A combination of the words “aviation” and “electronics.”

From the time avionics were invented in 1903 until approximately 1930, pilots rarely used them, navigating instead by known landmarks on the ground. In the 1930’s, however, engineers began installing communications and navigation equipment in airplanes. The first system designed for airplane navigation was the direction finder (DF), also known as a homing beacon. In the late 1930’s, the government began installing the first range stations, which allowed pilots to follow a specific course. Before World War II (1939-1945), electronic equipment was large, heavy, and often required an extra person to operate; therefore, only large aircraft used avionics.

During World War II, both Allied and Axis forces developed radio detection and ranging, or radar. In addition, the Allies developed the identification, friend or foe (IFF) system. The IFF system became the air traffic control (ATC) transponder. Throughout the 1940’s, engineers made many improvements in the size and reliability of avionics. During the late 1940’s and early 1950’s, the very high frequency omnidirectional range beacon was developed, which was a great improvement to the original range stations.

In the 1960’s, radios became lighter and smaller, mostly due to the application of the transistor to avionic equipment. The first avionics to use transistors were hybrids, or radios containing both vacuum tubes and transistors. In the 1970’s, manufacturers introduced the first reliable solid-state avionics, using semiconductor devices rather than electron tubes. Simultaneously, avionics using digital systems were introduced. These developments allowed for even smaller, lighter, and easier to use systems. Consequently, small personal aircraft of the 1970’s were able to have more complex avionics than could the large airliners of the 1950’s.

The introduction of the microprocessor and database technology in the 1980’s created a revolution in the avionics industry. For the first time, pilots could use long-range navigation systems, such as loran-C and Omega, for aircraft navigation. This new technology also allowed for increasingly smaller, lighter, and even easier to use avionics.

The 1990’s brought the introduction of satellite navigation, known as the Global Positioning System (GPS). By the end of the decade, the U.S. government decommissioned the Omega navigation system, which GPS had made obsolete.

In the early twenty-first century, improvements in microprocessors allowed many more improvements in avionics systems. Three-dimensional moving map displays and low-cost electronic flight instrumentation are a few of the improvements to come about in the first decade of the third millennium.

Avionics assist the pilot to navigate the aircraft in several ways. Many different navigation systems help pilots find their way across the globe and locate runways.

The automatic direction finder (ADF) indicates the direction of special radio navigation stations and AM broadcast stations. This system receives radio signals in the low- and medium-frequency bands. An indicator in the instrument panel simply points toward the source of the radio signals.

The very high frequency omnidirectional range beacon system provides the pilot with directional information relative to a course. This system receives radio signals in the very high frequency range from a station on the ground. The system is made up of a radio receiver connected to a device that converts the radio signal to visual information. The pilot chooses a bearing to fly, and a special indicator in the panel shows whether the airplane is to the left or right of a course, also known as a radial, that passes through the navigation station.

Loran-C provides pilots with long-range area navigation. The name “loran-C” is an abbreviation of “long-range navigation,” with the “C” representing the fact that the current system is the third generation of loran. Originally, loran-C worked as a maritime navigation system; however, with microprocessor and database technology, it became available to pilots. Loran-C does not require the pilot to use a navigation station as a reference point, as do the very high frequency omnidirectional range beacon and the automatic direction finder. Instead, the pilot simply chooses an origin and destination within the loran-C coverage area, and the loran-C guides the pilot directly from the origin to the destination. The system consists of a low-frequency receiver, computer, database, and an indicator. The receiver listens for pulses from a set of transmitting stations, and the computer measures the time delay between pulses to determine position.

The Global Positioning System (GPS) provides pilots with a worldwide area navigation system. Although GPS is similar in design to the loran-C, it is much more accurate. Twenty-four GPS satellites orbit the earth and provide pilots with three-dimensional navigation signals. Often, the GPS system will work with a moving map display to show exactly where the airplane is. The system consists of an ultrahigh frequency receiver, computer, database, and indicator. The receiver listens for pulses from the satellites, and the computer measures the time delay between pulses to determine position. With wide- and local-area augmentation systems, GPS can be used as the sole means of navigation.

The Instrument Landing System (ILS) gives pilots guidance toward runways and consists of three major components. The first, the aircraft’s localizer transmitter, is integrated with the VHF omnirange. When the pilot selects a special ILS channel, the VHF omnirange system switches to localizer mode. Now, instead of having several courses to choose from, the pilot has only one, which will lead to the end of the runway. The course directing indicator (CDI) will indicate whether the course is to the pilot’s left or right.

The second ILS component, the glide slope, provides pilots with vertical guidance to the end of the runway. The glide slope consists of a UHF receiver and circuitry that converts navigation signal information to visual information. When the pilot selects an ILS channel with the VHF omnirange system, the glide slope automatically becomes active and provides information on the CDI to indicate whether the pilot is above or below the proper glide path.

