3. In the T-39, ADF bearing information is displayed on a needle of the Electronic
Horizontal Situation Indicator (EHSI).
The EHSI (Figure 2) displays aircraft magnetic heading with navigational bearing data and
consists of a rotating compass card, two bearing pointers called No.l and No. 2 needles and a
range indicator. The compass card is actuated by the aircraft compass system. Aircraft magnetic
heading is displayed beneath the top index or "fiducial" marker. The No. 1 needle displays ADF
information. The T-34's Radio Magnetic Indicator (RMI) is similar to a EHSI but no distance
information is displayed.
When the aircraft compass system is functioning correctly, the ADF needle will display
magnetic bearing to a selected NDB station. Although the ADF system still generates a relative
bearing, the EHSI automatically solves the formula (magnetic bearing equals magnetic heading
plus relative bearing) by constantly positioning the aircraft's magnetic heading at the top of the
NOTE: In the event of a malfunction of the aircraft's compass system, the No. 1 needle will not
indicate a magnetic bearing but will still point toward the
station and continue to provide relative bearing.
RADIO AIDS TO NAVIGATION 1-3