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CFI Instrument Practical Test Standards, FAA-S-8081-9B, June 2001

II. Technical Subject Areas

A. Aircraft Flight Instruments and Navigation Equipment

    2. Flight instrument systems and their operating characteristics

      e. Automatic direction finder (ADF)
        1) Description
          a) ADF determines and indicates the relative bearing (RB) from the aircraft to a nondirectional beacon (NDB) on the ground
          b) ADF needle points to the NDB, indicating the RB to the NDB station
          c) Magnetic heading (MH) + relative bearing (RB) = magnetic bearing TO the station
          MH + RB = MB TO station
          ADF
        2) NDB components
          a) NDB transmits in range of 190-535 kHz
          b) Most ADF will also tune to AM radio frequencies (550-1650 kHz) but these do not continuously identify themselves and are more susceptible to sky wave propogation
          c) NDB stations may transmit voice, e.g. the AWOS information
          d) Before relying on ADF, identify the NDB station by its 2-letter (compass locators) or 3-letter Morse code identifier
        3) ADF components
          a) "Sense" antenna (nondirectional) receives signals equally from all directions
          b) "Loop" antenna (bidirectional) receives signals better from two directions
          c) When sense and loop antenna information is processed together, directional ambiquity is resolved
        4) Indicator instruments
          a) Fixed-card ADF or relative bearing indicator (RBI) always indicates zero at top of instrument
          b) Movable-card ADF allows pilot to rotate card to place aircraft's heading at top of instrument so needle head then indicates MB to the station (and tail indicates MB from the station)
          c) Radio magnetic indicator (RMI) automatically rotates azimuth card to keep aircraft heading at top of instrument
            i) Controlled by remote gyrocompass
            ii) Two needles can be used to indicate navigation information from both ADF and VOR receivers
        5) Using the ADF
          a) Tune and identify NDB
            i) Tune ADF receiver to appropriate NDB station with function switch in "Receiver" or "Antenna" postion
            ii) Identify by Morse code signal
            iii) Move function switch to ADF position
            iv) Check needle points to station before and after test function used to swing needle away
            v) Set low but audible volume to continuously monitor station signal
          b) Orientation
            i) ADF needle points TO the NDB station
            ii) RB + MH = MB TO station
          c) Station passage
            i) Near station: small heading changes cause large needle delfections
            ii) Passing station: needle shows erratic left/right oscillations or moves steadily toward a wingtip position (RB = 90° or 270°)
            iii) Abeam station: needle points to 90° or 270° position
            iv) Station passage occurs when needle shows a wingtip position or settles near the 180° position
            v) Time interval from near station to station passage varies with altitude (from a few seconds to 3 minutes at high altitudes)
          d) Homing
            i) Identify station Morse code signal
            ii) Fly aircraft on any heading required to maintain the 0° RB postion of the ADF needle
            iii) Aircraft follows a circuitous path to station on downwind side of direct track
          e) Tracking
            i) Use heading that maintains desired track (constant MB) to or from the station
            ii) To track inbound to station:
            • Turn to 0° RB heading
            • Maintain heading until 2°-5° needle deflection (drift angle) is observed
            • Turn in direction of needle deflection to intercept initial MB
            • Intercept angle depends on rate of drift, aircraft speed, station proximity
            • Standard intercept angle = 2 x drift angle
            • Track is intercepted when needle deflection = intercept angle
            • As track is intercepted, turn back in direction of needle until RB = estimated wind correction angle (WCA)
            • If needle deflects off-course, re-intercept track and turn to new RB = new estimated WCA
            iii) To track outbound from station:
            • Same principles as tracking inbound to station
            • Wind correction made toward needle tip deflection
          f) Intercepting bearings
            i) Inbound bearing
            • Turn to parallel desired inbound bearing
            • Note whether station is left or right of nose, double number of degrees of needle deflection to determine interception angle
            • Turn toward desired MB the number of degrees determined for the intercept angle
            • Maintain interception heading until needle is deflected same number of degrees as interception angle minus an appropriate lead
            • Turn and track inbound
            ii) Outbound bearing
            • Same procedure as for inbound except substitute 180° needle position for zero position
          g) Operational ADF errors
            i) Improper tuning and station identification
            ii) Not identifying malfunction of RMI slaving system or ignoring warning flag
            iii) Homing instead of tracking (relying solely on ADF instead of correlating with heading)
            iv) Poor orientation due to failure to follow proper steps in orientation and tracking
            v) Careless interception angles after rushing initial orientation procedure
            vi) Overshooting or undershooting MB often due to forgetting the course interception angle
            vii) Failure to maintain heading
            viii) Failure to understand ADF limitations
            ix) Over-correcting (chasing the ADF needle) due to failure to understand or recognize station approach
            x) Failure to keep heading indicator set to agree with magnetic compass
          h) GPS substitution for ADF (or DME)

      References:
        Instrument Flying Handbook, FAA-H-8083-15, 1999
        AIM 1-1-2
Greg Gordon MD, CFII
Updated: