SkyLicence
Chapter VIII

Instruments and Sensors - Pitot-Static, AHRS, IRS, Radio Altimeter, ADCs

SkyLicence study guide with diagrams.

Diagram — Instruments and Sensors - Pitot-Static, AHRS, IRS, Radio Altimeter, ADCsASI reads zero during taxiPitot cover left onNo pitot pressure reaches theinstrumentASI fluctuates during turbulenceWater in pitot lineWater movement causes erraticpressure changesASI shows slow decrease whilealtimeter steadyPitot line leakPitot pressure bleeds off; staticsystem unaffectedVSI shows descent in level flightStatic line leakAmbient pressure enters, simulatingpressure increaseAltimeter reads lower than actualStatic system leakHigher static pressure insideinstrumentAltimeter reads correctly at onealtitude but not anotherFaulty altimetermechanismInstrument not responding correctlyto pressure changesASI reads high at low speedsClogged pitot drainholeWater accumulation adds pressure

Pitot-Static System Testing

Pitot-Static — Leak and Blockage Effects on Instruments Pitot-static — effects of leaks and blockages on instruments PITOT TUBE Total pressure (dynamic + static) PORT Pitot line LEAK ASI Air- speed ↓ Speed decreases BLOCKED STATIC PORT Ambient pressure Static line LEAK BLOCKED ALTIMETER Leak: ↓ altitude Blocked: frozen VARIOMETER Leak: ↓ descent Blocked: frozen SUMMARY TABLE OF FAULTS FAULT AFFECTED INSTRUMENTS OBSERVED EFFECT Pitot leak Airspeed indicator (ASI) only Indicated speed slowly decreases Static leak Altimeter and variometer Altimeter ↓ altitude; VSI shows descent Static port blocked Altimeter and variometer Instruments "frozen" at altitude of blockage Pitot blocked Airspeed indicator (ASI) Zero speed on the ground Leak Blockage Pressure flow

A comprehensive pitot-static system test involves:

5.Leak check: Pressurize the system and observe pressure decay over time
6.Altitude verification: Compare altimeter readings against test set at multiple altitudes
7.Airspeed verification: Compare ASI readings against test set at multiple airspeeds
8.VSI verification: Check rate indications and lag characteristics

Important: A pitot leak affects only airspeed indications. A static leak affects altimeter and VSI indications. A blocked static port causes instruments to "freeze" at the altitude where blockage occurred.

Attitude and Heading Reference Systems (AHRS)

AHRS vs IRS — Attitude Reference AHRS vs IRS — Attitude Reference AHRS System (Attitude & Heading Reference System) SENSORS (Solid State) MEMS / Laser Gyroscopes Accelerometers (gravity + linear) Magnetometer (Earth's field) FUSION PROCESSOR Pitch/Roll calc + Magnetic heading OUTPUTS → PFD (Attitude, Heading, Rate) → AFCS / Autopilot Systems ✓ Compact • Lightweight • Fast align (level ground req.) IRS System (Inertial Reference System) SENSORS (High Precision) Ring Laser Gyroscopes (RLG) High-precision Accelerometers Stabilized Inertial Platform INERTIAL NAVIGATOR Double integration → Pos/Velocity OUTPUTS → FMS (Position, Velocity, Attitude) → PFD + Autonomous Navigation ⚠ Autonomous • No ext. aids • Long alignment (static)

Principles of Operation

AHRS combines accelerometers, gyroscopes, and magnetometers to provide aircraft attitude (pitch and roll) and heading information. Unlike traditional gyroscopic instruments, AHRS uses solid-state sensors and digital processing.

Accelerometers: Sense linear acceleration, including gravity. In level flight, accelerometers detect the direction of gravity to establish the vertical reference.

Gyroscopes: Measure angular rates (rate gyros). These are integrated to determine orientation changes.

Magnetometers: Measure Earth's magnetic field to provide heading reference.

