There are three basic kinds of instruments classified by the job they perform: flight instruments, engine instruments, and navigation instruments. There are also miscellaneous gauges and indicators that provide information not included in these classifications, especially on large complex aircraft. Flight control position cabin environmental systems, electrical power, and auxiliary power units (APUs), for example, are all monitored and controlled from the cockpit via the use of instrument systems.
Flight Instruments
The instruments used in controlling the aircraft’s flight attitude are known as the flight instruments. There are basic flight instruments, such as the altimeter that displays aircraft altitude; the airspeed indicator; and the magnetic direction indicator (magnetic compass), a form of compass. Additionally, an artificial horizon, turn coordinator, and vertical speed indicator are flight instruments present in most aircraft. Over the years, flight instruments have come to be arranged in a standard layout on instrument panels in most aircraft. This basic T-arrangement for flight instruments is shown in Figure 1.
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| Figure 1. The basic T arrangement of analog flight instruments. At the bottom of the T is a heading indicator that functions as a compass but is driven by a gyroscope and not subject to the oscillations common to magnetic direction indicators |
The top-center position directly in front of the pilot and co-pilot is the basic display position for the artificial horizon even in modern glass cockpits (those with solid-state, flat-panel display systems).
Original analog flight instruments are operated by air pressure and the use of gyroscopes. This reduces dependence on electrical power, which enhances safety in the event of electrical failure. Development of sensing and display techniques, combined with advanced aircraft electrical systems, has made it possible to develop reliable electrically operated primary and secondary instrument systems. Nonetheless, often a pneumatic altimeter, a gyro artificial horizon, and a magnetic direction indicator (magnetic compass) are retained somewhere in the instrument panel for redundancy. [Figure 2]
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| Figure 2. This electrically operated flat screen display instrument panel, or glass cockpit, retains an analog airspeed indicator, a gyroscope-driven artificial horizon, and an analog altimeter as a backup should electric power be lost, or a display unit fails |
Engine Instruments
Engine instruments are those designed to measure operating parameters of the aircraft’s engine(s). These are usually quantity, pressure, and temperature indications. They also include instruments for measuring engine speed. The most common engine instruments are the fuel and oil quantity and pressure gauges, tachometers, and temperature gauges. Figure 3 shows various engine instruments found on reciprocating and turbine-powered aircraft.
| Reciprocating engines | Turbine engines |
|---|---|
| Oil pressure | Oil pressure |
| Oil temperature | Exhaust gas temperature (EGT) |
| Cylinder head temperature (CHT) | Turbine inlet temperature (TIT) or turbine gas temperature (TGT) |
| Manifold pressure | Engine pressure ratio (EPR) |
| Fuel quantity | Fuel quantity |
| Fuel pressure | Fuel pressure |
| Fuel flow | |
| Tachometer | Tachometer (percent calibrated) |
| N1 and N2 compressor speeds | |
| Carburetor temperature | Torquemeter (on turboprop and turboshaft engines) |
Figure 3.Common engine instruments. Note: For example purposes only. Some aircraft may not have these instruments or may be equipped with others.
Engine instrumentation is often displayed in the center of the cockpit where it is easily visible to the pilot and co-pilot. [Figure 4] On light aircraft requiring only one pilot, this may not be the case. Multi-engine aircraft often use a single gauge for a particular engine parameter, but it displays information for all engines through the use of multiple pointers on the same dial face.
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| Figure 4. An engine instrumentation located in the middle of the instrument panel is shared by the pilot and co-pilot |
Navigation Instruments
Navigation instruments are those that provide information used by the pilot to guide the aircraft along a definite course. This group includes compasses of various kinds, some of which incorporate the use of radio signals to define a specific course while flying the aircraft en route from one airport to another.
Other navigational instruments are designed specifically to direct the pilot’s approach to landing at an airport. Traditional navigation instruments include a clock and a magnetic compass. Along with the airspeed indicator and wind information, these can be used to calculate navigational progress. Radios and instruments sending locating information via radio waves have replaced these manual efforts in modern aircraft Global Position Systems (GPS) use satellites to pinpoint the location of the aircraft via geometric trilateration. This technology is built into some aircraft instrument packages for navigational purposes. Many of these aircraft navigational systems are discussed in Communication and Navigation section. [Figure 5]
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| Figure 5. Navigation instruments |
Instruments can also be classified according to the principle upon which they operate. Some use mechanical methods to measure pressure and temperature. Others utilize magnetism and electricity to sense and display parameters. Still others depend on gyroscopic principles for their primary operation. Modern instruments utilize solid-state sensors and computers to process and display important information. The operating principles used for sensing parameters are explained next, followed by an overview of many engine, flight, and navigation instruments.
What is the "Basic-T" arrangement in an aircraft cockpit?
How are engine instruments typically organized in a multi-engine aircraft?
Why do modern aircraft still carry some analog or pneumatic instruments?
What is the difference between flight instruments and navigation instruments?
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