Aircraft Instrument Systems

Overview

This chapter covers the fundamental principles, construction, operation, maintenance, and troubleshooting of aircraft instrument systems. It encompasses pitot-static systems, gyroscopic instruments, magnetic compasses, and engine/airframe indicating systems. The content focuses on the knowledge required for an Airframe & Powerplant (A&P) mechanic to properly inspect, maintain, troubleshoot, and return to service aircraft instruments in compliance with Federal Aviation Regulations (FARs) and industry-accepted practices.

1. Pitot-Static Systems

The pitot-static system is the foundation of several critical flight instruments: the airspeed indicator (ASI), altimeter, and vertical speed indicator (VSI). These instruments operate by sensing and comparing dynamic (ram) and static (ambient) air pressures.

1.1 System Components

Pitot Tube: An open-ended tube, typically mounted on the wing or fuselage, that faces into the airstream. It captures ram air pressure (impact pressure) and delivers it to the airspeed indicator. Many pitot tubes are electrically heated to prevent ice blockage, which is mandatory for flight in known icing conditions.
Static Ports: Small, flush-mounted openings, usually located on the fuselage sides, that sample ambient (static) air pressure. They supply static pressure to the ASI, altimeter, and VSI.
Alternate Static Source: A backup static port, often located inside the cabin, that can be selected if the primary static ports become blocked. It provides a backup source of static pressure, though it may introduce slight instrument errors due to the different pressure environment inside the cabin.
Lines and Connections: Tubing and fittings that connect the pitot tube and static ports to the instruments. These must be leak-tight and free from obstructions.

1.2 Operating Principles

Airspeed Indicator (ASI): The ASI is a differential pressure gauge. It measures the difference between pitot (ram) pressure and static pressure. This differential pressure is proportional to the dynamic pressure of the airflow, which is converted to an indicated airspeed. A leak in the static system (allowing higher-than-ambient pressure in) or a leak in the pitot line (allowing ram pressure to bleed off) will result in a low airspeed reading. A blocked pitot tube (with static pressure venting through the drain hole) will cause the ASI to read zero. A blocked static system will cause the ASI to act like an altimeter, reading erroneously high during climbs and low during descents.
Altimeter: The altimeter is an aneroid barometer. It measures absolute static pressure and converts it to an altitude indication based on a standard atmosphere model. The barometric pressure setting (Kollsman window) allows the pilot to compensate for non-standard atmospheric pressure. The altimeter is required to be tested and inspected every 24 calendar months per 14 CFR 91.411. The maximum allowable error at sea level is ±75 feet.
Vertical Speed Indicator (VSI): The VSI measures the rate of change of static pressure. It uses a calibrated leak (a metered orifice) in the instrument case. When the aircraft climbs, the static pressure inside the case decreases more slowly than the pressure inside the sensing diaphragm, causing a deflection that indicates a climb. A leak in the instrument case or its connecting lines can cause a false climb or descent indication even when the aircraft is stationary.

1.3 Maintenance, Testing, and Regulations

Regulatory Requirements:
14 CFR 91.205: Mandates the required instruments and equipment for VFR and IFR flight. This includes a magnetic direction indicator, airspeed indicator, altimeter, and tachometer (for all aircraft).
14 CFR 91.411: Requires that altimeters and static pressure systems be tested and inspected within the preceding 24 calendar months for IFR operations. The static system must have no leakage exceeding 100 feet per minute (fpm) after stabilization.
14 CFR 43, Appendix E: Outlines the specific tests and inspections required for altimeters and static systems, including the leak test procedure.
Troubleshooting and Repair:
Blocked Ports: The pitot tube and static ports should be inspected for blockage (e.g., mud dauber nests, ice, debris). Cleaning a blocked static port is considered preventive maintenance under 14 CFR 43, Appendix A(c). After cleaning, a static system test is required to verify system integrity.
Leak Testing: If a pilot reports erratic or erroneous readings and the ports are clear, the next logical step is a leak test using a pitot-static test set. This involves applying a vacuum/pressure to the system and monitoring for leakage. The leak must be located and repaired.
System Checks: After any maintenance that opens the pitot-static system (e.g., replacing an instrument), a leak check is required before returning the aircraft to service.
Instrument Replacement: When replacing an ASI, it is critical to verify that the new instrument has the proper markings (e.g., white arc, green arc, red line) as required by the aircraft's type design and 14 CFR 91.205.

