Electronic flight instrument system
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An electronic flight instrument system (EFIS) is a flight deck instrument display system in which the display technology used is electronic rather than electromechanical. EFIS normally consists of a primary flight display (PFD), multi-function display (MFD) and engine indicating and crew alerting system (EICAS) display. Although cathode ray tube (CRT) displays were used at first, liquid crystal displays (LCD) are now more common.
The complex electromechanical attitude director indicator (ADI) and horizontal situation indicator (HSI) were the first candidates for replacement by EFIS. However, there are now few flight deck instruments for which no electronic display is available.
EFIS installations vary greatly. A light aircraft might be equipped with one display unit, on which are displayed flight and navigation data. A wide-body aircraft is likely to have six or more display units.
Typical EFIS displays and controls can be seen at this B737 technical information web site. The equivalent electromechanical instruments are also shown here.
An EFIS installation will follow the sequence:
- Data processors
A basic EFIS might have all these facilities in the one unit.
Display units 
Primary flight display (PFD) 
On the flight deck, the display units are the most obvious parts of an EFIS system, and are the features which give rise to the name "glass cockpit". The display unit taking the place of the ADI is called the primary flight display (PFD). If a separate display replaces the HSI, it is called the navigation display. The PFD displays all information critical to flight, including calibrated airspeed, altitude, heading, attitude, vertical speed and yaw. The PFD is designed to improve a pilot's situational awareness by integrating this information into a single display instead of six different analog instruments, reducing the amount of time necessary to monitor the instruments. PFDs also increase situational awareness by alerting the aircrew to unusual or potentially hazardous conditions — for example, low airspeed, high rate of descent — by changing the color or shape of the display or by providing audio alerts.
The names Electronic Attitude Director Indicator and Electronic Horizontal Situation Indicator are used by some manufacturers. However, a simulated ADI is only the centerpiece of the PFD. Additional information is both superimposed on and arranged around this graphic.
Multi-function displays can render a separate navigation display unnecessary. Another option is to use one large screen to show both the PFD and navigation display.
The PFD and navigation display (and multi-function display, where fitted) are often physically identical. The information displayed is determined by the system interfaces where the display units are fitted. Thus, spares holding is simplified: the one display unit can be fitted in any position.
LCD units generate less heat than CRTs; an advantage in a congested instrument panel. They are also lighter, and occupy a lower volume.
The MFD (multi-function display) displays navigational and weather information from multiple systems. MFDs are most frequently designed as "chart-centric", where the aircrew can overlay different information over a map or chart. Examples of MFD overlay information include the aircraft's current route plan, weather information from either on-board radar or lightning detection sensors or ground-based sensors, e.g., NEXRAD, restricted airspace and aircraft traffic. The MFD can also be used to view other non-overlay type of data (e.g., current route plan) and calculated overlay-type data, e.g., the glide radius of the aircraft, given current location over terrain, winds, and aircraft speed and altitude.
MFDs can also display information about aircraft systems, such as fuel and electrical systems (see EICAS, below). As with the PFD, the MFD can change the color or shape of the data to alert the aircrew to hazardous situations.
Engine indications and crew alerting system (EICAS) / electronic centralized aircraft monitoring (ECAM) 
EICAS (Engine Indications and Crew Alerting System) displays information about the aircraft's systems, including its fuel, electrical and propulsion systems (engines). EICAS displays are often designed to mimic traditional round gauges while also supplying digital readouts of the parameters.
EICAS improves situational awareness by allowing the aircrew to view complex information in a graphical format and also by alerting the crew to unusual or hazardous situations. For example, if an engine begins to lose oil pressure, the EICAS might sound an alert, switch the display to the page with the oil system information and outline the low oil pressure data with a red box. Unlike traditional round gauges, many levels of warnings and alarms can be set. Proper care must be taken when designing EICAS to ensure that the aircrew are always provided with the most important information and not overloaded with warnings or alarms.
ECAM is a similar system used by Airbus, which in addition to providing EICAS functions also recommend remedial action.
Control panels 
The pilots are provided with controls, with which they select display range and mode (for example, map or compass rose) and enter data (such as selected heading).
Where inputs by the pilot are used by other equipment, data buses broadcast the pilot's selections so that the pilot only needs to enter the selection once. For example, the pilot selects the desired level-off altitude on a control unit. The EFIS repeats this selected altitude on the PFD and by comparing it with the actual altitude (from the air data computer) generates an altitude error display. This same altitude selection is used by the automatic flight control system to level off, and by the altitude alerting system to provide appropriate warnings.
Data processors 
The EFIS visual display is produced by the symbol generator. This receives data inputs from the pilot, signals from sensors, and EFIS format selections made by the pilot. The symbol generator can go by other names, such as display processing computer, display electronics unit, etc.
The symbol generator does more than generate symbols. It has (at the least) monitoring facilities, a graphics generator and a display driver. Inputs from sensors and controls arrive via data buses, and are checked for validity. The required computations are performed, and the graphics generator and display driver produce the inputs to the display units.
