Gages and instruments used by the pilot to monitor the condition of an aircraft and the condition of flight.

The earliest aircraft had no instruments at all. Pilots controlled the airplane and the engine using their senses of sight, hearing, and touch. As airplanes grew more complex, pilots needed more instruments to control the planes and monitor the engines. In addition, pilots required instruments to help them navigate and to maintain control of the aircraft in fog or clouds.

The first instruments installed in aircraft monitored the crafts’ engines and fuel. By World War I (1914-1918), aircraft had compasses, inclinometers, and simple altimeters to help pilots navigate and maintain control.

In 1928, Paul Kollsman invented the first sensitive altimeter. A year later, on September 24, 1929, Army lieutenant James H. “Jimmy” Doolittle, using Kollsman’s altimeter and other instruments, demonstrated that an aircraft could be successfully controlled by reference to instruments alone. With a safety pilot in the forward cockpit, Doolittle climbed into the rear cockpit and covered it so he could not see out. Then he took off, flew a 15-mile triangular course, and landed. For the first time, an aircraft had been flown by reference to instruments alone. Although engineers improved instrument accuracy and reliability, the basic design of flight instruments remained the same from the 1930’s to the 1960’s.

During the 1960’s and 1970’s, as the transistor and, later, the integrated circuit came into general usage, instruments began to change dramatically. The instrument could be mounted away from the cockpit, and the information could be displayed on a simple indicator. In the 1980’s, as microprocessors came into general usage, the indicators could be replaced with cathode ray tubes and liquid crystal displays.

The most basic instrument used for navigation is the magnetic compass. The magnetic compass uses two small magnets attached to a floating compass card inside a container filled with kerosene. These magnets point toward the earth’s magnetic north pole. The compass card has letters and numbers printed on it that allow the pilot to determine the direction of flight. The movement of the airplane during flight causes the magnetic compass to swing back and forth. This limits the pilot’s ability to determine flight direction with precision.

The static system is designed to measure the ambient air pressure surrounding the aircraft. A static port consisting of small holes drilled through the side of the aircraft is connected to tubing that leads to the pressure-sensing instruments. The pitot tube is usually a cylindrical device with a hole at one end, installed so that the end with the hole faces forward. The other end is connected to a hose that leads to airspeed sensing instruments. With this arrangement, as the aircraft moves forward, it will create a positive air pressure within the pitot tube. Used together, the pitot tube, the static port, and the hoses associated with each are known as the pitot-static system.

Three flight instruments are based on measuring air pressure and are connected to the pitot-static system. These are known as pitot-static instruments and, in general, need no external power source.

The airspeed indicator is connected through hoses to both the pitot tube and the static port. The basic function of the airspeed indicator is to compare air pressure caused by aircraft movement to ambient air pressure. Within the instrument, a small set of bellows connects to the hose leading to the pitot tube. The bellows are also mechanically connected through gears and springs to a needle on the face of the airspeed indicator. The case of the instrument is connected to the hose leading to the static port. As the aircraft moves through the air, the pressure in the pitot tube inflates the bellows. As the bellows expand, the needle will move to indicate airspeed. Airspeed indicators can be calibrated in nautical miles per hour (knots), miles per hour, or both.

The altimeter is connected through hoses to the static port. The basic function of this instrument is to measure barometric air pressure. If a tube is placed into a mercury reservoir, the atmospheric pressure will force the mercury up into the tube. Measuring the length of tubing filled with mercury will give an indication of the atmospheric pressure. At sea level, the length of tube filled will be approximately 29.92 inches of mercury. At 20,000 feet, the length would only be 13.75 inches of mercury. This pressure is commonly known as barometric pressure.

Inside the altimeter is a sealed pressure capsule connected to needles on a dial calibrated in feet of altitude. As the aircraft climbs skyward, the capsule expands, causing the needles to indicate an altitude above sea level. The altimeter is only accurate when the pilot sets the altimeter for the local barometric pressure. For example, before takeoff, the pilot gets a weather report that indicates the local barometric pressure and then enters the pressure into the altimeter. Once entered, the altimeter will read the field elevation, or altitude above sea level.

The vertical speed indicator is connected through hoses to the static port. The function of this instrument is to measure the rate of altitude change. Inside the vertical speed indicator is a pressure capsule with a calibrated leak. This capsule is connected to a needle on the face of the instrument. As the aircraft increases or decreases in altitude, the capsule will expand or contract, and air will either leak in or out of the capsule, causing the needle to indicate the rate of either climb or descent.

The gyroscopic instruments work on the principle that a spinning wheel will remain rigid in space. The gyroscopic instruments are constructed around a spinning wheel called a gyroscope. Once spinning, the gyroscope will remain rigid; therefore, it is mounted on special devices called gimbals. The gimbals allow the aircraft to move freely around the rigid gyroscope. Gyroscopic instruments are different from pitot-static instruments in that they require a power source. These instruments may be either air or electrically powered.

Unlike the magnetic compass, the directional gyro remains stable in spite of aircraft movement. The gimbal of the directional gyro connects to a circular compass card. A number or letter under a lubber line at the top of the instrument indicates the direction in which the aircraft is pointed.

