SkyLicence
Chapter VII

Advanced Avionics - TCAS, Wx Radar, FMS, Autopilot, EFIS, EICAS

SkyLicence study guide with diagrams.

Diagram — Advanced Avionics - TCAS, Wx Radar, FMS, Autopilot, EFIS, EICASRA FAILFailed/disconnected TCAS antenna orcoaxial cableCheck antenna connections and cableintegrity firstTCASFAILTCAS computer failure or transponderinterface issueRun BIT; check transponder statusXPDRFAILMode S transponder failureCheck transponder power and operationalmode

Key Troubleshooting Principle: When a TCAS BIT fails for the transponder interface, the most likely cause is a faulty Mode S transponder or the transponder being in standby mode. The TCAS computer itself may be serviceable.

TCAS II — TA and RA TCAS II — TA and RA RA Zone TA Zone CLIMB AIRCRAFT A (Mode S) RA Zone TA Zone DESCEND AIRCRAFT B (Mode S Intruder) MODE S COORDINATION Complementary RAs TA (Traffic Advisory) • Traffic Alert (~40 seconds) • "TRAFFIC, TRAFFIC" (No maneuver) RA (Resolution Advisory) • Resolution Advisory (~25 seconds) • Vertical command: CLIMB / DESCEND TCAS II Logic • Interrogates Mode S Transponder • Calculates Tau (Time to CPA)

Built-In Test (BIT) Considerations

The TCAS BIT checks multiple interfaces separately:

Transponder interface BIT: Checks communication between TCAS computer and Mode S transponder
Antenna BIT: Checks TCAS antenna performance
Computer BIT: Checks TCAS processor integrity

2. Weather Radar System

System Components

The weather radar system consists of:

Receiver/Transmitter (R/T) Unit: Generates and receives RF energy
Antenna: Steerable parabolic dish with sweep and stabilization
Antenna Drive Motor: Provides azimuth sweep
Control Panel: Mode selection, gain, tilt, range
Display Interface: Data bus connection to EFIS

Operational Modes

WX (Weather): Standard weather detection mode
WX/TURB (Weather/Turbulence): Detects both precipitation and turbulence
MAP (Mapping): Ground mapping mode
TEST: Built-in test pattern verification

Antenna Tilt and Beam Position

Weather Radar — Antenna Tilt and Beam Position Weather Radar — Antenna Tilt and Beam Position Effect of tilt on precipitation detection during cruise TILT TOO HIGH (+15°) GROUND Aircraft Horizon +15° NO RETURN Beam passes above the clouds Adjustment error — NOT a malfunction TILT TOO LOW GROUND Aircraft -15° GROUND ECHOES Numerous parasitic returns masking the weather Incorrect tilt adjustment CORRECT TILT (0° to -5°) GROUND Aircraft DETECTION Precipitation correctly displayed Optimal setting for cruise TILT ADJUSTMENT — PRINCIPLES AND DIAGNOSIS Condition Observation Probable cause Technician action Tilt too high (+15°) No ground return Beam above the clouds Correct the tilt setting Tilt too low (-15°) Excessive ground echoes Beam reflected by the ground Increase the tilt Correct tilt (0° to -5°) Clear weather returns Beam passes through clouds No action required Note: The optimal tilt varies with cruise altitude, terrain, and the selected display range.

The antenna tilt angle is critical for proper weather detection:

Tilt too high (+15°): Beam points above horizon; no ground returns on ground testing
Tilt too low: Excessive ground clutter
Proper tilt: Optimized for weather detection at cruise altitude

Weather Radar Troubleshooting Scenarios

Important Relationship: The weather radar overlay on the EFIS map requires a dedicated data bus connection between the weather radar receiver/transmitter and the EFIS symbol generator. A fault in this bus will prevent overlay display even when the radar is functioning normally.

3. Flight Management System (FMS)

System Functions

The FMS integrates navigation sensors (GPS, IRS, VOR, DME) with a navigation database to provide:

LNAV (Lateral Navigation): Horizontal flight path guidance
VNAV (Vertical Navigation): Vertical profile guidance including climb, cruise, and descent
Performance Management: Speed and altitude optimization
Navigation Database: Waypoints, airways, procedures, and airspace data

Navigation Database Management

The FMS navigation database is organized by cycles (typically 28-day updates). Loading an incorrect database cycle means:

The FMS will still operate normally
Waypoints, procedures, and airspace data may be incorrect or outdated
A "DB MISMATCH" message occurs only if the database is incompatible with the FMS software version

Waypoint Sequencing Logic

FMS — Waypoint Sequencing and Abeam Point FMS — waypoint sequencing and abeam point Normal sequencing operation — not to be confused with a malfunction Direction of flight → WPT 1 (current) WPT 2 (next) WPT 3 ABEAM POINT Abeam NO-SEQUENCING ZONE SEQUENCING ZONE AIRCRAFT AIRCRAFT (sequencing completed) NORMAL SEQUENCING OPERATION • The FMS only sequences after passing the abeam point of the current waypoint. • Sequencing may also occur at a defined distance from the waypoint (depending on configuration). ⚠ THIS IS NOT A MALFUNCTION • Non-sequencing before the abeam point is a normal and expected FMS behavior. • No maintenance action is required. • Verify that LNAV mode is active. Chapter 7 — Advanced Avionics | FMS — Waypoint Sequencing | AME Training — Transport Canada Current waypoint Next waypoint Abeam point Before abeam — no sequencing After abeam — sequencing

The FMS sequences to the next waypoint only when the aircraft has passed the current waypoint's abeam point or a defined distance from it. This is normal operation and not a failure condition.

