B1-05.01 ELECTRONIC INSTRUMENT SYSTEMS

• The purpose is to describe typical arrangements of electronic instrument systems, and typical cockpit layouts.

Early Instrument Systems

• Early aircraft had limited instruments: a clock (optional) and a compass acceptable for daytime flights in clear weather.
• An altimeter and a simple attitude instrument got added next.
• Early instrument panels were disorganized, leading to pilots needing to relearn instrument positions for different aircraft.
• After 1950, aircraft cockpit instrument systems were arranged in the Basic T configuration.
• Aircraft had an Airspeed Indicator (ASI), Attitude Indicator (AI) and altimeter in a row, which made the information immediately identifiable.
• The compass was located below these instruments.
• This arrangement enabled pilots to fly in poor weather without disorientation.
• Additions included a turn coordinator (or a turn and slip indicator) and a Vertical Speed Indicator (VSI).
• They were added at the bottom left and bottom right, respectively.
• These additions provided more accurate and immediate information to the pilot.
• Turn coordinators show wing level position for fewer corrections.
• Larger aircraft like the 747 Classic use the Basic T configuration.
• The rad alt, standby instruments and special features such as ILS, VOR guidance indicators and a rising runway make it different.
• Additional functionality renamed the Artificial Horizon and Directional Gyro to the Attitude Direction Indicator (ADI) and the Horizontal Situation Indicator,(HSI) respectively.
• Standby instruments and engine instrumentation are placed in the center of the panel so both pilots can easily read.

Electronic Instrument Display Technology

• Common types of electronic displays in aircraft include light-emitting diodes and liquid crystal displays. Older displays can use Cathode ray tube technology but this technology is becoming rarer due to reliability, physical size, and service life.
• Signals from sensors and digital data buses do not generate a display on a screen, they must be interpreted by processors and electronic devices.
• The display units then generate signals to show data.

Arrangements of Electronic Instrument Systems

• The Electronic Flight Instruments System (EFIS) provides flight crew with the necessary information.
• EFIS displays primary aircraft operational information on display screens instead of analogue instruments.
• The EFIS reduces instrument clutter and maintenance requirements, increasing reliability and serviceability.
• A typical EFIS setup has four interchangeable displays: an EADI and EHSI for both the captain and first officer.
• There are three symbol generators, two control panels, and two source selection panels.
• An additional (center) symbol generator provides redundancy for primary symbol generator failure.
• In aircraft with an avionics digital data bus and multifunction displays the EFIS displays are not restricted to displaying EADI and EHSI.
• EFIS setups typically include two Primary Flight Displays (PFDs) and two Navigation Displays (NDs).
• Modern display systems offer large, uncluttered displays for better crew efficiency and situational awareness.
• Formats allow displays to be used interchangeably as a Primary Flight Display (PFD), a Multifunction Display (MFD), or an EICAS/ECAM display.
• PFDs include attitude, air data, navigation references, and Traffic Collision Avoidance System (TCAS) resolution advisories.
• M FDs include navigation maps, weather radar, TCAS traffic, and maintenance data.
• Operation is broken down into: Electronic Flight Instruments System (EFIS) with PFD and MFD, and the Engine Indication and Crew Alerting System (EICAS – Boeing) or Electronic Centralised Aircraft Monitoring (ECAM – Airbus).
• Central displays are commonly EICAS or ECAM, but all six displays can be interchanged.
• Current aircraft use the Basic T configuration system to reduce training times.

EADI and EHSI

• Improvements in display screen manufacturing led to the replacement of mechanical ADI and HSI units with the Electronic Attitude Direction Indicator (EADI) and Electronic Horizontal Situation Indicator (EHSI).
• The EADI integrated airspeed (IAS), including stall/overspeed warnings, altitude and rate of climb, autopilot, and other information, consolidated onto one screen.
• The name was changed to the Primary Flying Display (PFD).
• The EHSI Integrated with ADF, ILS, VOR and flight plan Map Information as well as color coding. It provided easier and more consistent mode recognition.
• Including aircraft speeds (GS and TAS) and heading/track.
• Annunciation of which NAV equipment supplies data.
• The name was changed to Navigation Display (ND).
• Navigation and terrain information can be integrated with a worldwide mesh terrain database, weather radar, and traffic data.
• PFD and ND can provide Resolution Advisories in the event of a collision threat.

