Most aircraft are equipped with a standard set of flight instruments which give the pilot information about the aircraft's attitude, airspeed, and altitude.
Most aircraft have these eight basic flight instruments:
- Gives the aircraft's height (usually in feet or meters) above some reference level (usually sea-level) by measuring the local air pressure. It is adjustable for local barometric pressure (referenced to sea level) which must be set correctly to obtain accurate altitude readings.
- Attitude indicator (also known as an artificial horizon)
- Shows the aircraft's attitude relative to the horizon. From this the pilot can tell whether the wings are level and if the aircraft nose is pointing above or below the horizon. This is a primary instrument for instrument flight and is also useful in conditions of poor visibility. Pilots are trained to use other instruments in combination should this instrument or its power fail.
- Airspeed indicator
- Shows the aircraft's speed (usually in knots) relative to the surrounding air. It works by measuring the ram-air pressure in the aircraft's pitot tube. The indicated airspeed must be corrected for air density (which varies with altitude and humidity) in order to obtain the true airspeed, and for wind conditions in order to obtain the speed over the ground.
- Shows the aircraft's heading relative to magnetic north. While reliable in steady level flight it can give confusing indications when turning, climbing, descending, or accelerating due to the inclination of the earth's magnetic field. For this reason, the heading indicator is also used for aircraft operation. For purposes of navigation it may be necessary to correct the direction indicated (which points to a magnetic pole) in order to obtain direction of true north or south (which points to the earth's axis of rotation).
- Heading indicator
- Also known as the directional gyro, or DG. Sometimes also called the gyrocompass, though usually not in aviation applications. Displays the aircraft's heading with respect to magnetic north. Principle of operation is a spinning gyroscope, and is therefore subject to drift errors (called precession) which must be periodically corrected by calibrating the instrument to the magnetic compass.
- Turn and bank indicator
- Also called the turn and slip indicator. Displays direction of turn and rate of turn. Internally mounted inclinometer displays 'quality' of turn, i.e. whether the turn is correctly coordinated, as opposed to an uncoordinated turn, wherein the aircraft would be in either a slip or a skid. Replaced in the late sixties and early seventies by the newer turn coordinator, the turn and bank is typically only seen in aircraft manufactured prior to that time.
- Turn coordinator
- Displays rate and direction of roll while the aircraft is rolling; displays rate and direction of turn while the aircraft is not rolling. Internally mounted inclinometer also displays quality of turn. Replaced the older turn and bank indicator.
- Vertical speed indicator
- Also sometimes called a variometer. Senses changing air pressure and displays that information to the pilot as a rate of climb or descent, usually in feet per minute.