Direct-To (DIR) Function Requirements

For a DIR function to execute successfully:

49.The waypoint must exist in the FMS navigation database
50.The waypoint must be suitable for direct-to navigation (some waypoints like holds or procedure turns are excluded)
51.GPS or other position sensors must be available for navigation

VNAV Computation Requirements

VNAV — Vertical Profile Computation Requirements VNAV — vertical profile calculation and required data VERTICAL DESCENT PROFILE FL350 FL300 FL250 FL200 FL150 FL100 FL50 Cruise FL350 VNAV descent path Approach TOD Descent start WP1 WP2 WP3 ALT: FL240 250 kt max ALT: FL140 220 kt max ALT: 3000 ft 180 kt max DATA REQUIRED FOR VNAV ⚠ WITHOUT THIS DATA NO VNAV PROFILE CAN BE CALCULATED 1. Cruise altitude Initial altitude for calculating the descent slope. 2. Top of descent point (TOD) Position where the descent must begin to comply with the profile. 3. Speed and altitude constraints Limits imposed at waypoints (e.g., 250 kt max, FL240). 4. Flight plan waypoints Waypoints defining the lateral trajectory (LNAV). ✓ LNAV can work without VNAV But VNAV requires ALL the data above TOD required WP constraints TOD Waypoints Descent path Cruise VNAV = Vertical Navigation

VNAV requires specific pilot-entered data:

Cruise altitude
Top of descent (TOD) point
Speed constraints at waypoints
Altitude constraints at waypoints

If these parameters are not entered or are incorrect, the FMS cannot compute a VNAV profile even though LNAV functions correctly.

FMS Troubleshooting Scenarios

4. Autopilot System

System Architecture

The autopilot system consists of:

Autopilot Computer: Processes commands and generates servo signals
Flight Director: Provides visual guidance commands on the PFD
Servo Actuators: Physically move control surfaces (pitch, roll, yaw)
Servo Clutches: Engage/disengage servo to control surface
Engage Switches: Pilot controls for autopilot activation
Mode Selectors: Altitude hold, heading select, approach, etc.

Control Loop Fundamentals

Autopilot — Closed-Loop Control Fundamentals Autopilot — closed control loop COMMAND Selected altitude e.g. 10,000 feet Reference value Command signal Σ Comparator Command − Feedback ERROR Error signal Pitch deviation Amplified by gain GAIN ADJUSTMENT Gain K Incorrect gain → over-correction Amplification SERVO CONTROL Elevator Pitch actuator Excessive hysteresis → delay Command AIRCRAFT RESPONSE Pitch / attitude Altitude variation SENSORS Altimeter / ADIRU Actual aircraft state VERTICAL OSCILLATION — ALT HOLD MODE Selected alt. Climb Descent Climb Descent Over-corrections due to gain/hysteresis CAUSES OF INSTABILITY Incorrect gain: amplification too strong Excessive servo motor hysteresis Delay in the feedback loop Closed control loop Sensor feedback is compared to the command to generate the error signal LEGEND Signal Feedback Fault Flow Transport Canada — AME Training (TEA) — E-ELECTRONICS ch7: Advanced Avionics — Autopilot

The autopilot operates as a closed-loop control system:

Input: Selected mode parameters (e.g., selected altitude)
Feedback: Actual aircraft state from sensors
Error Signal: Difference between desired and actual state
Servo Command: Corrective signal to eliminate error
Gain Settings: Determine responsiveness and stability

Autopilot Failure Modes

Flight Director vs. Autopilot Relationship

The flight director and autopilot share sensor inputs but have separate output paths:

Flight Director: Generates visual commands (no physical control movement)
Autopilot: Generates physical control movement via servos

When the flight director displays correct commands but the autopilot won't engage, the fault is in the servo system, not the autopilot computer or sensors.