Cockpit Layout

• The Boeing system uses the Engine Indicating and Crew Alert System (EICAS) for airframe and engine data.
• Upper and lower screens display primary and secondary engine data and cautions regarding system status.
• System synoptics are also displayed on these screens.
• The Airbus system uses the Electronic Centralised Aircraft Monitor (ECAM) to provide engine, system, and synoptic data.
• The primary and secondary screens show engine data and system status.

Multifunction Displays and Digital Data Bus

• Digitally based microprocessor electronics in instruments allow information display on one or more Multifunction Displays (MFDs) or Digital Display Indicators (ODIs).
• Systems communicate via a digital data bus.
• A symbol generator connected to the bus communicates with other devices via sensors (ARINC 429 and 629) or a Bus Controller (MIL-STD 1553).
• Systems connect to a data bus, transmitting info for indication requirements.
• Some simple setups may have limited transmittal on the data bus.
• Systems still pass information via analogue to digital converters (ADCs) and receive instructions through digital to analogue converters (DACs).
• Data buses are usually duplicated, where the second bus will take over if one fails
• Separated buses in parallel are called either channels A and B, or channels 1 and 2.
• Common failures are shorted and uninstalled terminals, LRU failures and continuous transmission.

Symbol Generators

• Symbol generation is the core of the electronic display system, with commercial aircraft having at least two.
• The symbol generator unit (SGU) receives sensor inputs, and processes and transmits the information to the electronic displays.
• Symbol generators provide the analogue, discrete, and digital signal interfaces.
• They also perform power control, symbol generation, and system monitoring.
• Within the electronic instrument display system, symbol generators get information and send it to the display, selected by the aircrew.
• Symbol generators receive data from the data bus.
• This allows back-up units to replace failed units.
• Display systems use up to three separate channels of a data bus, still functioning if two fail.
• The symbol generator processes given information before sending it.
• Information is sent from the symbol generator to the I/O processors, where the different data is extracted and storage locations are allocated in the RAM.
• Recalling information from RAM, it is displayed by the display controller.

Display System Data Types

• There are two types of data to be written on the screen: raster and stroke
• The raster data are overwritten onto the Weather Radar (WXR) Memory as the WXR is also raster information.
• The memory has a pixel for pixel space correlation on the screen, containing colour and brightness information.
• First the Weather Radar (WXR) writes in the WRX memory directly from the WXR.
• Then the data are written from the symbol generator over the top.
• Stroke data are a method of drawing on the screen so that circles and lines do not have a jagged appearance.
• This requires a location on the screen where a line must be drawn at the end of each row of pixels of the raster scan.
• The Raster Generator is the master timing device for both types of information.

Electronic Centralised Aircraft Monitoring(Airbus)

• ECAM displays are usually located in the centre of the instrument panel where the analogue engine instruments were traditionally located.
• In older models, the ECAM displays only display data generated by Flight Warning Computers (FWC) and ECAM symbol generators.
• Displays in front of the pilot are dedicated to providing EFIS information.
• ECAM is not restricted to centre displays, it can be displayed where the EFIS displays would go.

ECAM/EICAS

• The ECAM control panel features two display brightness controls, and push-button switches.
• Synoptic display illuminate upon pressing their diagram.
• The Flight Warning Computer integrates a maintenance panel for system testing of displays and generators.
• Unlike ECAM, the EICAS system does not incorporate individual symbology generators, being performed by computers
• EICAS monitors data, dispatch info, stores maintenance data, has color displays and alert messages.