5.15 Typical electronic digital aircraft systems

Module 5: Digital Techniques and Electronic Instrument Systems

5.15 Typical Electronic-Digital Aircraft Systems

Overview CMS & BITE: Computer Maintenance System & Built-In Test

Equipment ACARS: Aircraft Communication Addressing and Reporting System EFIS: Electronic Flight Instrument System EICAS & ECAM FBW: Fly-By-Wire FMS: Fly Management System GPS: Global Positioning System INS / IRS: Inertial Navigation System / Inertial Reference System TCAS: Traffic Collision Avoidance System DFDR / CVR: Digital Flight Data Recorder / Cockpit Voice Recorder

On-Board Maintenance Facilities CMU BITE ACMS

On-board Maintenance Facilities Data on the aircraft are acquired by:

BITE Built-in Test Equipment:

A system is composed of LRUs, which can be computers, sensors, probes, actuators etc. which perform specific functions.

A part of each system is dedicated to functions such as: monitoring, testing and troubleshooting.

This part of the system is the Built-in Test Equipment. BITE can (a) perform error detection test (b)

isolation: identify the possible failed LRUs and give a snapshot of the system when the failure occurred, (c) memorization: record the error in a memory device.

The information are sent to the Centralized Maintenance Computer.

BITE Concept

Testing with BITE Several kinds of tests:

Power-up test: Ensuring compliance with safety objectives. It is performed only on ground, because they disturb normal

operation. They are performed after long power cuts (more than

200msec). If the aircraft is airborne the test is limited to a few items to

enable a quick return to operation of the system: CPU test memory test ARINC test I/O test configuration test

However, when we normally refer to power-up tests, we assume the aircraft is on ground.

Testing with BITE Cyclic tests (also called operation test):

They are carried out permanently, because they do not disturb normal operation.

Examples: Watchdog test (i.e. CPU reset). RAM test.

System tests: Tests available to the maintenance staff, for troubleshooting purposes. Similar to ground power-up tests, but more complete. Examples: Tests performed after the replacement of an LRU.

Specific tests: Available only to specific systems. They are performed to generate stimuli to other devices, such as

actuators or valves. They have major effect on aircraft (such as automatic moving of flaps

etc.) They are performed only on ground by maintenance staff.

BITE Inside a Computer

Make a power up test when the computer starts-up

Operate normally and perform the cyclic tests.

If we are on ground, provide the option to do system or specific tests. Otherwise, continue with normal operation.

ACARS

Aircraft Condition Monitoring System Monitoring of engine, APU,

performance monitoring and troubleshooting.

Collects, records and processes aircraft system data.

ACMS data can be forwarded to the MCDU, to a printer, transmitted through ACARS etc.

ACMS architecture It consists of two parts:

Data Management Unit (DMU): Handles and stores system data.

Flight Data Interface Unit (FDIU): Provides DMU with data from the engines, systems etc.

DMU(Data Management Unit)

Electronic Library System Collection and presentation of technical and

operational material relating to aircraft, in a digital form.

Can be accessed by flight crews and maintenance staff through computers.

It’s a database of guides and manuals, stored in a computer and accessed by an LCD touch screen.

Usually, an ARINC 744A printer is connected to the library system, for document printing. ARINC 744A is the standard airborne printer

protocol.

Aircraft Communication Addressing and Reporting System

ACARS: An air-ground

communication data linked network.

It is used to transmit or receive automatically or manually generated reports to or from the ground station.

ACARS protocol was designed by ARINC.

Communication can be transferred through ground VHF stations or SATCOM (Satellite Communication).

Why ACARS? Prior to ACARS development all

communications between aircraft and ground were VHF or HF voice communications.

To reduce crew workload and ensure data integrity, developed ACARS communication system.

VHF Usage

AM: KHz (Not used in aviation). HF: 3 – 30MHz (Used in aviation for longer

range, e.g. when flying above Antarctic). VHF: Above 30MHz (Normally used in aviation).

Note: Higher frequencies are more easily absorbed by objects.

Automatic ACARS messages ACARS interface with FMS (Flight Management

Systems) enables the automatic receiving of weather reports from the ground.

Major flight phases (OOOI): Out of the gate, Off the ground, On the ground,

and Into the gate messages. Engine reports in real time can be

automatically sent from the airplane to the airliners. In case of failures during flight, real time

information can be uploaded by the manufacturers associated with the fault.

Non-automatic messages: Interaction between the crew and the ground. Communication between the flight crew and

the ground is made through FMS (it’s similar to email). Messages examples:

Weather Winds Clearances …

After Air France Flight 477 it has been discussed to make ACARS an online black-box, to retain flight information in case of lost black-box.