Approach Mode Considerations

During an autopilot approach:

Localizer capture: Requires functioning localizer receiver and antenna
Glideslope capture: Requires functioning glideslope receiver and antenna
Mode arming: Both localizer and glideslope must be armed; if approach mode isn't armed, neither will capture

5. Electronic Flight Instrument System (EFIS)

EFIS and EICAS — Display Architecture EFIS and EICAS — Display Architecture DATA SOURCES ADC (Air Data Computer) Pitot / Static / TAT AHRS / IRS Inertial Reference Attitude / Heading / Accel. FMS Flight Management Nav / Perf / Flight Plan ENGINE SENSORS FADEC / EEC N1, N2, EGT, FF, Press. CONTROL PANEL EFIS / EICAS Mode / Range Select PROCESSING & BUS SYMBOL GENERATORS (SG) Data Conversion → Vector Graphics DIGITAL DATA BUS ARINC 429 AFDX / Ethernet MIL-STD-1553 CREW DISPLAY (CRT / LCD) PRIMARY FLIGHT DISPLAY 250 240 230 FL350 FL340 FL330 HDG 270° NAVIGATION DISPLAY WPT1 WPT2 MAP MODE EICAS / ECAM N1 ENG 1 N1 ENG 2 EGT EGT ALERTS Source: Transport Canada AME Training - Advanced Avionics Ch.7

System Components

Symbol Generator (SG): Primary processor generating display symbology
Display Units (DU): Cathode ray tube or LCD screens
Display Backlight Inverter: Provides illumination for CRT displays
Data Buses: ARINC 429 or similar for data transfer

Primary Flight Display (PFD) Information Sources

Attitude: Attitude and Heading Reference System (AHRS) or Inertial Reference System (IRS)
Heading: AHRS flux valve/magnetometer
Airspeed: Air Data Computer (ADC)
Altitude: Air Data Computer (ADC)
Vertical Speed: Air Data Computer (ADC)

Failure Mode Analysis

Key Relationship: When both PFDs show the same failure (e.g., ATT FAIL), the fault is in the common source (AHRS/IRS). When only one PFD shows a failure, the fault is in the display-specific component (symbol generator or display unit).

Symbol Generator Power-Up

The symbol generator requires electrical power to operate. If it fails to power up:

108.Check the circuit breaker (most likely cause)
109.Check the power supply
110.Verify other aircraft systems are normal (to rule out general power failure)

6. Engine Indication and Crew Alerting System (EICAS)

System Functions

EICAS provides:

Engine parameter display: N1, N2, EGT, oil pressure, oil temperature, fuel flow
Crew alerting: Warnings, cautions, and advisories
System synoptic displays: Bleed air, electrical, hydraulic, fuel
Maintenance data: Fault recording and display

Alert Levels

Warning (Red): Immediate action required (e.g., BLEED TRIP, DUCT LEAK)
Caution (Amber): Awareness required, action may be needed (e.g., GEN OFF, FUEL IMBAL, L OIL PRESS)
Advisory: Information only

EICAS Message Interpretation and Troubleshooting

Troubleshooting Philosophy for EICAS Warnings

When an EICAS warning or caution appears:

126.Do not assume sensor failure - The indication may be real
127.Verify the actual condition before replacing components
128.Physical inspection is the first step for warnings like DUCT LEAK
129.Check circuit breakers for electrical-related cautions like GEN OFF
130.Consult maintenance manual limits for conditions like FUEL IMBAL

Important Relationships Between Systems

Data Bus Interconnections

Modern avionics systems communicate via digital data buses (typically ARINC 429):

Weather radar to EFIS symbol generator (for overlay display)
FMS to autopilot (for LNAV/VNAV guidance)
TCAS to EFIS (for traffic display)
ADC to EFIS, FMS, and autopilot (for air data)

Sensor Sharing

Multiple systems share common sensors:

AHRS/IRS: Provides attitude and heading to EFIS, autopilot, and FMS
ADC: Provides airspeed, altitude, and vertical speed to multiple systems
GPS: Provides position data to FMS and other navigation systems

Failure Propagation

A failure in one system can affect multiple displays:

AHRS failure: Affects both PFDs (ATT FAIL)
ADC failure: Affects airspeed and altitude on both PFDs
Symbol generator failure: Affects only one display unit

Normal vs. Failure Conditions

Understanding what constitutes normal operation is critical:

TA-ONLY mode below 1,000 ft AGL: Normal, not a failure
FMS not sequencing before abeam point: Normal operation
VNAV not computed without pilot data: Expected behavior
Incorrect database cycle: System operates but with potentially incorrect data

Summary of Troubleshooting Principles

157.Verify the actual condition before replacing components
158.Check circuit breakers first for electrical failures
159.Physical inspection is appropriate for mechanical/ducting warnings
160.Common source failures affect multiple displays (e.g., AHRS failure)
161.Localized failures affect only one display (e.g., symbol generator failure)
162.Normal operations must be distinguished from failures (e.g., TA-ONLY mode)
163.Data bus faults can prevent system integration (e.g., radar overlay)
164.Pilot-entered data is required for some FMS functions (e.g., VNAV)

This comprehensive understanding of advanced avionics systems, their interrelationships, and troubleshooting approaches will prepare the AME to diagnose and resolve the most common system failures encountered in modern aircraft maintenance.

Chapter 6Navigation Systems - VOR, ILS, DME, ADF, GPS, GNSS, TransponderChapter 8Instruments and Sensors - Pitot-Static, AHRS, IRS, Radio Altimeter, ADCs

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