Radio Magnetic Indicator
The Radio Magnetic Indicator (RMI) (Figure 16) is a system containing a north
seeking element stabilized by a gyro wheel. It incorporates the features of a magnetic
compass in that it indicates the magnetic heading of the aircraft. However, the erratic
nature of the magnetic compass is overcome by the stabilization of the gyro.
Contrary to the operation of the other instruments on the panel, the RMI rotates about
its center and the pointers remain fixed. The face, or "compass card," is calibrated in
degrees. The instrument may be used as a directional gyro when the magnetic
influence is cut out of the system. Alternating current (AC) is required for the
operation of the RMI. A complete description of this system can be found in
Use and Limitations. The RMI is the wing attitude crosscheck instrument for both
straight and level and straight climbs or descents. For level turns and climbing or
descending turns, the RMI is used as a performance instrument. It will give you an
indirect indication of wing attitude with the balance ball centered. With the balanced
ball out of trim, heading will drift, resulting in the RMI no longer acting as either a
wing crosscheck or performance instrument. In attitude instrument flight, it is
essential that our instruments portray an accurate indication of the aircraft's attitude.
It is imperative the balance ball is centered so that the RMI can provide us with
accurate attitude information.
Figure 16 Radio Magnetic Indicator
INTRODUCTION TO BASIC INSTRUMENTS 1-9