How is an ACARS message propagates? Through a VHF network:

Only applicable to land masses, where a VHF ground network is installed.

Used for up to 200miles transmission range. Through an HF network:

Used in areas such as the poles and oceans. Completed in 2001.

Through satellites: Provides world-wide coverage.

The message passes through an ACARS network (through ARINC’s servers) to the operator’s center. The operators center can be either CAA or a flight operator. ACARS service providers are used to propagate the

message to the destination.

ACARS messages categories 2 types of messages:

Air Traffic Control: Messages from / to ATC. (e.g. clearance). Aeronautical Operation Control: Messages from / to the base

(flight operation department). (e.g. fuel consumption, engine performance etc.)

The message format is defined by a specific ARINC protocol. Each message contains an address label.

Message Example:

ACARS mode: H Aircraft reg: D-AIRL [A321-131]Message label: 1L [Off message] Block id: 9 Msg no: M23AFlight id: LH3394 Org: LH06LT [Munich, Germany-Athen, Greece] [Lufthansa]Flight distance: appr. 1511km Flight time: appr. 1.7 hoursMessage content: 00016234212AN((628D8UVPCR(GKTRRUBW

ACARS vs. CPDLC Controller–pilot data link

communications It is built on ACARS. The aim is to reduce

voice congestion. It’s another communication system

between the flight crew and the flight controller.

Similarities with ACARS: Uses VHF, HF and Satellite. Text messages.

Differences with ACARS: Designed only for communication

between the flight crew and the controller. However, in Boeing 777 CPDLC

messages can be sent to the company, as well.

The future of ACARS ATN: Aeronautical

Telecommunications Network An integrated

network inspired from the Internet architecture.

ACARS uses character messages, while ATN uses binary format.

EFIS

Electronic Flight Instrument System EFIS:

PFD (Primary Flight Display) ND (Navigation Display)

EFIS Overview

Airbus 320 Primary Flight DisplayFlight Mode Annunciator

The flight mode annunciator (FMA), shows:

Autopilot operation AP/FD vertical and lateral

modes Approach capabilities AP, FD, A/THR engagement

status.

Green color is “engaged”. Blue color is “armed”. White is related to approach indications in column 4. Magenta are target speed, altitude etc.

Flight Mode Annunciator

Airspeed and Altitude Indications

Speed after 10sec.

PFD Errors and Messages

Boeing 777 Primary Flight Display

Boeing 777 Primary Flight Display

Airbus 320 Navigation Display

ND Warnings and Messages

Boeing 777 Navigation Display

Boeing 777 Navigation Display

Boeing 777 Navigation Display

Boeing 777 Navigation Display

ECAM

EIS Components

DMC: Digital Management Computer or Symbol Generator: Generates

data in a compatible format with the display units.

Contain CPUs, RAM, display drivers, raster generators etc.

In case of failure of DMC1 or DMC2.

System Data Acquisition concentrator

EICAS Control Panels

Crew Alerting System Examples

Fly-By-Wire

Fly-By-Wire Fly-By-Wire (FBW) is a system that replaces

the conventional manual flight controls of an aircraft with an electronic interface.

Flight control computers determine how to move the actuators at each control surface to provide the ordered response.

The movements of flight controls are converted to electronic signals transmitted by wires.

Fly-By-Wire allows automatic signals sent by the aircraft's computers to perform functions without the pilot's input, as in systems that automatically help stabilize the aircraft

Conventional Flight Control Systems vs. Fly-By-Wire Mechanical systems are heavy, non-accurate,

prone to failures and errors. They have limited ability to compensate for changing aerodynamic conditions.

Fly-By-Wire implies a purely electrically signaled control system. Computer configured controls: A computer system

is interposed between the operator and the final actuator.

Fly-By-Wire examples: Side-sticks Control yokes …

Fly-By-Wire Philosophy

The mechanical system that controls the servomechanism, which moves the surface is replaced by a computer.

Advantages of Fly-By-Wire Due to the complex calculations that

computers can make, they can make decisions without the pilot input. e.g. Automatic stability systems.

Safety: More than one wires can be easily used to ensure

the propagation of a signal. More than one computers can be easily used, to

ensure proper operation when one computer fails. BITE

Weight Saving

History of Fly-By-Wire Tupolev ANT-20 in 1930:

The first airplane, where long runs of mechanical systems were replaced by electrical systems.

Concorde (1986):  Mechanical servo valves

were replaced with electrically controlled servo valves, operated by an analogue electronic controller.