The final ILS component, the marker beacon, then turns on a light in the cockpit as the aircraft passes over certain checkpoints during the approach to the airport. A special receiver in the airplane is tuned to 75 megahertz and will listen for special signals from marker transmitters placed along the localizer course.

Distance measuring equipment (DME) uses radar principles to measure the distance between the aircraft and special navigation stations on the ground. The DME displays distance, speed, and time to or from the navigation station. The aircraft system consists of a transmitter and a receiver. The UHF transmitter sends pairs of pulses to a ground station, which the ground station then sends back to the aircraft. The DME will measure the time elapsed from when the pulses were sent to when they return and will calculate distance, speed, and time.

There are many communications systems on board aircraft. In small airplanes and helicopters, the system will consist of a VHF transceiver for the pilot to communicate with air traffic controllers. Similar to a citizen’s band radio, this more powerful system can have up to 2,280 channels. Many aircraft also have an intercom with which to communicate with other crewmembers and passengers.

In addition to the VHF transceiver and intercom, some aircraft may have high-frequency transceivers or satellite transceivers to allow long distance communication on transcontinental flights. Although similar in purpose, the design of these two systems is quite different. The high-frequency (HF) transceiver transmits and receives frequencies between 3 and 30 megahertz. Radio frequencies within this range have the ability to stay in the earth’s atmosphere and travel around the world. The satellite system uses ultrahigh frequencies and an antenna that swivels to remain pointed at a communications satellite in orbit above the earth. The signal travels from the airplane to the satellite and is then relayed to any place on Earth.

Another communications system is the aircraft communications and reporting system (ACARS), a private, low-speed, digital communications system used by the airlines to communicate between the aircraft and the operations center. Aircraft may also include passenger address systems that allow the pilots to speak to passengers and a radio telephone system that allows passengers to call friends, relatives, and business associates.

Air traffic controllers use two systems to track the movements of aircraft: the primary surveillance radar and the secondary surveillance radar. The primary surveillance radar uses a powerful transmitter and a large rotating antenna to send strong bursts of microwave energy into the air. The microwave energy reflects off the aircraft, returns to the large antenna, and shows up as a dot on the air traffic controller’s radar display. However, not all aircraft reflect microwaves well, and such aircraft may not show up on the radar display.

For this reason, all private and commercial aircraft are required to have special equipment on board that acts as part of the secondary surveillance radar system. Secondary surveillance radar sends a pulse code to a special radio in the aircraft called an ATC transponder. The ATC transponder replies with its own pulse code, which may contain a variety of information, such as altitude, speed range, and assigned codes, that will show on the air traffic controller’s radar screen.

Aircraft can also perform surveillance on each other. Airliners and large business aircraft use a system called transponder-based collision avoidance system (TCAS). A TCAS-equipped aircraft sends a pulse code to which other aircraft with ATC transponders reply. A special instrument in the first aircraft displays the location of the second, indicates collision threats, and recommends a flight direction to avoid collision.

Many aircraft are equipped with weather-surveillance systems. These come in two varieties, active and passive. The active system uses radar. Mounted in the nose of the aircraft, the antenna points forward and sweeps back and forth. The radar transmits energy in front of the aircraft, and water droplets reflect this energy back to the radar antenna. Rain will display on a screen in the instrument panel of the aircraft.

The passive weather-surveillance system uses a special loop antenna to detect the electrical activity associated with thunderstorms and air turbulence. The activity is shown on an indicator in the instrument panel of the aircraft. Both systems help pilots avoid dangerous weather and, in some cases, can be combined into a single, comprehensive weather-avoidance system.

Many aircraft are equipped with autopilots, which fly an aircraft automatically while the pilot accomplishes other tasks. The simplest autopilot is a single-axis system. The single-axis autopilot controls the airplane on only one of the axes of flight. For example, a wing leveler will keep the wings level, but the pilot will be responsible for keeping the nose level, and keeping the tail in line. The dual-axis autopilot controls two axes of flight, keeping both the wings and the nose of the aircraft level, for example.

The three-axis autopilot maintains control of the aircraft in all axes or directions. Often, two- or three-axis systems are interconnected with navigation and flight-management systems, and may include features such as throttle control and ground steering. In these cases, the autopilot is considered an integrated flight-control system.

There are many systems designed for passenger entertainment and convenience. Many aircraft have special telephones that passengers may use to make telephone calls. In addition, multichannel sound systems deliver several styles of music from which passengers may choose. In larger airliners, video systems allow passengers to watch movies or play video games. In some aircraft, passengers can keep track of the flight’s progress by viewing a moving map display. In addition, business jets may have a local area network, printers, and modems to allow passengers to work while in flight.

• 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 space flight.
• Eismin, Thomas K. Aircraft Electricity and Electronics. 5th ed. Westerville, Ohio: Glencoe, 1994. A beginner’s text that starts with the fundamentals of electricity and ends with electric instruments and autoflight systems.
• Helfrick, Albert. Principles of Avionics. Leesburg, Va.: Avionics Communications, 2000. A very complete avionics text that includes history.