by Jonathan M. Stern
The third gyroscopic instrument is the heading indicator. The heading indicator, also called the directional gyro, is used because a magnetic compass only works accurately when the airplane is flying straight and level in unaccelerated flight.
Any time the airplane is banked, pitched, accelerated, or decelerated, the magnetic compass may give an erroneous reading. The heading indicator solves this problem by using a gyroscope instead of magnets. The heading indicator, however, has its own error. Bearing friction causes the heading indicator to precess, or creep, from the heading to which it was set. Therefore, the heading indicator should be reset to the magnetic compass every 10 or 15 minutes, but only when the airplane is straight and level in unaccelerated flight.
Whether the heading indicator precesses on Flight Simulator is controlled by a setting in the Sim/Realism menu. There is a setting for Gyro Drift that must be selected to permit the heading indicator to precess.
The Lear 35A has a heading indicator slaved to the magnetic compass. It is unnecessary in the Lear to reset the heading indicator.