More sophisticated analogue computers were used in early versions of F-16.

Digital Fly-By-Wire In digital fly-by-wire flight

control systems, the signal processing is done by digital computers and the pilot literally can "fly-via-computer".

The programming of the digital computers enables flight envelope protection. Aircraft protection, reduced

pilot’s workload. FADEC: Full Authority Digital

Engine Control Permits control of flight surfaces

and engine autothrottles to be fully integrated.

FADEC advantages FADEC contains a digital computer and a unit that

controls the engine. Allows maximum performance to be obtained from

the engine. Protection from dangerous situations such as low-

speed stall or overstressing by flight envelope protection. the flight control systems commands the engines to

increase thrust without pilot intervention. In economy cruise modes, the flight control systems

adjust the throttles and fuel tank selections more precisely than all but the pilots.

Further Fly-By-Wire developments Fly-by-optics

Signal is transferred by light instead of current. Power-by-wire

Having eliminated the mechanical transmission circuits in fly-by-wire flight control systems, the next step is to eliminate the bulky and heavy hydraulic circuits.

Fly-by-wireless Intelligent Flight Control System

In case of a failure leading to a crash, computers make complex calculations to adjust the flight controls in a proper position to save the aircraft. It is believed that enchantments are mostly software

upgrades to the existing infrastructures.

FMS

Flight Management System: Introduction FMS basic operation:

Compares the pilot selected flight plan with the actual horizontal and vertical aircraft position.

In case of difference between them, the FMS makes a steering and thrust command.

The FMS input and output device is the: CDU: Control Display Unit or MCDU: Multifunction

Control Display Unit

FMS operations The pilot sets the flight plan through the

MCDU. A database with airports, runways, waypoints is

used. FMS automatically selects optimal parameters

e.g. climb ration, optimal speed etc. Shows information about the flight plan on

MCDU. Exchanges information and commands the

Autopilot / Autothrottle Flight System AFS. Accepts DME and VOR inputs. Gives information to the EFIS displays.

FMS Description Navigation:

FMS uses information form its database to automatically tune the navaids (ILS, VOR, DME).

Database must be updated every 28 days. Performance:

The FMS calculates the shortest possible flying time at the lowest fuel consumption. Can give predictions of fuel quantities and arrival times at future points in the flight plan.

Guidance: The FMS compares the desired position of the aircraft according

to the flight plan, with the actual aircraft position. If there is a difference, FMS commands the AFS to bring the aircraft to the desired position.

Position and velocity are calculated using the IRS, GPS, VOR and DME.

EFIS Display: FMS is the primary source of information displayed on EFIS.

Setting up the FMS INIT:

Set the take off runway and destination. Set fuels. Insert the waypoints.

F-PLN: Check or modify the flight plan. Eliminate discontinuities.

Performance: Set flaps, weather and other information

that affects performance for each flight phase.

Flight plan is displayed on ND.

FMS block diagram

Flight Management Computer : Performs the Navigation and performance calculations. Stores the database and the selected flight plan. Tunes to navaids. Commands the AFS. Makes EFIS display calculations

Performance and in-flight displays

GPS

GNSS GNSS (Global Navigation Satellite System) is

an umbrella term for systems which are used to navigate and determine current position based on signals received from dedicated navigation satellites.

4 most important GNSS systems: GPS GEONASS Galileo Compass

Principles of Operation All satellite navigation systems use

the same principle as DME (Distance Measuring Equipment): The receivers measure the time it

takes for a radio signal (around 1.5GHz) to travel from a transmitter in a satellite at a known point in space.

The receiver’s computer calculates the distance for more than one satellites.

Satellites and Space Segment There are 6 orbital planes with 4

satellites in each plane. Each plane is inclined 55

degrees relative to the equator. In the American GPS (NAVSTAR),

there are 24 satellites at 11,000nm moving around the globe and return to the same position after 12 hours.

Errors in Transmission After the third

measurement, one of the two possible points can be discarded, since it is far from the earth surface.

Possible errors that degrade the accuracy: Atmospheric conditions Noise due to sunspot

activity. Satellite clock drift:

Variations of the clock of the satellite transmitter clock. (1nsec drift causes 1ft. distance error).

Calculations from a 4th satellite are needed to eliminate the effects of these errors.

The European GPS System A system of 30 satellites (under

development). Higher accuracy that the NAVSTAR (down

to less than a meter). .

Supplementary Systems needed for aircraft Navigation

Aircraft-based Augmentation Systems: Sensors on the aircraft to detect

the quality of the GNSS data received and correct them if necessary.