2. Gyroscopic Instruments

Gyroscopic instruments use the principles of gyroscopic rigidity and precession to provide attitude, heading, and turn rate information. They can be powered by a vacuum system, a pressure system, or electricity.

2.1 Gyroscopic Principles

Rigidity in Space: A spinning gyro resists any force that attempts to change its axis of rotation. This property is used in the attitude indicator and heading indicator to maintain a stable reference.
Precession: The tilting of the gyro's axis of rotation when a force is applied to it. The resulting movement is 90 degrees from the point of force application in the direction of rotation. Precession is the basis for the turn coordinator and turn-and-slip indicator.

2.2 Instrument Types and Power Sources

Attitude Indicator (Artificial Horizon): Displays the aircraft's pitch and bank attitude relative to the horizon. It uses rigidity to maintain a fixed reference. It has an erection mechanism that automatically corrects the gyro to the vertical. A stuck erection mechanism can cause erroneous bank and pitch indications, even when parked. It is powered by vacuum or electricity.
Heading Indicator (Directional Gyro): Displays the aircraft's heading. It uses rigidity to maintain a fixed reference in the horizontal plane. It is subject to precession (drift) and must be periodically re-aligned to the magnetic compass. It is powered by vacuum or electricity. Excessive precession (e.g., >15° in 30 minutes) is often caused by worn gimbal bearings or a sticking gimbal.
Turn-and-Slip Indicator / Turn Coordinator: Indicates the rate of turn and the quality of coordination (slip/skid). It uses precession. The turn-and-slip indicator is typically vacuum-driven, while the turn coordinator is typically electrically driven. The ball in these instruments is a balance indicator and is only centered during coordinated flight; it is not necessarily centered when the aircraft is parked on a level surface.
Caging Mechanism: Some gyroscopic instruments (especially heading indicators) have a caging mechanism to lock the gyro in place during ground operations and spin-up. If the instrument is uncaged before the gyro reaches operating speed (typically 2-3 minutes), it can tumble and give erroneous readings. A malfunctioning caging mechanism can also cause tumbling.

2.3 Power Systems and Troubleshooting

Vacuum Systems: Typically produce 4.5 to 5.5 in. Hg of suction. The system includes a vacuum pump, regulator, filter, and relief valve. If a vacuum-driven instrument is not spinning, the first step is to verify the vacuum system is producing the required suction. A faulty pump, regulator, or a clogged filter can cause insufficient suction.
Electric Systems: Electric gyros require electrical power to operate. If an electric instrument is not functioning, the first step is to verify that it is receiving power and that the circuit breaker is not tripped. A faulty power supply within the instrument (e.g., burned-out motor) is a common cause of failure.
Troubleshooting Logic: A systematic approach is essential. For any inoperative gyroscopic instrument, the troubleshooting sequence should be:

Verify the power source (vacuum or electrical).
Check the instrument's caging mechanism.
Verify the instrument's internal operation.
Replace the instrument only after all external causes have been ruled out.

3. Magnetic Compass

The magnetic compass is a required instrument for all aircraft (14 CFR 91.205). It provides a basic heading reference and is used to correct the heading indicator.

3.1 Operating Principles

The compass uses a magnetized needle that aligns with the Earth's magnetic field. It is subject to two types of error:

Deviation: Caused by magnetic fields generated by the aircraft's electrical equipment, metal structure, and other components. This error is corrected by a compass swing.
Variation: The angular difference between true north and magnetic north, which varies by geographic location.

3.2 Compass Swing and Deviation Cards

Compass Swing: A procedure performed on a compass rose (a calibrated area on an airport) to determine and correct compass deviation. The aircraft is aligned on various headings, and the mechanic adjusts the compensating magnets inside the compass to minimize deviation.
Deviation Card: A card installed near the compass that lists the remaining deviation on various headings. It is part of the instrument's required documentation. An illegible or missing deviation card renders the compass unairworthy.
When Required: A compass swing is required after:
Installation of new avionics or electrical equipment that could introduce magnetic interference.
Any maintenance that could affect the aircraft's magnetic field.
When the deviation card is illegible or missing.
When the compass is replaced or repaired.
Regulatory References: The requirement for a compass correction card is specified in 14 CFR 23.1547. The procedure is detailed in AC 43.13-1B, Chapter 12.

4. Engine and Airframe Instruments

These instruments monitor the health and performance of the aircraft's systems.

4.1 Engine Instruments

Tachometer: Indicates engine speed (RPM). It can be mechanical (driven by a flexible shaft) or electrical. A mechanical tachometer reading zero during engine operation is most likely caused by a broken or disconnected drive shaft. The tachometer is required equipment for all aircraft.
Fuel Quantity System: Capacitance-type systems measure the dielectric constant of the fuel between probe plates. When the tank is full, capacitance is high. A short to ground in the probe wiring can bypass the capacitance, causing the indicator to read empty regardless of actual fuel level.

Fuel Flow System: Turbine-type flow transmitters use a small turbine that spins in the fuel flow. If the turbine is stuck or damaged, the transmitter will not generate a signal, and the indicator will read zero.
Exhaust Gas Temperature (EGT) Gauge: Measures the temperature of exhaust gases. Rapid fluctuations, when other engine parameters are stable, are most likely due to a faulty probe or a loose wiring connection.

4.2 Airframe Instruments

Bonding: Proper bonding of the instrument panel to the aircraft structure ensures a common ground and minimizes electrical noise, which can cause erratic instrument readings. Poor bonding is a primary concern for the proper operation of electrical and electronic instruments.

5. General Maintenance Practices and Regulations

14 CFR 43.9: Requires a logbook entry for any maintenance performed, including a description of the work and the signature and certificate number of the person performing the work.
14 CFR 43, Appendix A: Defines preventive maintenance tasks that an A&P can perform, including cleaning static ports and performing a compass swing.
14 CFR 65.81: Grants an A&P mechanic the privileges to perform maintenance, preventive maintenance, and alterations.
14 CFR 65.101: Authorizes an A&P with an instrument rating to perform instrument repairs and calibrations.
AC 43.13-1B: The FAA's accepted methods, techniques, and practices for aircraft inspection and repair. It provides detailed guidance on instrument systems, including leak testing, compass swings, and troubleshooting.
Troubleshooting Best Practices: Always start with the simplest and most likely cause. Verify the power source, check for leaks, and inspect connections before condemning a component. This prevents unnecessary parts replacement and downtime.
Instrument Case Integrity: Any damage to an instrument's case or glass that could compromise its seal (e.g., a cracked glass face on a vacuum-driven instrument) requires immediate replacement, as it can lead to a vacuum leak and instrument failure.

6. Common Relationships Between Concepts

Pitot-Static Leaks and Airspeed: A leak in the static system (high pressure in) or pitot line (low ram pressure) both result in a low airspeed indication.
Static System Leaks and Altimeter: A leak in the static system causes the altimeter to read lower than the test set's reference altitude during a pitot-static check.
Vacuum Power and Gyro Operation: A vacuum-driven gyro instrument will not spin up to speed if the vacuum system is not producing the required suction (4.5-5.5 in. Hg).
Electrical Power and Gyro Operation: An electric gyro instrument will not erect or operate if it is not receiving electrical power.
Magnetic Interference and Compass Deviation: Installation of new avionics can introduce magnetic fields, requiring a compass swing to correct deviation.
Blocked Static Port and VSI: A blocked static port can cause a VSI to show a continuous climb or descent indication, even when the aircraft is parked.
Caging and Gyro Tumbling: Uncaging a gyroscopic instrument before it has reached operating speed can cause it to tumble and give erroneous readings.

Chapter 8: Aircraft Instrument Systems