Critical Failure Modes

Accelerometer Bias: A bias in an accelerometer causes a fixed attitude error. For example:

A bias in the lateral accelerometer produces a fixed roll error
A bias in the longitudinal accelerometer produces a fixed pitch error
Heading remains correct because magnetometers provide heading reference

Gyroscope Precession: Causes slow drift in heading over time. This is a natural characteristic of mechanical gyroscopes. In modern AHRS, this is compensated by accelerometer and magnetometer updates.

Alignment Requirements: AHRS requires the aircraft to be stationary and reasonably level during alignment. Non-level parking can cause alignment failure because the system detects excessive tilt.

Troubleshooting Approach

When an AHRS shows correct heading but attitude errors:

27.Check for accelerometer bias (most likely cause of fixed errors)
28.Verify aircraft is truly level
29.Check for magnetic interference (affects heading, not attitude)
30.Consider gyroscope precession (causes drift, not fixed errors)

Inertial Reference Systems (IRS)

Principles of Operation

IRS uses accelerometers mounted on a stabilized platform (or in a strapdown configuration) to measure acceleration in three axes. By integrating acceleration to velocity and velocity to position, the system determines aircraft position, velocity, and attitude without external references.

Alignment Process: IRS alignment requires:

IRS — Alignment Process and Motion Sensitivity IRS — alignment procedure and motion sensitivity Step 1 — Stationary aircraft Earth rotation detected ✓ STATIONARY Step 2 — Position coordinates LATITUDE / LONGITUDE 45° 30' N Latitude 73° 45' W Longitude VALIDATE Step 3 — Required level surface Level required — unleveled aircraft = failure Alignment duration — approximately 10 minutes System detects true north and earth rotation ⚠ Disturbances — alignment failure Wind Towing Ground activity Vibrations ✓ ALIGNMENT SUCCESSFUL • True north determined • Position calculated by inertial integration ✗ ALIGNMENT FAILURE • Measurement corrupted by motion • Restart the complete procedure TECHNICAL NOTES — MAINTENANCE • The IRS uses laser or fiber-optic gyroscopes and accelerometers to detect the Earth's rotation (15°/hour). • Any disturbance during alignment invalidates the inertial reference — the system displays "ALIGN FAULT" and must be reset. • Verify that the parking brakes are set, ladders are removed, and the parking area is clear before starting the alignment.
37.Aircraft stationary to sense Earth's rotation rate
38.Accurate position entry (latitude/longitude)
39.Level surface (within limits)
40.Approximately 10 minutes for completion

Alignment Failure Modes

Critical Insight: IRS alignment is extremely sensitive to movement. Even slight movement from wind, towing, or ground crew activity can prevent successful alignment. The system must sense Earth's rotation to establish true north, and any additional motion corrupts this measurement.

Navigation Mode

Once aligned and switched to NAV mode, IRS provides:

Present position (latitude/longitude)
Ground speed and track
Attitude (pitch, roll, heading)
Body rates and accelerations

Radio Altimeter

Principles of Operation

Radio altimeters (also called radar altimeters) measure height above terrain by transmitting radio waves toward the ground and measuring the time delay for the reflected signal to return. The system operates in the E-band frequency range (4.2-4.4 GHz).

Key Characteristics:

Measures actual height above terrain (not pressure altitude)
Effective over most terrain types (water, sand, snow, forest)
Provides accurate readings from 0 to approximately 2,500 feet
Essential for autoland systems and ground proximity warning

Normal Operation and Common Misconceptions

Normal Indications:

On the ground: Indicates 0 feet (or minimal height due to antenna-to-ground distance)
Over water: Functions normally (water is an excellent reflector)
Over snow: Functions normally (snow reflects radio waves)
Over forest: Functions normally (trees provide adequate reflection)

Common Installation Issues:

