Aircraft Instrument Systems Principles

Aircraft Instrument Systems Principles

• Pressure is the force differential between two points.
• Atmospheric Pressure is the force exerted by the air surrounding us.
• Absolute Pressure is measured relative to a total vacuum and often used for comparisons on aircraft.
• Gauge Pressure measures the difference between atmospheric pressure and the pressure being measured and is the most common type used.
• Differential Pressure compares two different pressures.

Measuring Instruments

• Aneroid Wafer measures the difference in pressure between a sealed chamber and the ambient pressure.
• Bellows-type measures the difference between the pressure inside a bellows and outside.
• Bourdon tube-type is commonly used to measure high pressure.

Aircraft Specific Instruments

• Manifold Pressure Gauge indicates the engine's power output.
• Engine Pressure Ratio Indicator measures thrust produced by axial flow turbine engines.
• Pressure Switches activate warning lights or sequence system activities based on fluid pressure changes.

Altimeters

• Altimeters are barometers that measure absolute air pressure.
• Types include:
  • Three-Pointer Altimeter: Commonly used in general aviation aircraft.
  • Encoding Altimeter: Contains an electronic pressure sensor that sends a digital code to the transponder.
  • Servo Altimeter: Uses a built-in compensation system for specific aircraft and minimizes errors.
  • Radio Altimeter: Displays altitude based on radio signals.
• Altimeter Tests:
  • Scale Error: Altimeter must indicate the same altitude as a master indicator within a specified tolerance.
  • Hysteresis: Altitude readings should match when increasing and decreasing at the same pressure level.
  • After Effect: Altimeter should return to the same indication after testing.
  • Friction: Must have minimal difference in readings before and after vibration.
  • Case Leak: Altimeter should not leak more than a specified amount.
  • Barometric Scale Error: Correlation between the barometric scale and altimeter pointers must be accurate.

Airspeed Indicators

• Airspeed Indicators measure the difference between pitot (ram air) pressure and static pressure.
• True Airspeed Indicator accounts for altitude and air temperature to display true airspeed.
• Maximum Allowable Airspeed Indicator shows the never-exceed speed of an aircraft with red markings on the indicator.
• Machmeter is an airspeed indicator adjusted for altitude, displaying airspeed relative to the speed of sound.
• Combination Airspeed Indicator displays airspeed, Mach, and maximum allowable airspeed.

Vertical Speed Indicator

• Vertical Speed Indicator measures the rate of change of static pressure and indicates climb or descent.
• Instantaneous Vertical Speed Indicator includes accelerometers for immediate indication of vertical speed changes.

Temperature Measuring Instruments

• Temperature Instruments measure temperature increase or decrease using expansion or contraction of liquids, solids, or gases.
• Non-Electrical Instruments:
  • Liquid Expansion: Rarely used on aircraft.
  • Solids Expansion: Bimetallic strip thermometers are commonly used for outside air temperature.
  • Gas Expansion: Bourdon tube thermometers utilize expanding gas for temperature measurement.
• Electrical Instruments:
  • Resistance Instruments: Measure changes in resistance of temperature-sensing bulbs.
  • Voltage Instruments: Measure voltage produced by metals when exposed to heat.
  • Thermocouple Instruments: Used for measuring high temperatures.

Mechanical Movement Measurement

• Accelerometer: Indicates G force acting on the aircraft, including maximum negative and positive Gs.
• Synchroscope: Syncs RPMs of twin-engine aircraft propellers.
• Tachometer: Displays engine RPMs.

Gyroscopic Instruments

• Gyro is a rotating mass, a key component in the early "blind flight" instruments.
• Spinning Gyroscope Characteristics:
  • Rigidity in Space: A spinning gyro remains fixed in its spinning plane even when mounted on gimbals.
  • Precession: When an outside force tries to tilt a spinning gyro, the response is as if the force was applied 90 degrees further in the direction of rotation.
• Heading Indicator (Gyrocompass): Shows the aircraft's heading relative to a fixed horizontal reference.
• Attitude Indicator (Artificial Horizon): Indicates the aircraft's orientation relative to the Earth's horizon.
• Turn Indicator: Measures the rate of rotation of an aircraft around its vertical axis.
  • Turn and Slip Indicator: Includes "needle and ball" parts for indicating turning motion.
  • Turn Indicator: Translates turning motion into a rate of turn indication.

Power Sources for Gyroscopic Instruments

• Vacuum: Used in older aircraft instruments.
• Electric: More modern instruments.
• Pressure: Some specialized instruments.

International Standard Atmosphere (ISA)

• A common reference for temperature and pressure in aviation.

Pressure

• Standard Pressure: Found at sea level and various elevations, subject to change with weather.
• Hectopascals (hPa): Modern term for "millibars".