USE OF A CDI
Almost without exception, all military aircraft make use of a radio compass system as the
primary navigation instrument. In the T-34C, this system' ,s cockpit display is the radio
magnetic indicator or RMI. In its role as a compass, you have used this instrument since the
Familiarization stage. In the Radio Instrument stage, you will combine this usage with the
interception of aircraft position as depicted on the RMI's VOR and TACAN needles.
The RMI's principal shortcoming is its small size. Just as it is difficult to resolve small
heading differences on the face of this instrument, it is also difficult to recognize small
deviations from your desired course as depicted on the VOR and TACAN needles, hence the
The course deviation indicator (CDI) is the major component of the IND-350; its other
components are the omni bearing selector (OBS) and TO-FROM indicator (also known as the
It is important to re-emphasize here that, in the event of a discrepancy between RMI and
CDI, the RMI is to be relied upon for course information.
To properly use the IND-350 you must first twist the desired course with the OBS. Then set
the VOR-TACAN switch to the appropriate position. The TO-FROM indicator will then show
whether the course selected will take you TO or FROM the navigational facility tuned. If you
are within ten degrees of the course selected, the course deviation indicator will provide a picture
of relative position.
Figure 39 presents a typical situation. The selected course inbound to the station is 360
degrees and the aircraft is 5 degrees left of course (imagine your aircraft on the tail of the
VOR needle, and at the center of the CDI). You must therefore, turn right (toward the CDI bar)
to establish the aircraft on the desired course. Note that the CDI provides an easily interpreted
picture of course deviation and correction, but actual aircraft position (185 degree radial) is more
easily seen on the RMI.
Thus, as a general. rule, you will use the RMI for orientation, and the CDI as an initial
indication of approaching the assigned course (specifically within 10 degrees of course)
and fine tuning once established on course. Keep this dictum in mind and you should have few
problems with disorientation.
3-30 RADIAL TRACKING AND COURSE CONTROL