The heading indicator is used to facilitate the use of the magnetic compass. It provides a stable heading indication without all the errors presented by the compass.
Internal friction inside the instrument causes friction, resulting in precession. This causes the instrument to drift from the proper heading over time. Additionally, the earth rotates through a full 360 degrees every day. This alone will result in 15 degrees of heading indicator drift per hour, even without any friction in the instrument. Heading indicator drift is corrected by resetting the instrument against the compass heading every 10 or 15 minutes.
Slaved Heading Indication
Some aircraft are equipped with heading indication systems that match the heading indicator to magnetic north automatically. This is accomplished with an electric motor which operates on a signal from a magnetic north sensing device called a magnetometer. The heading indicator can then be referred to as either slaved to the magnetometer or operating as a free gyro.
The magnetometer is normally placed in a remote location, such as a wingtip, to reduce or eliminate magnetic interference from the airplane's electrical system. The device contains a flux valve, which provides an electronic reference to magnetic north.
A pushbutton allows selection of slaved or free gyro modes of operation. When depressed, the system is slaved. When the button is in the outer position, the system is in the free gyro mode.
A slaving meter indicates the difference between the displayed heading and the actual magnetic heading. Deflection of the needle to the right indicates a clockwise error of the compass card, while a left deflection indicates a counterclockwise error.
During turns, it is normal for the slaving meter to show a full deflection to one side. It is also normal for the needle to move from one side to the other. Correct the error when the needle remains fully deflected left or right during level flight.
To accomplish this correction, place the gyro into free mode by pushing the pushbutton out. With the system in free gyro mode, press the clockwise heading drive button to rotate the compass card to the right. This is a clockwise adjustment which eliminates left compass card error.
Pressing the counterclockwise heading drive button will produce a counterclockwise adjustment. This drives the compass card to the left, which eliminates right compass card error.
Horizontal Situation Indicator
Some aircraft combine the VOR and heading indicator into a single instrument called the HSI. An HSI is heading indicator with integrated VOR CDI and glide slope indications.
Radio Magnetic Indicator
An RMI is a slaved compass with one or two needles. These needles are used to give bearings to NDBs and VORs. The compass card of an RMI indicates magnetic heading.
When used with an NDB or VOR, magnetic bearing is indicates by the display. When used with a VOR, the tail of the needle indicates on what radial the aircraft is currently located.
An RMI needle will point to 180 degrees when tuned to a VOT, indicating the aircraft is due north of the station.