(a) No person may operate a civil aircraft under IFR using the VOR system
  of radio navigation unless the VOR equipment of that aircraft--
    (1) Is maintained, checked, and inspected under an approved procedure; or
    (2) Has been operationally checked within the preceding 30 days, and was
  found to be within the limits of the permissible indicated bearing error set
  forth in paragraph (b) or (c) of this section.
    (b) Except as provided in paragraph (c) of this section, each person
  conducting a VOR check under paragraph (a)(2) of this section shall--
    (1) Use, at the airport of intended departure, an FAA-operated or approved
  test signal or a test signal radiated by a certificated and appropriately
  rated radio repair station or, outside the United States, a test signal
  operated or approved by an appropriate authority to check the VOR equipment
  (the maximum permissible indicated bearing error is plus or minus 4 degrees);
  or
    (2) Use, at the airport of intended departure, a point on the airport
  surface designated as a VOR system checkpoint by the Administrator, or,
  outside the United States, by an appropriate authority (the maximum
  permissible bearing error is plus or minus 4 degrees);
    (3) If neither a test signal nor a designated checkpoint on the surface is
  available, use an airborne checkpoint designated by the Administrator or,
  outside the United States, by an appropriate authority (the maximum
  permissible bearing error is plus or minus 6 degrees); or
    (4) If no check signal or point is available, while in flight--
    (i) Select a VOR radial that lies along the centerline of an established
  VOR airway;
    (ii) Select a prominent ground point along the selected radial preferably
  more than 20 nautical miles from the VOR ground facility and maneuver the
  aircraft directly over the point at a reasonably low altitude; and
    (iii) Note the VOR bearing indicated by the receiver when over the ground
  point (the maximum permissible variation between the published radial and the
  indicated bearing is 6 degrees).
    (c) If dual system VOR (units independent of each other except for the
  antenna) is installed in the aircraft, the person checking the equipment may
  check one system against the other in place of the check procedures specified
  in paragraph (b) of this section. Both systems shall be tuned to the same VOR
  ground facility and note the indicated bearings to that station. The maximum
  permissible variation between the two indicated bearings is 4 degrees.
    (d) Each person making the VOR operational check, as specified in paragraph
  (b) or (c) of this section, shall enter the date, place, bearing error, and
  sign the aircraft log or other record. In addition, if a test signal radiated
  by a repair station, as specified in paragraph (b)(1) of this section, is
  used, an entry must be made in the aircraft log or other record by the repair
  station certificate holder or the certificate holder's representative
  certifying to the bearing transmitted by the repair station for the check and
  the date of transmission.

  (Approved by the Office of Management and Budget under OMB control number
  2120-0005)