B1-05.15 TYPICAL ELECTRONIC DIGITAL AIRCRAFT SYSTEMS 1

Typical Electronic/Digital Aircraft Systems (5.15)

• These systems often include Built-In Test Equipment (BITE).

Integrated Test Equipment

• In modern aircraft, the ability to troubleshoot is equally as important as repair or inspection.
• Self-contained diagnostics for electronic/avionics systems are known as BITE (Built-In Test Equipment).

BITE Systems

• Simple BITE systems use a go/no-go LED (red/green) on the equipment's black box or LRU.
• More complex systems use a multi-character display to monitor multiple LRUs and can test associated wiring.
• Current self-diagnostics use a centralized monitoring system with flight deck displays for test activation.
• Faults detected by BITE systems are monitored in one location, and can include paper printouts or data transmission to maintenance during flight.
• Boeing’s advanced system is known as CMCS (Central Maintenance Computer System).
• Airbus’ advanced diagnostic system is called Centralized Fault Display System (CFDS).
• Advanced systems are easier to use and understand, and they incorporate enhanced BITE features.
• Commercial airliners may have several BITE units monitoring various systems, each receiving inputs from individual components.
• Boeing 757/767 aircraft, for example, have BITE systems on approximately 50 LRUs.
• Individual systems often have dedicated BITEs within the LRU being monitored, accessed in the electronics equipment bay or a similar location.

BITE Philosophy

• Aircraft systems are generally composed of Line Replaceable Units (LRUs) like black boxes, sensors, or actuators.
• LRUs, controlled by digital computers, are permanently monitored for safety and troubleshooting.
• In each system, a dedicated computer section handles BITE functions.
• BITE continuously monitors LRU circuits, inputs, outputs, and links.
• During failures, BITE can identify the failed LRU, determine if the failure is intermittent or permanent, capture a system environment snapshot, and save details in non-volatile memory.

BITE Function

• BITE information in system memory is sent to a centralized maintenance device.
• Tests can be initiated from a centralized maintenance device, with benefits such as:
  • A single interface location (cockpit).
  • Easy fault identification.
  • Reduced troubleshooting duration.
  • Simplified technical documentation.
  • Standardized equipment.
  • Ability to divide BITE into 4 groups.

Power Up Test

• This test is a safety measure to ensure compliance.
• Duration depends on the system's operational status.
• Executed on the ground after power cuts exceeding 200 milliseconds.
• In-flight, the test is limited to enable a quick return to system operation.
• A power up test typically tests:
  • Microprocessor.
  • Memories.
  • Data lines and I/O circuits.
  • Configuration.

Cyclic Tests

• Cyclic tests, known as In Operation Tests, are carried out continuously without disturbing system operation.
• Tasks include:
  • Watchdog test: verifies that the microprocessor can be restarted if the software fails.
  • RAM test: tests random access memory.
  • Data line (ARINC 429) message validity.

System Test

• Maintenance crew uses this test to troubleshoot.
• Performed after system restoration (LRU replacement) to check system integrity, similar to a power up test.

Specific Tests

• Some systems have specific tests available to stimulate command devices like actuators or valves.
• These tests can significantly affect the aircraft like movement of slats or flaps or engine dry cranking.
• LRU and BITE concepts greatly reduce aircraft maintenance downtime.
• Once repairs are done, a complete operational check is performed.
• The BITE system monitors and verifies correct operation after repair.

Maintenance Control Display Unit (MCDU)

• The MCDU contains a screen, keyboard, function keys, mode keys, and line keys for sending commands to connected systems.
• The MCDU allows access to the following system components:
  • FM (Flight Management System)
  • ACMS (Aircraft Condition Monitoring System)
  • CMS (Central Maintenance System)
  • SAT (Satellite Communication System)
  • ATSU (Air Traffic Service Unit).

MCDU Utilization

• Accessing specific systems to review status or perform tests using the MCDU requires keystrokes.
• Upon power up, the MCDU displays the STATUS PAGE.
• Select MCDU Menu to display the MCDU MENU page.
• Select the line key next to CMS; the MAINTENANCE MENU will be displayed.
• Select the line key next to SYSTEM REPORT/TEST to display those pages. Scroll keys allow selection of the desired page.

Simple BITE Circuit

• Many systems built in the 1980s or later have some type of BITE circuitry.
• Some independent systems, like radio altimeters, include BITE circuitry in the main LRU.
• Tests are performed to press the test button while monitoring the LEDs, appropriate display with correct indications, and verifying the LED and display operation.
• The BITE circuitry performs simple tests on the transceiver, antenna, and the radio altimeter display.
• Similar tests are common on many LRUs found on any aircraft.

Self-Test BITE

• Some systems use a BITE feature initiated from a flight deck switch.
• The Ground Proximity Warning System (GPWS) uses a test button to test the computer and interfacing system. The test feature is available during-flight and on the ground.
• A successful test involves the following sequence after pressing and holding the test button:
  • The pilots BELOW G/S light illuminates and remains lit.
  • The GLIDESLOPE aural sounds once, followed by the WHOOP – WHOOP – PULL UP aural at reduced volume 2 to 8 times if the TEST button is held.
  • The pilots PULL UP light illuminates during the WHOOP – WHOOP – PULL UP aural.
  • The aural will sound again at full volume after releasing the TEST button after the initial sound.

Aircraft Communications Addressing and Reporting System (ACARS)

ACARS Introduction

• ACARS is a radio teletype mode developed in the early eighties as a digital data link for commercial and business aircraft by Aeronautical Radio.
• Designed to reduce the flight crew's workload, by using computer to make exchanges of routine reports and messages easy.
• ACARS provides a capability for on-board maintenance systems to transmit information to a ground station, including faults and performance data, reducing delays.
• Ground personnel can log fault data and make informed maintenance decisions.
• Via communications uplink, maintenance can interrogate aircraft systems, schedule repairs, and alert the destination station.
• Technicians, tooling, and parts are ready upon landing, reducing turnaround time.

ACARS Components

• Airborne components include the Management Unit (MU), Control Unit (CU) and a printer.
• The MU formats flight data collected from the Control Unit, aircraft, and event sensors.
• The MU generates a GMT clock signal and controls message transmissions/receptions via the VHF transceiver.
• The flight crew interfaces with ACARS through the Control Unit (CU), which has an alphanumeric keyboard to enter information or respond to requests.
• The ACARS MU and related subsystems are monitored by a BITE circuit, continuously monitoring ACARS components and reporting failures.
• A test can be initiated using the MU test switch.
• During the test, all four lights should illuminate, then extinguish before the appropriate light (green-OK, red-failed system) illuminates.

Data Bus Fundamentals

• Modern microprocessor replaced mechanical instrument with modern electronic systems.
• Information is displayed via a Multi-Function Display (MFD) or Digital Display Indicators (DDI).
• Signals sent to system components are linked through a digital data bus.
• Information is digitally transmitted to symbol generators from sensors (ARINC 429/629) or the Bus Controller (MIL-STD 1553).

Electronic Centralized Aircraft Monitoring (ECAM)

ECAM Introduction

• ECAM, used by Airbus aircraft (A320 and subsequent), displays information from major systems on two display units.
• Displays provide system status, warnings, cautions, and failures along with systems status and synoptic diagrams.
• Maintenance staff can view system status, diagrams of major systems, and failure indications recorded in the flight warning computers' memory.
• Displays are centre instrument panel, replacing analogue engine gauges in older models.
• With data bus technology, the ECAM display can be displayed in front of the Pilot, in place of EFIS if selected to.

ECAM Displays

• ECAM operates normally and abnormally.
• Displays colour-coded warnings, caution/advisory messages, and system status.
• The uppermost screen displays primary engine data, basic system data, and all warnings, cautions/ancillary messages of the Warning Computers.
• The display shows primary engine information, basic system information, and all the warning, caution, and ancillary messages on the left; in the right it shows secondary details.

ECAM System Operation

• The main component is the Flight Warning Computer FWC.
• The FWC receives inputs from aircraft systems. Analogue-based systems route Signal Analogue to Digital Converter, or SADC into the FWC.
• Both FWC's monitor the other for errors/malfunctions.
• The FWC output is sent to ECAM symbol generators, which transform the signal for ECAM displays.
• Under normal situations, FWC provides output to one of the symbol generators at one time.
• Newer Airbus models incorporate symbology generators within display units instead of having separate avionics boxes.

Electronic Flight Instrument System (EFIS)

EFIS Purpose

• The purpose of the EFIS display system is to provide the flight crew with the information required to operate the aircraft.

Electronic Flight Instrument System (EFIS) components

• EFIS consists of Cathode Ray Tubes, displays per pilot, three symbol generators, two control panels, and two source select panels.
• A third symbol generator so that signals can be switched to either the Captain or First Officer's display unit if their primary symbol generator fails.
• Symbol Generators provide interfaces between aircraft navigation/sensor systems and display units/control panel. They produce signals to drive the deflection, provide power control, and monitor system.
• The display controller provides switches and buttons to select information sources relevant to the current phase of flight.
• The display is not restricted to the CRT, it can be displayed on whatever display is selected.

Primary Flight Display (PFD)

• Combines information from an EADI and an EHSI onto a single CRT.
• Displays attitude, navigation/compass, flight control, and primary air data that are received from analogue-digital converters interfacing non-digital equipment to the data bus.
• Other navigation systems include Traffic Avoidance System TCAS, Inertial Reference Units, and Air Data.

Multifunction or Navigation Display (MFD or ND)

• The MFD displays lateral navigation/compass, radar, TCAS, flight management, and diagnostic information.
• The MFD backups PFD/EICAS in case of failure.
• It receives same data bus inputs on the PFD, plus input from weather radar, EICAS control panel, Central Maintenance Computer, and Flight Management Computer.

Boeing's Engine Indicating and Crew Alerting System (EICAS)

EICAS Introduction

• Like Airbus, Boeing jets display aircraft system information electronically on Cathode Ray Tubes (CRT) or Liquid Crystal Displays.
• Introduced on Boeing 757/767 aircraft, EICAS displays engine and system data eliminating traditional instrumentation.
• EICAS helps reduce the amount maintenance required.
• Primary engine data is always on display. During failures, the crew's attention is drawn to the automatic display of messages indicating significance of the problem, including status and operating values.
• Along with Central Maintenance Computers CMC, EICAS monitors the aircraft ensuring there are no system failures.
• EICAS records all system faults for flight crew or engineering staff investigation.
• Via the ACARS system, transmits information to a ground station (aircraft system faults/performance data).

EICAS Upper Display

• Normally shows primary engine indications, crew alert messages, flaps and landing gear status, fuel quantity, and environmental control information.

EICAS Lower Display

• Normally shows auxiliary EICAS formats, which includes engine operation, system status, synoptics, and maintenance pages.
• During normal flight, the lower screen will be blank (information is only displayed when selected by the pilots for required secondary information).
• Reason that the screen is blank is reduce the glare from the display, since it is centrally located between the pilots

EICAS System Operation

• Only one computer is in operation, while the other acts as redundancy and is ready to take over/switched over manually.
• The EICAS system performs the function, but doesn't use separate symbology generators.
• EICAS gives maintenance personnel information without additional workload for the flight crew, achieving subsystem parameters automatically when malfunctions are detected.
• The flight crew can manually record data, eliminate manual handwritten recordings of system data, and improve communication between aircrew and maintenance crews.
• When a warning arrives, its level is accessed, sent to the relevant areas for aural and visual display.
• There is a dedicated Maintenance Control Panel for use by engineers to display data.
• System failures that occur during a flight are automatically recorded and can be displayed by engineers after landing.

EICAS Control Panel

• Allows control of EICAS functions and selection of display modes.

EICAS Maintenance Control Panel

• System failures occurring during flight are automatically recorder and can be retrieved after landing. Displays are unavailable during a flight

EICAS BITE Tests

• Used to test computers, display units (upper/lower), caution and warning displays, various interfaces.
• Initiated by pressing the test on the MCP, with the parking brake MUST be set. Can be initiated with MCP test button
• Each computer is tested individually.
• The EICAS message “LEICAS TEST” and “TEST IN PROGRESS” will appear on the upper screen until it is complete, as will the message "R EICAS TEST"

Fly-By-Wire Flight Control System

Fly-By-Wire General Arrangement

• The flight control system uses digital computers to completely manage the surfaces.

