11.5 Avionic Systems part

• Aircraft instrumentation provides data for pilots and engineers. Pilots need attitude and position data, while engineers check systems before flight.
• Accurate instrumentation is essential for safe, economical, and reliable aircraft operation.
• Instruments monitor flight, engines, and systems. Flight instruments are on the captain's and co-pilot's panels, engine indications are central, and hydraulic indicators are overhead

Instrument Classification

• Aircraft instrumentation is classified into five types: pressure, gyroscopic, compasses, mechanical, and electronic.
• Instruments are classified by use into four categories: flight, engine, navigation, and other systems.

Flight Instruments

• Flight instruments provide the aircraft's flight status to the pilot.
• Primary flight instruments include attitude, altitude, airspeed, and direction of flight.

Engine Instruments

• Engine instruments provide information on power plant status, power output, instrument vacuum/pressure, and electrical system health.

Navigational Instruments

• Navigational instruments provide navigational information such as path of flight, ground proximity warnings, and weather avoidance.

Miscellaneous Instruments

• Miscellaneous gauges and indicators provide information on systems like anti-ice, de-ice, heating, and air conditioning, more common on complex aircraft.

Basic T

• The "Basic T" or "Flight Tee" layout on the instrument panel includes key flight instruments.
• The layout is identical on the captain’s and co-pilot’s instrument panels
• Instruments in the Basic T: Airspeed Indicator (ASI), Attitude Directional Indicator (ADI), Altimeter, and Horizontal Situation Indicator (HSI).
• The Basic T standard has been used since the early 1950s.
• The attitude indicator is at the top center, ASI is to the left, altimeter to the right, and heading indicator is under the attitude indicator.

Pressure Measuring Instruments

• Aircraft instruments measure fluid pressures like air, fuel, and oil, either directly with bellows or via transducers converting pressure to electrical signals.
• Pressure is the force differential between two points.
• Standard sea level pressure is 1013.25 hPa at 15 °C, which is used for aircraft performance specifications.
• Three ways of measuring pressure: absolute, gauge, and differential.

Absolute Pressure

• Absolute pressure compares atmospheric pressure to a complete vacuum (zero reference).
• Atmospheric pressure can be measured in InHg, hPa, or PSI.

Gauge Pressure

• Gauge pressure is the difference between atmospheric pressure and the pressure being measured.
• Tyre pressure measurement is an example of gauge pressure.

Differential Pressure

• Differential pressure is the comparison between two different pressures.
• Airspeed measurement uses the difference between pitot and static pressure using a bellows.

Examples of Instruments Using Diaphragms or Bellows:

• Altimeter
• Vertical Speed Indicator (VSI)
• Machmeter
• Cabin differential pressure gauge
• Manifold pressure gauge

Pressure Sensing Elements

• Ensure correct pressure readings.
• Types of sensors include Bourdon tube, aneroid capsule or bellows, and pressure transducer.

Bourdon Tube

• Typically made of copper, brass, or bronze with an elliptical cross-section, curved into a half circle.
• Pressure inside the tube distorts the ellipse, straightening the curve and moving a pointer.

Direct-reading Pressure Gauges

• Mostly based on the Bourdon tube principle.
• Used in general aviation aircraft for fuel, oil, and hydraulic system pressure measurements.

Aneroid Capsule

• Used to compare absolute pressure with other pressures.
• It expands and contracts with changes in atmospheric or gas pressure.
• A barometric altimeter is an example of an absolute pressure instrument.

Special Pressure Instruments

• Manifold pressure gauges
• Engine-pressure ratio indicators
• Pressure switches
• Altimeters
• ASIs
• Vertical speed indicators
• Instantaneous vertical speed indicators

Pressure Transducers

• Converts pressure into an analogue electrical signal, or a digital signal via an A/D converter.
• Used in air data systems to relay altitude and speed compensation information.
• Hazardous fluids can be measured at their source without bringing them to the cockpit.
• Systems include an indicator and a transmitter unit.
• Transmission methods are mostly electrical, using DC (Desynn) and AC (Synchros).

Bourdon Tube Pressure Transducer

• Uses a Linear Variable Differential Transformer (LVDT).
• As pressure increases, the tube straightens, displacing an iron core within the LVDT.
• The output voltage is fed to a servo amplifier to drive a remote pointer.

Aneroid Capsule Transducer

• Similar to the Bourdon tube transducer but monitors two pressures (P1 and P2) using differential aneroid capsules.
• Differences in pressure cause movement of an iron core and generate a current to null the error signal.

