Aircraft fly under two general sets of rules. Under visual flight rules (VFR), pilots have a responsibility to watch and avoid other aircraft, using accepted rules of spacing. They must also remain clear of clouds. When the visibility becomes poor, the cloud ceiling (the height above Earth of the lowest layer of clouds) drops too low, or a pilot must fly very near or through clouds, pilots must switch to instrument flight rules (IFR), under which air traffic controllers direct the pilot. To fly under IFR conditions, a pilot has to pass a special flight examination and the aircraft must have the appropriate instruments. For safety reasons, airliners normally fly IFR, and many pilots choose to fly IFR even under VFR conditions.
Some of an airplane's most basic equipment and instruments that a pilot uses to fly under VFR are its radio, its altimeter, and its airspeed, attitude, and turn and slip indicators.
The airspeed indicator gives the speed of the plane relative to the air, not to the ground. For example, suppose a plane is traveling into a 50 mile per hour (MPH) headwind, a wind blowing in a direction opposite to the direction in which the plane is traveling. The plane's airspeed indicator says that the plane is traveling 250 miles per hour (MPH). In this case, the plane is traveling at a speed of 200 MPH relative to the ground.
The altimeter indicates the height of the plane above sea level. The attitude indicator shows the aircraft's position relative to Earth's horizon. It indicates whether the nose of the plane is pointed up or down relative to the horizon and whether the wings are level or tilted. The turn and slip indicator helps pilots determine the rate and direction of a turn.
Flying IFR requires a complicated array of instruments and navigational controls. These include the automatic direction finder (ADF) system, the distance measuring equipment (DME) transceiver, and the high frequency omnidirectional range (VOR) system. Working in concert with pilots to help them land safely in all conditions, air traffic controllers employ several navigational systems. The Instrument Landing System (ILS) sends radio signals to receivers on an airplane, indicating whether the airplane is on the correct approach path. Airport Surveillance Radar (ADR) gives controllers a view of all aircraft activity within about 50 miles of the airport. The Microwave Landing System (MLS) uses microwave beams that scan up and down and side-to-side to help pilots approach and land with great precision.
Today, more and more aircraft are now also equipped with receivers that intercept radio signals from satellites as part of the Global Positioning System (GPS). These receivers allow a pilot to know the aircraft's exact position and speed at every second. Versions of GPS receivers are now even available in automobiles, and as a hand-held system that a boater, hiker, or any curious individual can use to pinpoint his or her position at any time.
You can get more information about flight controls and instruments,
as well as about other aspects of flying, from the
Aeronautic Learning Laboratory for Science, Technology, and Research
site sponsored by the National Aeronautics and Space Administration (NASA).