Avionics

The onboard electronics used for piloting an aircraft are called avionics. Avionics include communications and navigation systems, autopilots, and electronic flight management systems (FMS). Onboard electronics that are unrelated to piloting tasks, such as video systems for passengers, are sometimes considered avionics as well. Many of these devices include embedded computers.

History

Radiotelephone (two way voice radio) systems have been installed in aircraft since before World War II, and have been widely used for mission coordination and air traffic control. Early systems used vacuum tubes, and because of their weight and size, were installed out of the way with only a control head in place in the flight deck. Standardization on VHF frequences occurred shortly after World War II, and transistor radio systems replaced the tube-based systems shortly afterward. Only minor changes have been made to these systems since 1960s.

The earliest navigation systems required the pilot or navigator to wear headphones and listen to the relative volume of tones in each ear to determine which way to steer on course.

Later, navigation systems developed along three separate paths:

• The NDB/ADF system
• The VOR system
• The ILS system

Non-Directional Radiobeacon

The NDB (non-directional radiobeacon) was the first electronic navigation system in widespread use. The original radio range stations were high-power NDBs, and followed nighttime routes previously delineated by colored light beacons. NDBs use the LF and MF bands, and are still in use today (2003) at smaller airports because of their low cost. DF (direction finder) and ADF (automatic direction finder) avionics can receive signals from these. A needle shows the pilot the relative heading toward the station compared to the centerline of the aircraft.

VHF Omni Range

The VOR system (VHF omni range) is less prone to interference from thunderstorms, and provides improved accuracy. It is the backbone of the air navigational system today (2003). VOR receivers allow the pilot to specify a radial, that is, a line extending outward from the VOR transmitter at a particular angle to magnetic north. Then, a course deviation indicator (CDI) shows the amount by which the aircraft is off the chosen course. Distance measuring equipment (DME) was added to many VOR transmitters and receivers, allowing the distance between the station and the aircraft to be shown.

Instrument Landing System

The instrument landing system (ILS) is a set of components used to navigate to the landing end of a runway. It consists of lateral guidance from a localizer, vertical guidance from a glideslope, and distance guidance from a series of marker beacons. Optional components include DME and a compass locator, the name given to an NDB placed at the start of the final approach course.

For a time, LORAN systems, which provide navigational guidance over large areas, were popular particularly for general aviation use. They have declined in popularity with the commercial availability of GPS service.

GPS receivers have made some inroads into the flight deck but remain a supplemental source of navigation.

Transponder

The transponder is a transceiver that receives 'interrogations' from air traffic control radar systems and replies with a digital code. This secondary radar reply permits the radar system to detect the aircraft more reliably and at greater distances than are possible with primary radar.

A basic 'mode A' transponder responds with a 4-digit code with each digit ranging from 0 to 7. This is called a 4,096 code transponder. This pilot sets the code according to the type and status of the flight or as directed by air traffic control.

A 'mode C' transponder also replies with the pressure altitude of the aircraft encoded to the nearest 100 feet. Modern 'mode S' transponders can respond with a longer digital identifier that is unique for each aircraft (thus allowing each aircraft to be uniquely identified even when there is no voice communication between the aircraft and air traffic control) and can receive digital traffic information from air traffic control radar systems and display them for the pilot.

An IFF transponder, "Identification, Friend or Foe", is used in military aircraft and has additional modes of operation beyond those used in civil air traffic control.

Auxiliary and diagnostic systems

Commercial transport aircraft are expensive, and only make money when they are flying. For this reason, efficient operators perform as much service as possible in-flight, and during the turn-around time in a terminal. To make this process possible, embedded computer systems test aircraft systems, and also collect information about faults in equipment that they control. This information is normally collected in an on-board maintenance computer, and sometimes transmitted ahead to help order spares. Although this sounds ideal, in real life, these self-test systems are often not considered flight-critical, and therefore they are sometimes unreliable, and trusted only to indicate that a device requires service.

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