Like personal computers, flight instrument systems need power-on-self-test facilities and continuous self-monitoring. Flight instrument systems, however, need additional monitoring capabilities:
- Input validation — verify that each sensor is providing valid data
- Data comparison — cross check inputs from duplicated sensors
- Display monitoring — detect failures within the instrument system
Former practice 
Traditional (electromechanical) displays were equipped with synchro mechanisms which would transmit, to an instrument comparator, the pitch, roll and heading that were actually being shown on the Captain's and First Officer's instruments. The comparator warned of excessive differences between the Captain and First Officer displays. Even a fault as far downstream as a jam in, say, the roll mechanism of an ADI would trigger a comparator warning.
The instrument comparator thus provided both comparator monitoring and display monitoring.
Comparator monitoring 
With EFIS, the comparator function is as simple as ever. Is the roll data (bank angle) from sensor 1 the same as the roll data from sensor 2? If not, put a warning caption (such as CHECK ROLL) on both PFDs. Comparison monitors will give warnings for airspeeds, pitch, roll and altitude indications. The more advanced EFIS systems, more comparator monitors will be enabled.
Display monitoring 
An EFIS display allows no easy re-transmission of what is shown on the display. What is required is a new approach to display monitoring that provides safety equivalent to that of the traditional system. One solution is to keep the display unit as simple as possible, so that it is unable to introduce errors. The display unit either works or does not work. A failure is always obvious, never insidious. Now the monitoring function can be shifted upstream to the output of the symbol generator.
In this technique, each symbol generator contains two display monitoring channels. One channel, the internal, samples the output from its own symbol generator to the display unit and computes, for example, what roll attitude should produce that indication. This computed roll attitude is then compared with the roll attitude input to the symbol generator from the INS or AHRS. Any difference has probably been introduced by faulty processing, and triggers a warning on the relevant display.
The external monitoring channel carries out the same check on the symbol generator on the other side of the flight deck: the Captain's symbol generator checks the First Officer's, the First Officer's checks the Captain's. Whichever symbol generator detects a fault, puts up a warning on its own display.
The external monitoring channel also checks sensor inputs (to the symbol generator) for reasonableness. A spurious input, such as a radio height greater than the radio altimeter's maximum, results in a warning.
Human factors 
At various stages of a flight, a pilot uses different combinations of data. Ideally, only the data in use would be displayed, but an electromechanical instrument has to be in view all the time. To improve display clarity, intricate mechanisms are used on ADIs and HSIs to remove superfluous indications temporarily, e.g., removing the glide slope scale when it is not being used.
With EFIS, some indications, e.g., engine vibration, might not be displayed under normal conditions. If limits are exceeded, then the reading will be displayed. In similar fashion, EFIS is programmed to show the glideslope scale and pointer only during an ILS approach.
If a failure of input data is detected, electromechanical instruments add yet another indicator to the display. Typically, a bar is dropped across the erroneous data. EFIS, on the other hand, removes invalid data from the display and substitutes an appropriate warning.
A de-clutter mode is activated automatically when the pilot's attention is required to be focused on a specific item. For example, if the aircraft is pitched up or down above a specified pitch, usually 30 to 60 degrees, the attitude indicator will de-clutter items from sight until the pitch is brought to an acceptable level. This allows the pilot to focus on the most important matter of aircraft control.
Although color has long been used in traditional instruments, it is restricted to aiding in identification of the data. There is no means of changing the color of any display component.
This restriction has been lifted with EFIS. For example, as an aircraft approaches the glideslope, a blue caption could indicate glide slope is armed; on capture the color might change to green.
On a typical EFIS system, the navigation needles are color-coded to reflect the type of navigation being used. Green needles are used for ground based navigation such as VORs, Localizers and ILS systems. Magenta needles are used for GPS navigation.
EFIS offers versatility by avoiding some of the physical limitations of traditional instruments. Thus, the same display which shows a course deviation indicator, can be switched to show the planned track provided by an area navigation or flight management system. If desired, the weather radar picture can then be superimposed on the displayed route.
The flexibility afforded by software modifications, minimises costs when new aircraft equipment and new regulations are introduced. The EFIS system can be updated with new software to extend its capabilities. Such updates introduced in the 1990s included ground proximity warning system and traffic collision avoidance system.
A degree of redundancy is available even with the simple two-screen EFIS installation. Should the PFD fail, transfer switching repositions its vital information to the screen normally occupied by the navigation display.
Advances in EFIS 
Recent advances in computing power and reductions in the cost of liquid-crystal displays and navigational sensors (such as GPS and attitude and heading reference system) have brought EFIS to general aviation aircraft. Notable examples are the Garmin G1000 and Chelton Flight Systems EFIS-SV.
Several EFIS manufacturers have focused on the experimental aircraft market, producing EFIS and EICAS systems for as little as US$1,000. The low cost is possible for several reasons, including steep drops in sensor prices and a lack of requirements to receive Federal Aviation Administration certification. This latter point restricts their use to experimental aircraft and certain other aircraft categories depending on local regulations. Uncertified EFIS systems are also found in Sport Pilot category aircraft, including factory built, microlight and ultralight aircraft. These systems can be fitted to certified aircraft in some cases as secondary or backup systems depending on local aviation authorities rules and regulations.
See also 
- Primary flight display and navigation display are the names used in the Federal Aviation Administration Advisory Circulars and also in ARINC Specification 725
- This driver is hardware, not software!
- Downstream and upstream refer to the direction of data flow; from sensor, to processor, to display
Further reading 
- Advisory Circular AC25-11A Electronic Flight Deck Displays, at the U.S. Federal Aviation Administration