Chapter 8: Instruments and Sensors - Pitot-Static, AHRS, IRS, Radio Altimeter, ADCs

Pitot-Static System Testing

A comprehensive pitot-static system test involves:

1. Leak check: Pressurize the system and observe pressure decay over time
2. Altitude verification: Compare altimeter readings against test set at multiple altitudes
3. Airspeed verification: Compare ASI readings against test set at multiple airspeeds
4. 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.

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Attitude and Heading Reference Systems (AHRS)

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:

1. Check for accelerometer bias (most likely cause of fixed errors)
2. Verify aircraft is truly level
3. Check for magnetic interference (affects heading, not attitude)
4. Consider gyroscope precession (causes drift, not fixed errors)

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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:

1. Aircraft stationary to sense Earth's rotation rate
2. Accurate position entry (latitude/longitude)
3. Level surface (within limits)
4. 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

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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)

• 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:

1. Verify antenna is clean and properly connected
2. Check transmitter/receiver unit (most likely component failure)
3. Verify electrical power supply
4. Check for damaged coaxial cables

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Air Data Computers (ADC)

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:

1. Verify pitot-static system is within limits (eliminate pressure source issues)
2. Check wiring between ADC and receiving system (most likely cause if pitot-static is good)
3. Verify ADC power supply
4. Check ADC internal calibration

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Important Regulations and Procedures

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

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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)

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Summary of Critical Troubleshooting Principles

1. Isolate the system: Determine which parameters are affected and which are normal
2. Check simplest causes first: Pitot covers, water in lines, loose connections
3. Verify input systems: Before suspecting a complex component, verify its inputs
4. Consider environmental factors: Movement during alignment, non-level parking, terrain effects
5. 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.

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