electronic Flight Instrument System: An Overview


The electronic flight instrument system (EFIS) is the electronic display technology on a flight deck display system. An EFIS typically consists of three main parts: the Engine Indicating and Crew Alerting System (EICAS), Primary Flight Display (PFD), and Multi-Function Display (MFD). early EFISs used cathode ray tube displays, although modern systems utilize a liquid crystal display.

The installation of an EFIS varies greatly depending on the type and size of aircraft. For example, a large, wide-bodied aircraft can be equipped with as many as six displays on which information such as flight and navigation data is displayed, while a light aircraft is unlikely to have more than one unit. During the installation of an EFIS, the displays go in first, followed by the control panels, and finally the data processors. A basic EFIS will contain all of these facilities in one unit.

The display units of a EFIS comprise three different displays: the primary flight display, navigation display or multi-function display, and the electronic centralized aircraft monitoring or engine indications and crew alerting system. The control panels comprise the array of controls where the pilots select the display mode and range (for example, a compass rose or map) and enter in data (such as a certain heading). The pilots only enter the information once, at which point the data buses broadcast the inputs for other equipment to use. For example, using the control unit, the pilot makes a selection of the preferred level-off altitude. The EFIS then repeats the selected altitude on the primary flight display, which generates an altitude error display by comparing it with the actual altitude. The altitude selection is also used by the altitude alerting system to provide the appropriate warnings and by the automatic flight control system for leveling off.

The data processors, also known as the display processing computer, display electronics unit, or symbol generator, produced the visual displays within the EFIS.
The processor receives the data inputs from the pilot and EFIS format selections, while also receiving signals from sensors. In addition to generating symbols, the symbol generator features monitoring facilities, a display driver (hardware not software), and a graphics generator. The input arrives through data buses and is then verified. The required computations are then performed and the display driver and graphics generator produce the entries to the display units. Similar to ordinary computers, monitoring in an EFIS requires continuous self-monitoring and power-on-self-test facilities. However, an EFIS requires additional monitoring competencies. These include input validation to validate the data from each sensor, data comparison to verify the inputs from the duplication sensors, and display monitoring to identify instrument system failures.

In older display systems of the electromechanical type, displays were fitted with synchro mechanisms which, when in transmission, displayed the pitch, roll, and heading, on an instrument comparator displayed on the captain’s and first officer’s instruments. The comparator notes any significant differences between the two displays. For example, in the event of an error like far downstream, which refers to the data flow direction (from the sensor to the processor to the display), the roll system triggers a warning in the comparator. Therefore, the instrument comparator provides both display and comparator monitoring, making it a critical part of the EFIS. Within the EFIS, the task of the comparator is to provide warnings for airspeeds, roll, pitch, and altitude indications. It takes the data from sensor one and sensor two and checks that they are the same. If the data differs, the comparator displays a warning on both flight displays. In advanced EFIS systems like those in large commercial aircraft, more monitors are enabled.

A drawback of EFIS displays is that they do not provide re-transmission of what is shown. To do this, a new approach similar to that of a traditional system is required. One solution to this is to keep the display system as simple as possible, ensuring that errors are minimal as possible and the units will simply work or not work. This will make any errors more obvious and allow the monitoring function to be moved upstream to the symbol generator output. In systems of this type, each generated symbol has two display monitoring channels, an internal and external. The internal channel, from the symbol generator, samples the output to the computer and display unit; for example, which roll attitude produces the indication. At this point, the computed roll attitude is compared to the input of the roll attitude with the symbol generator from the attitude and heading reference system (AHRS). Any differences in the two figures will be accompanied by error processing and initiate a warning on the display.

The external channel will carry out the same check on the symbol generator of the other side. The captain’s symbol generator checks that of the first officer, and the first officer’s symbol generator in turn inspects that of the captain. If any faults are detected, an alert is put up on the display. In addition to this, the external monitoring channel also checks the sensor inputs to the symbol generator to improve the reliability. A false input, such as a radio height greater than the radio altimeter’s maximum, results in a warning.

The EFIS is a critical aspect of the safe operation of any aircraft. As such, it is important to get your EFIS components from a trusted and legitimate source. At Just NSN Parts, owned and operated by ASAP Semiconductor, we can help you find all types of electronic flight instrument system components in addition to many other parts for the aerospace, civil aviation, and defense industries. Dedicated account managers are always available and ready to help you find all the parts and equipment you need, 24/7-365. For a quick and competitive quote, email us at sales@justnsnparts.com or call us at 1-714-705-4780.


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