AC Operated Remote Pressure Indication

• Transmitter consists of bellows, armature spindle, spring, and inductor coils.
• Pressurized oil expands the bellows, altering stator coil inductance, and the resulting current ratio is fed to the indicator.

Strain Gauges

• Measures the strain of an object and converts it to an electrical output.
• Foil strain gauges and silicon-based strain gauges are commonly used.

Pitot-static System

• Uses pressure-sensitive instruments to determine airspeed, altitude, and rate of climb.
• Combines static air pressure and dynamic pressure from the aircraft's motion.
• Pitot tube measures total pressure, while static port(s) measure static pressure. Dynamic pressure is the difference between the two.

Static Pressure System

• Supplies static pressure to the altimeter, ASI, and VSI.
• Connected to the static port via tubes and pipes.
• Static ports are located in areas with smooth airflow.

Static Ports

• Must be kept clean and smooth to prevent airflow disturbance, with a cover used during washing/repainting.

Static Source Error (SSE)

• Difference between measured and real static pressure due to fuselage shape and airspeed; modern aircraft use air data computer correction factors.

Sideslip Manoeuvre

• Generates higher pressure on one side of the fuselage; cross-porting tubes equalize static pressure from ports on each side.

Pitot Pressure System

• Used by the ASI to measure total pressure (dynamic + static).
• Pitot tube points into the airstream.

Pitot Tubes

• Located where they can measure undisturbed pitot pressure.
• The leading edge must be in good condition.
• Heating prevents ice buildup.
• A cover protects against water and foreign objects when parked.
• Some aircraft have a pitot-static tube that includes the static port.
• A baffle prevents water entry, and a drain hole removes water and dust.

Pipelines and Drains

• Pitot and static pressures are transmitted through corrosion-resistant metal pipelines.
• Flexible pipelines are used for connections to components on anti-vibration mountings.
• Pipeline diameter is related to the distance to eliminate pressure drop and time-lag.
• Draining holes in probes, drain traps, and valves eliminate water.

Pitot-static Drains

• Lines must be checked for water regularly, as it causes fluctuating indications or blockage.
• Water is collected by traps with spring-loaded valves at the lowest points of the tubes.
• Pressing the valves drains water, ensuring they reseat. Leak tests are needed for older drains without spring-loaded valves.

Pitot-static System Architecture

• Consists of an ASI, altimeter, and vertical speed indicator, supplied by a static pressure system with two static ports.
• Larger aircraft have duplicated systems.
• An alternate static source selector valve allows the pilot to use the co-pilot's static pressure system.

Pitot and Static Line Marking

• Pipelines are identified.

Indications of Pitot Tube Blockage

• The ASI displays inaccurate speeds.

Indications from Static Port Blockage

• The ASI functions but with likely inaccurate indications.

Testing of Pitot-static Systems

• Must be done with extreme caution using the proper maintenance manual.

Pitot Leak Test

• Ensure electrical pitot heat is inhibited before connecting test equipment.

Static Leak Test

• The static connector of the test box is connected to the static port using adapters and hoses. Do not step on or block the hoses.
• On some aircraft, the static ports are electrically heated. The heater elements can be activated by a ground/flight switch. Pull the circuit breaker (C/B) for the heating systems before connecting the test equipment.

Altimeter Settings

• Adjusts the sub-scale of a pressure altimeter to indicate the aircraft's height above a known reference.
• QNH: Pressure at sea level; altimeter reads airfield height above sea level.
• QFE: Pressure at the airfield level; altimeter reads zero on take-off/landing.
• QNE: Height indicated with the barometric scale set to 1013.25 mbar (29.92 inHg).

Transition Altitude

• Standard setting of 1013 hPa selected during climb at transition altitude.
• During descent, the setting is changed back to the QNH or QFE of the destination.
• Flying above transition altitude requires setting the altimeter to 1013.25 mbar (29.92 inHg)
• Request QFE before landing, usually at the transition level.

Summary of Altitude Terms

• The airport is located at 2712 ft above sea level, this means it has an elevation of 2712 ft
• An aircraft situated on the airfield has an altimeter indication of 2712 ft if the barometric correction is set to the actual pressure at sea level: QNH (In this example, it is 1030 hPa).
• If the actual pressure at the airport is selected (in this example, 935 hPa), the altimeter indication will be 0 ft; this setting is called QFE
• If an aircraft passes the airport at an altitude of 5319 ft, its altimeter will show this altitude if the barometric correction is set to QNH.
• If QFE is used, the indication will be 2607 ft. When the standard setting of 1013.25 hPa is used, the indicator shows 5000 ft, which corresponds to a flight level of 50.