This instrument is also known as the artificial horizon, or attitude indicator. Two gimbals within the attitude gyro are connected to a horizon-reference arm. Using two gimbals allows the aircraft to move freely in all directions around the rigid gyroscope. The reference arm rotates right and left and moves up and down. When the gyroscope is spinning and rigid in space, the horizon reference bar will remain level. As the airplane climbs, descends, or banks, the pilot can compare the position of the reference bar to an airplane symbol and bank index on the face of the instrument. In this manner, the pilot can then determine the attitude of the aircraft relative to the horizon.

There are two different types of rate gyros, the turn and slip indicator, and the turn and bank indicator. Both types feature an inclinometer on the face of the instrument that indicates whether or not the aircraft is sliding sideways. A sideways slide is known as either a slip or a skid.

The turn and slip indicator uses a gyroscope in a horizontally mounted gimbal connected to a needle in the face of the instrument. As the aircraft turns, the gimbal rotates and forces the needle to the left or right, depending on the direction of the turn. The faster the turn, the greater the deflection of the needle. In a turn and bank indicator, also known as a turn coordinator, the gimbal is mounted at an angle and connected to a symbolic airplane on the instrument face. This instrument senses both bank rate and turn rate.

All aircraft are equipped with a tachometer. A tachometer measures the rotation speed of the engine in revolutions per minute or in percent of maximum. In piston-powered aircraft, the tachometer may also include an hour meter to measure the time that the engine has been running. In helicopters, the tachometer will have two needles, one to measure engine speed and the other to measure rotor speed. Jet-powered aircraft have two tachometers labeled N₁ and N₂. The N₁ tachometer measures the speed of the low-pressure compressor, and the N₂ tachometer measures the speed of the high-pressure compressor.

Oil temperature and pressure gauges are also found on all aircraft. Many piston-powered aircraft are cooled by a combination of air and oil. By monitoring the oil temperature, the pilot can determine if the engine is operating within its proper temperature range. All engines are lubricated with oil, and the oil pressure gauge alerts the pilot to any changes in oil pressure that would indicate an engine problem.

Many aircraft are equipped with an exhaust-gas temperature gauge. This instrument measures the temperature of the exhaust gases, and pilots use this instrument to monitor the efficiency of the engine.

Piston-powered aircraft may be equipped with a manifold pressure gauge. This instrument is similar in construction to, yet less sensitive than, an altimeter. The manifold pressure gauge measures the air pressure within the intake manifold. For aircraft equipped with constant-speed propellers, this instrument is the only reliable way to measure the power output of the engine.

Jet engines will have instruments that measure pressure at both the low and high pressure compressors. These pressure gauges allow the pilot to monitor the performance of the engine.

Pilots monitor the condition of the electrical system in the aircraft by using an ammeter. Ammeters measure electrical current flow in amperes. Some aircraft also have a voltmeter. The voltmeter measures electrical potential in volts. By monitoring these instruments, the pilot can determine whether the battery is charging or discharging and whether the generator is working properly.

All aircraft are equipped with fuel quantity indicators, the equivalent of a fuel gauge in an automobile. Since many aircraft have more than one fuel tank, there may be more than one fuel gauge. In some cases, a single gauge can be used with a selector switch so that the pilot must measure fuel quantity one tank at a time.

Some aircraft are equipped with fuel flow gauges. These instruments monitor the rate at which the engine or engines are using fuel. Pilots use these gauges to monitor the condition of the engines and to plan when fuel stops will be necessary.

Electronic flight instrumentation systems (EFIS) can be used in place of every instrument except the magnetic compass. By nature, instruments with no moving parts are more reliable than their mechanical counterparts. Various techniques are used to replace the mechanical components of an instrument. For example, by using accelerometers coupled to microprocessors, engineers can duplicate the operation of the gyroscope. In addition, a laser beam shining through a ring of optic fiber can duplicate the operation of a mechanical accelerometer.

By using technology similar to computers and televisions, information can be displayed on cathode ray tubes or liquid crystal displays. In most EFIS designs, all of the flight instrument information is exhibited on one or two displays, while navigation, engine, and other information will be shown on other displays.

EFIS-equipped aircraft may a have special engine monitoring system called the engine indicating and crew alerting system (EICAS). With EICAS, engine data are not all displayed continuously. During normal operation, only a minimum amount of information is displayed. If a malfunction occurs, important information will appear automatically on the electronic display.

• Brown, Carl A. A History of Aviation. 2d ed. Daytona Beach, Fla.: Embry-Riddle Aeronautical University, 1980. A well-illustrated book that covers the history of flight from ancient times to the space age.
• Cessna Pilot Center. Manual of Flight. Denver, Colo.: Jeppesen, 1982. Part of Cessna’s integrated flight training system for pilots seeking their private pilot license.
• Eismin, Thomas K. Aircraft Electricity and Electronics. 5th ed. Westerville, Ohio: Glencoe, 1994. A beginner’s text starting with the fundamentals of electricity and ending with electric instruments and autoflight systems.
• Helfrick, Albert. Principles of Avionics. Leesburg, Va.: Avionics Communications, 2000. A very complete avionics text that includes history.
• Jeppesen Sanderson. Instrument Rating Manual. 7th ed. Englewood, Colo.: Jeppesen Sanderson, 1993. A textbook designed to assist pilots to prepare to add an instrument rating to their pilot license.
• Treager, Irwin E. Aircraft Gas Turbine Engine Technology. 3d ed. Westerville, Ohio: Glencoe, 1996. Written for the aircraft maintenance technician, with a comprehensive view of gas turbine engine technology.

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