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.
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 five separate paths:
- NDB/ADF systems
- VOR systems
- ILS systems
- ATCRBS Transponders
- Distance Measurement Equipment
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. 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. NDBs use the LF and MF bands, and are still in use today (2005) at smaller airports because of their low cost but their use is quickly being supplanted by GPS. This is due mostly from the higher cost of ADF equipment in the aircraft and maintaining the NDB stations.
VHF Omni Range
The VOR system (VHF omni range) is less prone to interference from thunderstorms, and provides improved accuracy. It is still 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 .
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.
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.
Distance Measurement Equipment (DME) is used to give the pilot the information of its distance away from the VOR station, thus with a bearing and distance from a particular known VOR station a pilot can fix his exact position. Such systems are refereed to as VOR/DME. A military version of the DME, which is widely used in the US, is the TACAN. Such systems are known as VOR-TAC. Needless to mention, the frequencies for the VOR and DME or VOR and TACAN are paired by international standards, thus once a pilot tunes onto a particular VOR frequency the airborne equipment will automatically tunes on the co-located DME or Tacan.
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.
Avionics have changed significantly with the advent of the GPS receiver and "glass cockpit" display systems.
The use of the Global Positioning System has changed aircraft navigation both in the en-route phase and apporach (landing) phases of flight.
Aircraft have traditionally flown from one radio navigation aid ("navaids") to the next (e.g., from VOR to VOR). The paths between navaids are called airways. While this is rarely the shortest route between any two airports, the use of airways was necessary because it was the only way for aircraft to navigate with percision in instrument conditions. The use of GPS has changed this, by allowing "direct" routing, allowing aircraft to navigate from point to point without the need for ground-based navigation. This has the potential to save significant amounts of both both time and fuel while en-route.
However, "direct-to" routing causes non-trivial difficulties for the air traffic control (ATC) system. ATC's basic purpose is to maintaining appropriate vertical and horizontal separation between aircraft. The use of direct routing makes maintaining separation harder. A good anaology would be vehicular traffic: Roads are comparible to airways. If there were no roads and drivers simply went directly to their destination, significant chaos would ensue (e.g., large parking lots without barriers or lines). ATC does give clearance for direct routing on occasion, but its use is limited. Projects like free flight propose to computerize ATC and allow greater use of direct routing by identifing potential conflicts and suggesting manuvers to maintain separation. This is much like the existing Traffic Collision Avoidance System, but on a larger scale and would look further forward in time.
GPS has also significantly changed the approach phase of flight. When horizontal visibility and vertical cloud ceilings are below visual flight rules (VFR) minimums,
aircraft must operate under instrument flight rules (IFR). Under IFR, aircraft must use navigational equipment for horizontal and vertical guidance. This is particularly important in the apprach and landing phases of flight. The path and procedure used to land on a particular runway is called an approach procedure (sometimes just "approach").
IFR approaches traditionally required the use of ground-based navaids such as VOR, NDB and ILS. GPS offers some significant advantages over traditional systems in that no ground-based equipment is required, reducing cost. This has allowed many smaller airports that cannot justify ILS equipment to now have instrument approaches. GPS receivers for aircraft are also less expensive, uses a single small antenna, and requires virtually no calibration.
The downside to GPS approaches is that they have higher minimum visibility and ceiling requirements; ILS typically have requirements a cloud ceiling no lower than 200 feet above ground level and horizontal visibility greater than 1/4 mile, while GPS minimums are typically never less than 400 feet and 1 mile. Currently (early 2005), GPS approaches offer horizontal guidance only. Vertical guidance is possible, but GPS accuracy in the vertical is not as high as in the horizontal. To solve this problem, the FAA has implemented the Wide Area Augmentation System (WAAS). GPS receivers with WAAS capability have typical vertical accuracy of 2-3 meters. This is sufficient for ILS-type approaches, i.e., those with vertical navigation. GPS/WAAS receivers certified for vertical navigation GPS approaches are slowly coming to the market.
Although the FAA was initially slow to allow the use of GPS in IFR approaches, the number of published GPS approaches is climbing significantly. However, because ILS has lower minimum visibility and ceiling requirements, ILS remains the "best" type of approach, and the FAA has committed to maintaining ILS installations.
Advances in computing power and flat panel LCD displays have made the glass cockpit possible. Glass cockpits are loosely defined as aircraft flight decks where information is presented on one or more electronic displays. They offer significantly lower pilot workloads and improved situational awareness over traditional "steam gauge" flight decks.
Glass cockpits were first introduced on airliners and military aircraft. Recently, they have started to appear in general aviation aircraft such as the Cirrus Design SR20 and Lancair designs.