Altimeters

• A barometer that measures the pressure of the surrounding air.
• It has evacuated bellows measured by gears and levers that translate dimensional changes into pointer movement.
• In western countries, aircraft altitude is measured in feet (Imperial Standard), whereas Russia and China use metres.
• The altimeter shows zero feet at FL 00 – QNE, Sea level – QNH, and The runway – QFE

Types of Altimeters in Aircraft:

• Barometric altimeter
• Radio altimeter
• Global Navigation Satellite System (GNSS), Global Positioning System (GPS), Galileo, etc.
• Laser altimeter
• Most common altimeters in use are the barometric and the Radio Altimeter (Rad Alt).

Altimeter Function

• All altimeters measure the static pressure of the atmosphere. At the standard atmosphere, the static pressure at sea level is 1013.25 hPA At 18 000 ft, the pressure is only 50% of that at sea level
• Temperature compensation is applied to stop temperature changealtering the elasticity of the capsule.
• Read in terms of pressure altitude. When conditions differ from ICAO standards, a temperature correction must be made to determine the density altitude.

Servo Assisted Altimeter

• Mechanical altimeters suffer from friction in their bearings and mechanical linkages and lose accuracy as altitude increases.
• Servo altimeters replace the mechanical linkage between the capsules and pointer with an electrical servo mechanism.

Operation

• Capsules expand or contract, the I bar moves in relation to the E bar producing an "error" signal to the Amplifier (Amp), which will drive the three-phase servo motor.

Standby Mode Operation

• Altimeter returns to standby mode under power failure, servo motor failure, amplifier failure, or detection of circuit failure.

The servo altimeter with ADC:

• In these systems, the pneumatic inputs from the static vents and pitot probes are fed directly to the ADCs. Piezo-electrical transducers convert this pressure into a digital electrical signal, which is used to compute the aircraft’s altitude.

Vertical Speed Indicator (VSI)

• Also known as the Rate of Climb (ROC) indicator.
• Establishes a rate of ascent or descent to reach an altitude within a given time.
• Differential pressure gauge that indicates the rate of altitude change in thousands of feet per minute from static pressure variations.

Mechanism

• Housed in an airtight case connected to the aircraft’s static system.
• Static pressure is fed directly to the inside of a capsule.
• The exterior capsule is connected to the inside casing of the instrument via a calibrated metering unit.
• A change in altitude creates a pressure differential, which is measured to gauge the rate of climb or descent.
• In level flight, the pressure inside the case is the same as that inside the capsule (pointer remains stationary).

Response to Static Pressure

• When the aircraft descends, static pressure in the capsule and case increases. Due to the metering unit restriction, the increase in case pressure lags the increase in static pressure in the capsule. The capsule, therefore, expands to indicate a rate of descent.
• When the aircraft climbs, static pressure in the capsule and the case decreases. Due to the metering unit restriction, the decrease in case pressure lags the decrease in static pressure in the capsule. The capsule, therefore, contracts to indicate a rate of climb.

Types of Display

• Non-Logarithmic and Logarithmic Displays
• Rate of 250 ft/min descent
• The logarithmic display is more accurate at lower rates of descent.

Instantaneous Vertical Speed Indicator (IVSI)

• Standard VSI is prone to errors due to friction in the mechanism.
• IVSI provides an extra operating force to compensate for this low operating force.
• Incorporates an accelerometer unit (vertical acceleration pump or "dashpot") that responds quickly to a change in altitude.

EVSI (Electronic Vertical Speed Indicator)

• Replaced conventional VSI (and other instruments) with electronic versions.
• Uses an Inertial Vertical Speed (IVS) signal from the Inertial Reference Unit (IRU).

Pressure Sensors

• Inside the ADC, the various pressure inputs (pitot and static) are translated into an electrical signal, instead of directly driving the mechanical components of the VSI (and other pressure instruments).

Common Sensors

• Piezoresistive elements
• Bonded strain gauge
• Capacitive sensor

The Piezoresistive Element

• Is connected to a Wheatstone bridge. The applied pressure presents the load to the diaphragm, which provides bending stresses.

Bonded Strain Gauge

• The bonded strain gauge operates on the same principle as the piezoresistive sensor, however, it is used for larger strain measurements.

