The altimeter shows the aircraft’s altitude above sea-level by measuring the difference between the pressure in a stack of aneroid capsules inside the altimeter and the atmospheric pressure obtained through the static system. It is adjustable for local barometric pressure which must be set correctly to obtain accurate altitude readings. As the aircraft ascends, the capsules expand as the static pressure drops therefore causing the altimeter to indicate a higher altitude. The opposite occurs when descending.
The 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.
The 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, temperature and humidity) in order to obtain the true airspeed, and for wind conditions in order to obtain the speed over the ground.
The compass 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).
The 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. In many advanced aircraft (including almost all jet aircraft), the heading indicator is replaced by a Horizontal Situation Indicator (HSI) which provides the same heading information, but also assists with navigation
The turn indicator (also known as turn and slip) 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. The original turn and bank indicator was replaced in the late 1960s and early ’70s by the newer turn coordinator, which is responsive to roll as well as rate of turn. The turn and bank indicator is seen typically in aircraft manufactured only prior to that time, or in gliders manufactured in Europe.
Vertical speed indicator
The VSI (also sometimes called a variometer). Senses changing air pressure, and displays that information to the pilot as a rate of climb or descent in feet per minute, meters per second or knots.
Additional panel instruments that may not be found in smaller aircraft
Course deviation indicator
The CDI is an avionics instrument used in aircraft navigation to determine an aircraft’s lateral position in relation to a track, which can be provided by a VOR or an Instrument Landing System.
This instrument can also be integrated with the heading indicator in a horizontal situation indicator.
Radio Magnetic Indicator
An RMI is generally coupled to an automatic direction finder (ADF), which provides bearing for a tuned Non-directional beacon (NDB). While simple ADF displays may have only one needle, a typical RMI has two, coupled to different ADF receivers, allowing for position fixing using one instrument.