Figure 1 - P-8010009

INSTRUMENT NAVIGATION

Figure 1

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

compass card.

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