Angled antenna: Causes slant range error, indicating higher than actual height (common in helicopter installations)
Radio Altimeter — Slant Range Error from Angled Antenna Radio altimeter — slant range error due to tilted antenna Air Ground 5 ft 10 ft θ Antenna Actual reflection point Oblique reflection point Shadow zone — no direct return RADIO ALTIMETER Transmitter 4.2–4.4 GHz Receiver — return time Output: AGL height 10 ft (displayed) Coaxial cable SLANT RANGE ERROR: The tilted antenna measures the slant range (diagonal path) instead of the actual vertical height. Display = 10 ft for 5 ft actual. LEGEND Actual height Slant range Radio beam RADIO ALTIMETER Transmitter 4.2–4.4 GHz Receiver — return time Output: AGL height 10 ft (displayed) Actual height: 5 ft Longer path
Dirty antenna: Reduces signal strength but typically causes loss of signal rather than offset errors
Damaged coaxial cable: Causes signal loss or intermittent operation

Self-Test Failure Troubleshooting

When a radio altimeter fails self-test:

71.Verify antenna is clean and properly connected
72.Check transmitter/receiver unit (most likely component failure)
73.Verify electrical power supply
74.Check for damaged coaxial cables

Air Data Computers (ADC)

ADC — Air Data Flow to Consumer Systems ADC — air data flow to consuming systems LEGEND Raw data Calculated outputs Fault propagation INPUT SENSORS Pitot tube Total pressure (Pt) (dynamic + static) Static ports Static pressure (Ps) TAT probe Total temperature ⚠ SENSOR FAULT (e.g., pitot blockage) ADC Air Data Computer Inputs: Pt, Ps, TAT Internal processing: • Sensor error correction • Aerodynamic calculations • Position compensation • Conversion to ARINC 429 standard signals DEGRADED MODE If invalid data → error flags on outputs CALCULATED PARAMETERS Indicated airspeed (IAS) Airspeed indicator True airspeed (TAS) Corrected for density Mach number TAS / speed of sound Pressure altitude Altimeter (ISA) Vertical speed Variometer (VSI) Air density ρ (for TAS) COCKPIT DISPLAYS EFIS PFD (main display) • IAS / TAS / Mach • Altitude / VSI • Temperatures EIS Engine display Parameters derived from ADC Standby Airspeed indicator Altimeter VSI (direct) CONSUMING SYSTEMS Autopilot • Altitude hold • Speed hold FMS • Navigation • Predictions Management system • Flight envelope • Protection Flight recorder • ADC parameters • Reference ⚠ FAULT IMPACT AP/FMS receive erroneous data → behavior Fault propagation: erroneous data transmitted downstream to systems AP / FMS: possible erroneous modes TECHNICAL NOTE: An upstream fault (sensor or line) propagates to all ADC outputs. The technician must verify the integrity of pitot/static lines and sensor operation before any downstream troubleshooting.

Principles of Operation

ADCs receive pitot and static pressure inputs and compute various air data parameters:

Indicated airspeed (IAS)
True airspeed (TAS)
Mach number
Pressure altitude
Vertical speed
Outside air temperature (with temperature probe input)
Air density

Installation and Maintenance Requirements

New ADC Installation:

Must be approved for installation (typically CAN-TSO approval)
Installation must be performed under an AMO for Part IV/VII aircraft
System testing required to verify proper operation

Used ADC Installation:

Must be inspected and tested for safe operation
Requires maintenance release (e.g., Form 1)
Must meet manufacturer's specifications

Troubleshooting Approach:

When an ADC provides incorrect data to other systems:

98.Verify pitot-static system is within limits (eliminate pressure source issues)
99.Check wiring between ADC and receiving system (most likely cause if pitot-static is good)
100.Verify ADC power supply
101.Check ADC internal calibration

Important Regulations and Procedures

Pitot-Static System — ASI, Altimeter, VSI Pitot-Static System — ASI, Altimeter, VSI Heater Drain Static Port Fuselage / Wing KNOTS Airspeed Ind. (ASI) ALTITUDE Altimeter FT/MIN Vert. Speed (VSI) Total Pressure (Pitot + Static) Static Pressure (Ambient) Legend & Operation Pitot Line (Dynamic Pressure) Static Line (Ambient Pressure) Heating Element (Anti-ice) Note: ASI requires both pressures. Maintenance Points (TEA) • Static leak = Indicated Alt < Actual • Blocked Pitot = ASI reads 0 on ground • Check drains & pitot heat regularly