Capacitive Sensor

• The capacitive type is a very accurate low-pressure sensing device. It measures atmospheric pressure through the change in voltage across the capacitive element.

Airspeed Indicator (ASI)

• Indicates aircraft speed relative to the air in knots (nautical miles per hour).
• A simple mechanical ASI is a differential pressure gauge. It indicates the difference between pitot pressure, fed from the pitot probe to the inside of a capsule, and static pressure, from the static vents fed to a sealed case surrounding the capsule.
• The airspeed can be calculated from the dynamic pressure (q).
• Total pressure – static pressure = dynamic pressure

Airspeed Limits

• Airspeed indicators are marked in accordance with a standard color-coded marking system so that the pilot can determine, at a glance, the current airspeed in relation to various safety-critical airspeeds.
• Red radial lines: Maximum and minimum limits of airspeed. The red line is the never exceed speed line; it represents the speed above which operation may result in structural failure of the airframe.
• Yellow arc: Precautionary ranges. Operation is permitted in this range but only in smooth air and with caution.
• Green arc: Normal operating ranges. The upper limit of this arc represents the maximum structural cruising speed VN0 which should not be exceeded unless the aircraft is operating in smooth air. The lower limit represents the aircraft stall speed in a pre-defined configuration VS. This is usually a power-off stall at maximum take-off weight in a clean configuration (flaps retracted, landing gear stowed).
• White arc: Ranges in which the flaps can be lowered. The limits of the white arc represent the maximum flap extension speed VFE at 85 Knots of Indicated Airspeed (KIAS) and the stall speed with flaps extended in the landing configuration VS0 at 40 KIAS.
• VSO – the stall speed in landing configuration with flaps and gear down.
• VS1 – the stall speed in clean configuration with maximum weight.
• VFE – the maximum allowable speed with flaps extended.
• VNO – the maximum airspeed for cruise flight.
• MMO – a limit designed to prevent aircraft from shock wave damage while approaching the speed of sound.
• VMO – a structural limit designed to prevent airframe damage from excess dynamic pressure.

ASI Blockages

• Blocked Pitot: A blockage in the pitot tube will cause any pressure in the capsule to become trapped there. As a result, the ASI reading will remain constant, whatever the actual speed, as long as altitude stays constant. A change in altitude will result in a change in static pressure. For example, in descent, the static in the case will increase but the static trapped in the capsule will stay the same. Consequently, the ASI will under-read. In a climb, the ASI will over-read.
• Blocked Static: As with a blocked pitot, a blockage in the static line will cause a constant reading in level flight. During descent, the pressure in the capsule will rise with the static part of the pitot pressure; however, the static pressure in the case will not change and the ASI will over-read.
• During an ascent, the pressure in the capsule will fall with the static part of pitot pressure; however, the static pressure in the case will not change and the ASI will under-read.

ASI Errors and Terminology

• Indicated Airspeed (IAS): The airspeed read directly from the Airspeed Indicator (ASI) on an aircraft, driven by the pitot-static system.
• True Airspeed (TAS): The speed of the aircraft relative to the air mass through which it is flying.
• Instrument Error (IE): Caused by constructional tolerances (built into the instrument during construction).
• Pressure or Position Error: Position Error (PE) is the difference between the calibrated airspeed and the IAS, due to the recorded static being unequal to the ambient pressure.

Speed Terminology

• Ground Speed (GS): The speed of the aircraft relative to the surface of the Earth.
• Airspeed Indicator Reading (ASIR): The uncorrected reading on a specified ASI.
• Indicated Airspeed (IAS): ASIR corrected for instrument error.
• Rectified Airspeed (RAS): IAS corrected for pressure error.
• Computed Airspeed (CAS) or VCAS: IAS corrected for static source error (position error).
• Equivalent Airspeed (EAS): IAS corrected for compressibility error.
• True AirSpeed (TAS): CAS corrected for density error.

Speed of Sound (Mach)

• Mach number (M) is the ratio between the aircraft’s TAS and the Local Speed of Sound (LSS).
• The velocity of sound in a gas is independent of changes in pressure but proportional to the square root of its absolute temperature
• Modern electronic Machmeters use information from an ADC system, which makes calculations using inputs from a pitot-static system.

Machmeters

• Machmeters effectively measure the airspeed with a differential bellows, as described earlier, for the ASI.
• Machmeters include Mach number indication, ASI, and PFD of an EFIS.
• The maximum allowed Mach number is marked with a red line on the Machmeter scale because it is a fixed value.

Air Data Computer (ADC)

• The computed signals from the Air Data Computer (ADC) or the Central Air Data Computer (CADC) are either analogue or digital servo outputs
• ADCs are used by aircraft to acquire and process data from pitot and static pressure sensors, data buses, and analogue inputs.
• CADC Inputs to VSI
• Its outputs were used by several systems such as autopilot and servo instruments.
• Modern aircraft have an ADC that calculates and delivers electrical signals to the indicators.

Air Data Modules

• Signals from the CADC are analogue or digital servo outputs.
• ADMs are for Airspeed, Altimeter, Mach, TAS, SAT (Satellite), and VSI.
• Modern digital glass cockpit aircraft use air data modules that convert air pressure to digital data, saving weight and maintenance costs.

Digital Air Data Computers (DADC)

• Analogue ADCs use pressure switches and sensors to feed electrical signals through transistors, chips, or resistors, where they obtain a set of analogue electrical signals. These are sent to gauges and systems. Digital ADCs take the same input but read it via piezo sensors to create a digital signal, to be processed and sent to gauges and systems. Occasionally, DADCs need to convert digital output signals to analogue to talk to older aircraft systems.

Digital Data Bus

• On modern aircraft, digital signals from the ADC travel along a data bus, such as the ARINC 429 and ARINC 629.

Air Data Modules

• Short, flexible hoses connect pitot probes to the pitot Air Data Module (ADM) and pitot Static Air Data Module (SADM)
• The pitot ADMs control the heat for the pitot probes and the Angle of Attack (AOA) sensors.

The Air Data Module (ADM)

• Measures absolute pressure from a pitot probe or static pressure port. All ARINC 629 ADMs are interchangeable and can be a pitot or a static ADM.
• The ADM gets 28 Vdc for power. The ADM calibrates the pressure and compensates for temperature. The ARINC 629 transmitter/receiver sends the data on a flight controls ARINC 629 bus.
• ADM has one pneumatic connector and two electrical connectors with captive screws.

The ADIRU

• Combines an Air Data Computer and Inertial Reference Unit.
• Redundancy management logic selects valid data from multiple sources.

Standby Instruments

• Standby instruments are provided with altitude and airspeed when main instruments fail. Standby instruments are sourced from a single pitot-static source

Gyroscopic Instruments

• Gyroscopic instruments provide indications of roll, pitch, and yaw.

Gyroscopes

• The Attitude Indicator (AI), or Artificial Horizon (AH), shows aircraft orientation relative to Earth's horizon.
• Vertical Gyros (VG) provide the attitude indication for the Attitude Director Indicator (ADI).
• Directional Gyro (DG) provides the heading reference signal for the Horizontal Situation Indicator (HSI).
• A rate gyro is used for the rate of turn indication.

Gyro Principles

• A gyro is a rotating mass that keeps the direction of its axis constant as long as no force acts on it.

Gyroscope Construction

• A Gyro, in its simplest form is a rotor that spins on a shaft.
• To keep the Gyro fixed in a point in space it must be mounted, and this is done by a gimbal ring or gimbal.
• Gyros for aircraft will in most cases have either one or two gimbals to provide an indication of the aircraft's attitude.
• Spinning: Plane of movement about its own (spin) axis (XX)
• Tilting: Plane of movement about a horizontal axis (YY), at right angles to the spin axis
• Veering: Plane of movement about the vertical axis (ZZ), perpendicular to the other axis

Classification of Gyroscopes

• Gyroscopes can classified in three ways: Degrees of Freedom, Spin Axis Orientation, Use- type of gyro.
• Basically, the number of gimbals a gyro has will dictate the degrees of freedom.

Classes of Gyros

• Space (Free) Gyro
• Aircraft in flight are still very much a part of the Earth, i.e., all references relate to the Earth's surface. With this in mind, the free gyroscope described earlier serves no useful purpose.
• The Horizontal and Vertical Gyros establish references against which pitch and roll attitude changes can be detected and a directional reference can be identified.
• Earth Gyro: the attitude indicator uses this type of gyro; it has its spin constrained to the Earth's vertical axis
• Rate Gyro (RG): where the spin axis is horizontal is constrained to measure the rate of rotation in one plane about an axis which is at right angles both to the spin axis and the axis of freedom. It is used for turning.

Key Properties of the Gyro

• Rigidity and Precession
• The rotor exhibits two properties that rely on its angular momentum: rigidity and precession.