VOR - checked every 30 days

§91.171 VOR equipment check for IFR operations.

1. No IFR using VOR system unless the VOR equipment of that aircraft --
   1. is maintained, checked, and inspected under an approved procedure; or
   2. or has been operationally checked within the preceeding 30 days, and was found to be within the limits of the permissible indicated bearing error
2. operationally checked means...
   1. At departure airport, use a TEST SIGNAL. Accurate to plus or minus 4 degrees.
   2. At departure airport, use a DESIGNATED VOR CHECKPOINT. Accurate to plus or minus 4 degrees.
   3. AIRBORNE CHECKPOINT. Plus or minus 6 degrees.
   4. AIRBORNE test
      1. Select a VOR radial that lies along the centerline of an established VOR airway;
      2. Select a prominent ground ponit along the radial, not more than 20 nautical miles from the VOR ground facility, and maneuver the aircraft directly over the point at a reasonably low altitude; and
      3. 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).