Aircraft Instrumentation and Automatic Flight Control Systems Notes

• Pressure Relationships: At sea level in a standard atmosphere ($1013.25\,hPa$), Calibrated Airspeed (CAS) is equal to True Airspeed (TAS).

  Air Data Instruments and Principles

• Pitot-Static System:     * The pitot tube orifice measures total pressure.     * A blockage of both the ram air input and the drain hole causes the Airspeed Indicator (ASI) to act like an altimeter.     * Ice blocking the ram air inlet only affects the ASI.     * Static pressure port damage in non-pressurized aircraft can be mitigated in an emergency by breaking the glass window of the rate-of-climb indicator.

• Altimeter Fundamentals:     * The instrument is fed by static pressure. It contains aneroid capsules containing a vacuum (or very low pressure) within a casing subjected to static pressure.     * Errors: Position error (static pressure variation near the source), barometric error, instrument error, and hysteresis (varies with time at a given altitude).     * Temperature Effects: When flying from warm to cold air, the altimeter will overread.     * Vibrating Device: Used to reduce friction in linkages.

• Airspeed Indicator (ASI):     * Calibration: Modeled on Saint-Venant’s formula, which accounts for air compressibility.     * Speed Scales:         * White Arc: $V_{SO}$ (lower limit) to $V_{FE}$ (upper limit).         * Green Arc: $V_{S1}$ (lower limit) to $V_{NO}$ (upper limit).         * Yellow Arc: $V_{NO}$ (lower limit) to $V_{NE}$ (upper limit).     * V-speeds:         * $V_{NO}$: Maximum structural cruising speed (do not exceed except in still air).         * $V_{NE}$: Never-exceed speed.         * $V_{LO}$: Maximum speed for landing gear operation.         * $V_{LE}$: Maximum speed with landing gear extended.         * The red/white hatched pointer indicates the Maximum Operating Speed ($V_{MO}$) relative to altitude.

• Mach Meter:     * Principle based on the ratio derived from $\frac{P_t - P_s}{P_s}$.     * The local speed of sound depends only on temperature. At sea level in standard atmosphere, it is $661\,kt$.

• Vertical Speed Indicator (VSI): Measures the rate of change of static pressure. Response time is improved by adding an accelerometer sensor.

• Air Data Computer (ADC): Provides corrections for position/pressure errors, hysteresis, and enables remote data transmission.

Gyroscopic Instrumentation

• Fundamentals: Properties include rigidity in space and precession. Efficiency is maximized by concentrating mass on the periphery with high rotation speeds.

• Laser Gyro: Features a longer life cycle, is influenced by temperature, and uses two light waves.

• Directional Gyro (DG):     * A gyro with 2 degrees of freedom and a horizontal spin axis.     * Subject to Earth rotation error; at $45^{\circ}N$, the reference turns by $10.5^{\circ}/\text{hour}$ to the right.     * Maximum Earth rotation error is $15^{\circ}/\text{hour}$ at the poles ($15^{\circ}/\text{hour} \times \sin(\text{latitude})$).

• Attitude Indicator (Artificial Horizon): Uses a gyro with 2 degrees of freedom and a vertical axis maintained by a gravity-type erecting system.

• Turn and Bank Indicator: Uses a rate gyro with 1 degree of freedom and a horizontal spin axis. The ball indicates coordination (e.g., in a left turn, needle left and ball right indicates a slip/insufficient rudder).

• Inertial Platforms: Pendulous systems require a Schuler period of approximately $84\,minutes$.

Magnetic and Slaved Compass Systems

• Magnetic Compass: Affected by magnetic masses, ferrous metals, and electrical currents.     * Turning Errors (Northern Hemisphere): During deceleration on Easterly/Westerly headings, an apparent turn to the South is indicated. When turning toward North, the compass undershoots; toward South, it overshoots.     * Deviation Correction: Quadrantal deviation is corrected using soft iron pieces.     * Compass Swinging: Carried out to reduce deviation and determine its value on various headings.

• Slaved Gyro Compass: Derives signals from a flux valve (earth's magnetic field detector). The system includes a directional gyro, azimuth control, and synchronizing control.