CARs Standard 571 Requirements

Maintenance Release Requirements

Minor Modifications (non-major):

No approved data required
Work performed per manufacturer's instructions
Maintenance release signed by authorized AME
Transport Canada approval NOT required

Major Repairs:

Approved data required
Must be reported to Transport Canada
Maintenance release signed by authorized AME
Testing required to verify repair

Major Modifications:

Approved data required (e.g., STC approved by Transport Canada)
Maintenance release signed by authorized AME
System testing per manufacturer's instructions
Results recorded

Recording Requirements

Standard 571.03 requires recording:

Brief description of work performed
Location and method of repair
Test results (e.g., leak check results)
Date of maintenance
Identification of person performing work
Deviation card values (for compass adjustments)

Specialized Maintenance

Standard 571.04 defines specialized maintenance as tasks requiring specialized equipment and training:

Calibration of pitot-static systems (typically requires AMO)
Compass compensation (requires qualified personnel)
IRS alignment procedures

Part-Specific Requirements

Part IV Aircraft (Commercial Operations):

Maintenance must be performed under an AMO
Approved parts required
Compass swing required after compass replacement

Part VII Aircraft (Airline Operations):

Maintenance under AMO
Approved parts (e.g., PMA)
Pitot-static system testing after pitot tube replacement

Used Parts Requirements (Standard 571.08)

Used parts must:

Be inspected and tested for safe operation
Have maintenance release (Form 1)
Meet manufacturer's specifications

Relationships Between Concepts

Pitot-Static and ADC Relationship

The pitot-static system provides raw pressure inputs to the ADC. Any fault in the pitot-static system will affect all ADC outputs. Conversely, if the pitot-static system tests within limits but the ADC provides incorrect data, the fault lies in the ADC or its interconnections.

AHRS and IRS Relationship

Both systems provide attitude and heading information, but through different technologies:

AHRS uses accelerometers, rate gyros, and magnetometers
IRS uses accelerometers on a stabilized platform with gyroscopic references
Both require stationary alignment
Both are sensitive to non-level parking

Radio Altimeter and Barometric Altimeter Relationship

Radio altimeter: Measures true height above terrain (essential for landing)
Barometric altimeter: Measures pressure altitude (essential for flight level separation)
Both are used together in autoland systems
Radio altimeter provides the final approach reference below 2,500 feet

Compass and Heading Systems Relationship

Magnetic compass provides primary heading reference
Heading indicator (HI) or directional gyro provides stabilized heading
Slaving systems correct gyroscopic precession using magnetic sensor input
Compass swing required after compass replacement to correct deviation
Deviation card records residual errors after compensation

System Integration

Modern aircraft integrate these systems through:

ADCs providing air data to flight management systems
AHRS/IRS providing attitude and heading to autopilots and displays
Radio altimeters providing height data to autoland and ground proximity systems
All systems contributing to the Electronic Flight Instrument System (EFIS)

Summary of Critical Troubleshooting Principles

178.Isolate the system: Determine which parameters are affected and which are normal
179.Check simplest causes first: Pitot covers, water in lines, loose connections
180.Verify input systems: Before suspecting a complex component, verify its inputs
181.Consider environmental factors: Movement during alignment, non-level parking, terrain effects
182.Follow regulatory requirements: Proper documentation, approved data, authorized personnel

This knowledge forms the foundation for diagnosing and maintaining the critical sensor systems that ensure safe aircraft operation in all phases of flight.

Chapter 7Advanced Avionics - TCAS, Wx Radar, FMS, Autopilot, EFIS, EICASChapter 9EWIS - Zonal Inspections, Chafing, Degradation, Aging Aircraft

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