• When the pilot moves the side-stick/rudder pedal, the electrical signals from the transducers and are then transmitted to the electrically operated actuators.

• No mechanical link between the pilot and controlled surface.

• Selected surfaces are dictated by a software program that manages the flight control system is designed.

• Factors affecting the flight control surfaces are considered prior to sending the signal.

• Benefits:

  • Windshear protection, reduced workload, lower costs, improved aircraft performance.
  • Reduced maintenance costs by removing the complex mechanical systems.

• Several different computers with specific functions provide redundancy and safety.

• Each computer will use different microprocessor types, suppliers, data buses, installations, and use a different supply + hydraulic sytsem.

Flight Management System (FMS)

FMS General Arrangement

• The FMS interfaces with the aircraft to aid the fuel quantity and flights control systems.
• Also connected and integrated with; air-to-ground data links, radio navigation, Internal Reference, Thrust Management, and the ECAM/EFIS systems.
• FMS components are centred around the Flight Management Computer (FMC).

Flight Management System (FMS)

• The FMS is used by the flight crew to reduce the crew workload.
• The FMC is effect the master cpu, and used in; inertial navigation, fight control, thrust management, air data, navigation sensors, and various EICAS computers.
• The FMS can automatically control the aircraft after take-off, up to roll out and landing.

FMC Operation

• One two or three FMCs are installed on one aircraft, with each FMC allocated to a particular space.
• All input data for the systems are shared across the FMCs
• A CDU or Command Display Unit is used to enter data, it is the sent to other fitted spaces.
• The output for a FMC is the shared across a CDU, which means that if a FMC fails, the aircraft can control the operation of the remaining FMCs

Control Display Unit (CDU)

• The CDU provides control of the FMC and allows access to FMC fault data.
• Printing of CDU display is also possible with in-flight printer.

FMS BITE

• Continuously monitor for using BITE software
• Automatically every power upp
• BITE tests can also be started using the central maintenance system.
• At test of all components, all systems each display specific messages.

Global Positioning System (GPS)

GPS Introduction

• Use of satellite to navigate vehicles of all kinds is ever growing

Airborne GPS Components

GPS configurations for aircraft: - Simple: A panel mounted unit - Remote mounted: Using a data bus to show the result, - Dedicated: Mutlifuntctional CDUs coupled with a computer

• GPS data is provided to the FMS for update to all systems

Receiver BITE Test

• The BITE test on receiver tests internal operation, and it's antenna.
• The test shows progress for approximately 36 seconds

Internal Reference System (IRS)

IRS Introduction

• IRS provides basic and complex data
• IRS can be used together with the total autopilot system The IRS is based on combining laser gyroscope and accelerometer technology
• IRS operation is separate to all other processes, with GPS updating taken periodically
• The IRU include circuity for signal processing, integration with other systems

IRS Components

• An aircraft can be installed with 3 IRSs This composes an inertial Reference Unit, which is a laser gyroscope and accelerometer, used in the navigation system
• Each installation has a Mode Selection Unit for operation
• An inertnial System Display Unit, one per operator

Standby Power Supply

• The IRS systems require a power supply to maintain operating status. Each IRS system has the ability to draw 115 to 28 VAC power.Each unit features a dedicated battery system

IRS BITE

• When energized, the IRU will run a BITE program
• The operator will need to enter aircraft location using CDU.
• Do not move the platform until the action is complete.
• The IRS will then be ready is completed.

Traffic Alert and Collision Avoidance (TCAS)

TCAS Introduction

• It is designed to provide advice to pilots on how to best avoid other traffic
• The equipment works independently from any aircraft support structure, forming a zone around the aircraft that is tracked for the potential of incidents.
• There are two warnings

Traffic Advisory (TA)

• Informs the crew of other intruder devices and aircraft

Resolution Advisory (RA)

• Used to give a visual directive to the crew, depending if the intent if to increase of decrease altitude.
• The system includes:
  • Computer
  • Display screen
  • Directional Antenna