Radio Altimeters and Electronic Flight Instrument Systems (EFIS)

• Radio Altimeter:     * Operates in the $4200\,MHz$ to $4400\,MHz$ range (UHF/SHF bands).     * Measures the true height between the ground and the lowest wheels using Frequency Modulation ($FM$).     * Precision is typically $\pm 2\,feet$ between $0$ and $500\,ft$.

• EFIS Components:     * Primary Flight Display (PFD): Main piloting instrument showing attitude, altitude, and Flight Mode Annunciator ($FMA$).     * Navigation Display (ND): Displays navigation and weather data.

• Flight Management System (FMS): A global 3-D flight management tool optimizing paths in both horizontal and vertical planes.

Automatic Flight Control Systems (AFCS)

• Flight Director (FD): Provides command bars on the ADI to help the pilot maintain optimal paths for heading or radio-electric axis interception.

• Autopilot (AP):     * A closed-loop servomechanism for stabilization and guidance.     * Fail-Operational: Performance is not deteriorated following a single failure.     * Fail-Passive: No failure effect, but causes system disconnection.     * Basic Modes: Includes pitch attitude hold, wings level, and heading hold.     * Auto-Trim: Functions to cancel elevator hinge moments and relieve the load on servo-actuators.

• Automatic Landing:     * Requires AP and auto-throttle ($ATS$) to function through at least the flare-out.     * Cat II Approach: Height information is mandatory from the radio altimeter.

• Thrust Management:     * EPR: Computed by dividing turbine discharge pressure by compressor inlet pressure.     * FADEC: Manages fuel flow, starting sequences, data transmission, and operational limits.

Warning and Recording Systems

• Altitude Alert System: Warns when approaching or deviating from a preselected altitude via visual and aural signals.

• Ground Proximity Warning System (GPWS):     * Active from $50\,ft$ to $2500\,ft$ radio altitude.     * Modes: High descent rate ("SINK RATE"), terrain closure, loss of altitude after take-off ("DON'T SINK"), and glide slope deviation.     * Inputs: ADC, Radio Altimeter, ILS Glide Slope, and Landing Gear/Flap positions.

• Traffic Collision Avoidance System (TCAS):     * Based on transponder replies from other aircraft.     * TCAS I: Provides Traffic Advisories ($TA$) only.     * TCAS II: Provides vertical Resolution Advisories ($RA$). Corrective $RA$ requires vertical speed modification; Preventive $RA$ warns against changing current rate.     * Symbolology: Red full square denotes $RA$; Yellow full circle denotes $TA$.

• Stall Warning: Based on Angle of Attack ($AoA$), configuration (flaps/slats), and sometimes Mach number. Utilizes wind vanes or pressure probes.

• Flight Recorders:     * Flight Data Recorder (FDR): Minimum last $25\,hours$ of data. Must start before the aircraft can move under own power.     * Cockpit Voice Recorder (CVR): Minimum last $30\,minutes$ of audio environment, radio communications, and interphone conversations.

Engine and Maintenance Instruments

• Pressure Gauges:     * Bourdon Tube: Used for high pressures (e.g., oil pressure).     * Bellows/Aneroid: Used for low pressures (e.g., manifold pressure, fuel pump).

• Temperature Gauge:     * Thermocouple: Two dissimilar metal conductors (Seebeck effect) used for Exhaust Gas Temperature ($EGT$) and Cylinder Head Temperature ($CHT$).     * Ratiometer: An accurate electrical thermometer that is independent of supply voltage variations.

• RPM Indicators (Tachometers):     * Can use 3-phase AC generators where frequency varies with RPM.     * Electronic tachometers measure impulse frequency from a notched wheel in a magnetic field.

• Fuel Quantity:     * Capacitance Gauges: Measure mass by utilizing the fuel’s dielectric constant (approx. $2 \times$ that of air). Indications are largely independent of temperature and aircraft attitude.

• Vibration Monitoring: Based on accelerometers and high/low frequency filters to measure vibration amplitude.