Satellite-based Augmentation Systems: Geostationary satellites detect

errors and correct GNSS signals transmitted to users.

They are limited to certain regions of the world.

WAAS (USA), EGNOS (Europe). Ground-based Augmentation

Systems: Ground stations around the

airports enhance positioning accuracy.

They are considered a long term replacement to ILS.

Example: Differential GPS: A ground-station propagates the GPS error to GPS receivers.

INS / IRS

Accelerometer Acceleration moves

the strings to the opposite direction of the movement.

The acceleration of indication can be integrated once to give velocity and once more to provide distance.

On the aircraft, induction is are used.

By knowing the starting position (IRS alignment) an aircraft can calculate the distance covered.

The Gyroscope principle When the rotor spins,

no matter how the plane rotates on the yaw, the plane at which gyro rotates remains the same. The gimbal will move

so, the spin axis remain the same, no matter how it will rotate.

Inertial Navigation System Mechanical

gyros: A gyro along

with an electrical system to measure the distance between the gyro spin axis and the gimbal movement.

Inertial Reference System Mechanical gyros are

replaced with laser gyros, for greater accuracy.

Movement of gimbal is measured with the difference between arrival times of two laser beams.

When rotation takes place, the orientation of the mirrors changes, thus the beams reach at different times the detector.

TCAS

TCAS: Traffic Alert Collision Avoidance System

System designed to reduce the incidence of mid-air collisions between aircrafts.

When another aircraft appears in the vicinity an automatic negotiation is being made between the 2 aircrafts to avoid collision.

Information are displayed in EHSI (Electronic Horizontal Situation Indicator)

A number next to each aircraft shows the height of each aircraft in comparison to this one.

TCAS Alerts Traffic Advisory (TA):

Pilots are instructed to initiate a visual search for the traffic causing the TA.

 If the traffic is visually acquired, pilots are instructed to maintain visual separation from the traffic.

Resolution Advisory (RA): Pilots are expected to

respond immediately to the RA.

This means that aircraft will at times have to maneuver contrary to ATC instructions or disregard ATC instructions.

Clear of Conflict (CC): Pilots shall promptly return to

the terms of the ATC instruction.

TCAS Block Diagram

TCAS AdvisoriesType Text Meaning Required action[1][2][5]

TA Traffic; traffic.Intruder near both horizontally and vertically.

Attempt visual contact, and be prepared to maneuver if an RA occurs.

RA Climb; climb. Intruder will pass belowBegin climbing at 1500–2000 ft/min

RA Descend. Descend. Intruder will pass above.Begin descending at 1500–2000 ft/min

RA Increase climb. Intruder will pass just below Climb at 2500 – 3000 ft/min.

RA Increase descent. Intruder will pass just above. Descend at 2500 – 3000 ft/min.

RA Reduce climb.Intruder is probably well below.

Climb at a slower rate.

RA Reduce descent.Intruder is probably well above.

Descend at a slower rate.

RA Climb; climb now.Intruder that was passing above, will now pass below.

Change from a descent to a climb.

RA Descend; descend now.Intruder that was passing below, will now pass above.

Change from a climb to a descent.

RAMaintain vertical speed; maintain.

Intruder will be avoided if vertical rate is maintained.

Maintain current vertical rate.

RAAdjust vertical speed; adjust.

Intruder considerably away, or weakening of initial RA.

Begin to level off.

RA Monitor vertical speed.Intruder ahead in level flight, above or below.

Remain in level flight.

RA Crossing.Passing through the intruder's level. Usually added to any other RA.

Proceed according to the associated RA.

CC Clear of conflict.Intruder is no longer a threat.

Return promptly to previous ATC clearance.

FDR & VDR

Flight Data Recorder (FDR) Flight Data Recorders store snapshots

of the following information: Altitude Heading Airspeed Acceleration Thrust on each engine Use of Autopilot Angle of attack Air temperature …

These information are from the same sources that supply the flight crew.

Recording begins with the start of the first engine and ceases at shut-down of the last engine.

Must survive impact velocity of 270knots.

Each snapshot is taken 1-2 times per second.

Can record from 17 – 25 hours continuously.

Crash survivable memory unit.

Underwater Locator Beacon (emitting for 30 days up to 20,000 ft.)

Power Supply

Cockpit Voice Recorder (CVR) Often referred as “black

box”. Records all the

communication transmitted or received by to / from the flight deck.

Voice communication between the flight crew.

All sounds in the cockpit, e.g. audio signals, sound alarms.

Must be capable of recording for at least 2 hours.

Recording begins with the start of the first engine and ceases at shut-down of the last engine.