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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

      a. VHF onmirange (omni-directional range) (VOR)
        1)Description
          a) The primary navigation aid (NAVAID) used by general aviation
          b) Provides 360° azimuth information accurate to within 1°
          c) If collocated with DME, then resultant VOR/DME provides both azimuth and distance information
          d) If collocated with military tactical air navigation unit (TACAN), then resultant VORTAC provides both azimuth and distance information
          e) Courses oriented from the VOR station are called radials
          f) VOR information is not influenced by aircraft attitude or heading
          g) Transmitted also:
            i) Morse code identification signal
            ii) Voice communication
          h) Classified by operational use and service volumes (A.I.M 1-1-8)
            i) Standard high altitude VOR service volume
            standard high altitude VOR service volume
            ii) Standard low altitude VOR service volume
            standard low altitude VOR service volume
            iii) Standard terminal VOR service volume
            standard terminal VOR service volume
          i)Check NOTAMs or A/FD for accuracy problems of specific VORs
        2) VOR components
          a) Ground station VOR ground station
            i) Automatic monitoring system
            ii) Identification
            • Morse code
            • Voice
            iii) VHF frequencies between 108.0 and 117.95 MHz (only even tenths between 108.0 and 112.0 to preclude conflict with ILS frequencies)
            iv) Accuracy of signal is generally within +/-1° (AIM)
          b) Airborne equipment
            i) Antenna
            ii) Receiver
            iii) Indicator instrument VOR indicator instrument
            • Omnibearing selector (OBS)
            • Course deviation indicator (CDI) needle
            • Ambiguity or TO/FROM indicator
            • Index
            • Flags or other weak signal indicator (OFF or blank TO/FROM indicator window)
            • May be part of HSI
        3) VOR operational use
          a) Orientation
            i) Tune VOR to appropriate frequency
            ii) Audibly identify station
            iii) Rotate OBS to center CDI needle with FROM indication
            iv) Read azimuth (radial) at index
            v) With a TO indication and centered CDI needle, index indicates magnetic bearing to the station
            vi) Above the station, in the cone of confusion, CDI needle deviates back and forth
            vii) Reverse sensing results if heading to the station with a FROM indication or from the station with a TO indication
            viii) A second VOR can allow simultaneous determination of location along a radial from the first VOR
          b) Tracking TO and FROM the station
            i) TO the station: Rotate the OBS until the CDI needle centers with a TO indication and fly course indicated, correcting for wind (left needle deflection indicates left crosswind)
            ii) FROM the station: Center CDI needle with FROM indication and fly course indicated, correcting for crosswind
          c) Course interception
            i) For initial orientaion, turn to fly parallel to and in same direction as desired course
            ii) Determine difference between desired and current radials
            iii) Double the difference to determine intercept angle beween 20° and 90°
            iv) Rotate OBS to set desired course at index
            v) Turn to intercept heading
            vi) Fly until CDI centers
            vii) Turn and track desired course inbound or outbound
            viii) For example:
            VOR course interception
            Course interception without initial turn to parallel:
              Determine present radial you are on
              Find difference between radial on and radial to intercept
              Double this difference to find intercept angle between 20 and 90°
              Turn to intercept new radial at this angle
          d) Helpful VOR navigation simulator applet
        4) VOR operational errors
          a) Careless identification of station
          b) Failure to check accuracy/sensitiviy
          c) Turning in wrong direction during orientation
          d) Failure to check ambiguity indicator, resulting in reverse sensing and corrections in the wrong direction
          e) Failure to initially parallel desired radial on a track interception problem
          f) Overshooting or undershooting radial on interception
          g) Overcontrolling corrections during tracking, especially close to the station
          h) Misinterpretation of station passage (voice transmission may cause transient fluctuation of TO/FROM indicator)
          i) Chasing the CDI needle resulting in homing instead of tracking
          j) Certain propellor RPM settings may cause CDI fluctuation of up to 6° (correct with slight RPM change)
        5) VOR receiver accuracy check
          a) Sensitivity check: note number of degrees of change as OBS is rotated to move CDI from center to last dot on either side (should be less than 10-12° on either side)
          b) 14 CFR Sec. 91.171 requires VOR accuracy check within 30 days prior to IFR flight
          c) VOR test (VOT) facility or repair station accuracy check
            i) Locations published in A/FD and IFR area and low altitude enroute charts
            ii) Repair station frequency normally 108.0
            iii) Identify with series of dots or continuous tone (contact FSS for specific VOR identification information)
            iv) Instrument should indicate 0° FROM (i.e., on the 0° radial) or 180° TO (reminder: "Cessna one-eighty-TO")
            v) Airborne check may be permitted for specific areas/altitudes stated in the A/FD
          d) Certified surface or airborne checkpoints
            i) Specific radials at specific points on surface (or airborne)
            ii) Error should be no more than 4° (6° airborne) after applying compass card correction
            iii) Dual system VORs (independent except for antenna) may be checked against each other: bearings to station should not differ by more than 4°

References:
Greg Gordon MD, CFII
Updated: