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 and the static pressure drops, causing the altimeter to indicate a higher altitude. The opposite effect occurs when descending. With the advancement in aviation and increased altitude ceiling the altimeter dial had to be altered for use both at higher and lower altitudes. Hence when the needles were indicating lower altitudes i.e. the first 360 degree operation of the pointers was delineated by the appearance of a small window with oblique lines warning the pilot that he is nearer to the ground. This modification was introduced in the early sixties after the recurrence of air accidents caused by the confusion in the pilot’s mind. At higher altitudes the window will disappear.
The attitude indicator (also known as an artificial horizon) shows the aircraft’s relation 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 (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 HSI which provides the same heading information, but also assists with navigation.
Vertical speed indicator
The VSI / rate of climb indicator 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.
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/ILS
This instrument can also be integrated with the heading indicator in a HSI
Radio magnetic indicator
An RMI is generally coupled to an automatic direction finder (ADF), which provides bearing for a tuned 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.