Aircraft instruments are vital components that provide pilots with crucial information for safe and efficient flight. Understanding how these Aircraft Instruments and Associated Systems function, their maintenance, and their various indications is fundamental for both pilots and aviation mechanics. This guide will delve into the intricacies of these systems, covering everything from basic flight instruments to advanced warning systems, making it an ideal resource for students studying this essential topic.
Understanding Aircraft Instruments and Their Core Functions
At the heart of an aircraft's operational capabilities lie its instruments, each designed to provide specific data points. These instruments are often interconnected, forming complex systems that ensure flight safety and efficiency. Proper understanding and maintenance of these systems are paramount.
Instruments Connected to the Static System
Several key flight instruments rely on the aircraft's static system to function accurately. The static system measures ambient atmospheric pressure, which is critical for determining altitude and vertical speed. The instruments connected to the static system are:
- The airspeed indicator
- The vertical speed indicator
- The altimeter
If a mechanic replaces any instrument connected to the static system, a mandatory check must be performed: the static system must be checked for leaks to ensure accuracy.
Repairs and Alterations of Aircraft Instruments
Due to the critical nature of aircraft instruments, strict regulations govern their repair and alteration. All repairs and alterations must be made by the instrument manufacturer or by an FAA-approved repair station certificated for the instrument. This ensures the highest standards of safety and accuracy.
Interpreting Aircraft Instrument Dial Markings
Instrument dial markings provide quick visual cues to the pilot about the operational status and limitations of the aircraft. Understanding these color-coded arcs and lines is essential for safe operation.
- A red radial line on the dial of an aircraft instrument marks a never-exceed condition.
- A yellow arc on the dial marks a caution range of operation.
- A green arc indicates the normal range of operation.
- A white arc on an airspeed indicator indicates the airspeeds at which the flaps may be lowered.
Slippage Marks and Range Markings
To prevent errors, instruments often have specific indicators. A white slippage mark that extends across the lower part of the instrument cover glass and the instrument case bezel shows whether the glass has slipped. A slipped glass would put the range marks over the incorrect numbers, leading to dangerous misinterpretations. Mechanics can find the required range markings for instruments in a particular aircraft in the Type Certificate Data Sheets for that aircraft.
Essential Electrical and Gyroscopic Systems
Many aircraft instruments rely on electrical power or gyroscopic principles for their operation. These systems are integral to navigation and flight stability.
Magnetic Compass and Deviation Error
The magnetic compass is a fundamental navigation instrument. When installing a magnetic compass, deviation error must be corrected by performing a compass swing. The maximum amount of deviation error allowed when a magnetic compass is installed in an aircraft is ten degrees. The compass uses a special water-clear fluid, similar to kerosene. To prevent interference with other instruments, many electrical instruments are mounted in steel cases, which concentrate the magnetic flux.
Aircraft Electrical Bonding
For proper electrical system function and safety, an aircraft instrument panel must be electrically bonded to the primary aircraft structure. The bonding straps carry the return current from the instruments into the aircraft structure, completing electrical circuits.
Gyroscopic Instruments and Power Sources
Gyroscopic instruments provide information on the aircraft's attitude and turn rate. The three sources of power used to operate gyro instruments in an aircraft are:
- Suction
- Pressure
- Electricity
Turn and Slip Indicator vs. Turn Coordinator
While both assist with turns, they differ in sensitivity:
- A turn and slip indicator is sensitive only to the yaw of the aircraft.
- A turn coordinator uses a canted gyro, making it sensitive in both the roll and yaw axes.
Engine Monitoring and Performance Instruments
Monitoring engine performance is critical for safe flight and engine longevity. Various instruments provide essential data on engine health and operation.
Cylinder Head Temperature and Thermocouples
Cylinder head temperature indicators rely on thermocouples. The length of the thermocouple leads must not be altered during installation. This is because an instrument operated by a thermocouple is a current-measuring device, and the resistance of the thermocouple and its leads must be kept at the value specified for the indicator to ensure accurate readings.
Manifold Pressure Gage
When the engine is not operating, a manifold pressure gauge should read the existing barometric pressure.
Turbine Engine Tachometers and EGT Systems
Turbine engine tachometers are calibrated in percentage of the takeoff RPM. Exhaust gas temperature (EGT) of a turbine engine is measured by an averaging system made up of a series of thermocouples arranged around the inside of the exhaust duct of the engine.
Oil Temperature Gage Issues
If the needle of an electrical oil temperature gauge pegs on the high side of the instrument dial, it likely indicates an open in the bulb circuit. This causes the instrument to see an infinite resistance, and the higher the resistance of the bulb circuit, the further the needle moves across the dial.
Fuel Quantity and Flow Systems
For an electronic-type fuel quantity indicating system, tubular capacitors extending across the fuel tank from top to bottom are used as sensors. For most small horizontally opposed fuel-injected aircraft engines, a flowmeter is often a pressure gauge that measures the pressure drop across the injector nozzles.
Dual Tachometers in Helicopters
A dual tachometer is used on a single-engine helicopter for a specific reason: one needle indicates the speed of the engine, and the other indicates the speed of the main rotor. When the needles are