FAA Federal Aviation Regulations (FARS, 14 CFR)

No person may operate an ultralight vehicle when the flight visibility or distance from clouds is less than that in the table found below. All operations in Class A, Class B, Class C, and Class D airspace or Class E airspace designated for an airport must receive prior ATC authorization as required in §103.17 of this part.

------------------------------------------------------------------------
Distance from
Airspace                 Flight visibility        clouds
------------------------------------------------------------------------
Class A.........................  Not applicable....  Not Applicable.
Class B.........................  3 statute miles...  Clear of Clouds.
Class C.........................  3 statute miles...  500 feet below.
1,000 feet above.
2,000 feet
horizontal.
Class D.........................  3 statute miles...  500 feet below.
1,000 feet above.
2,000 feet
horizontal.
Class E:
Less than 10,000 feet MSL.....  3 statute miles...  500 feet below.
1,000 feet above.
2,000 feet
horizontal.
At or above 10,000 feet MSL...  5 statute miles...  1,000 feet below.
1,000 feet above.
1 statute mile
horizontal.
Class G:
1,200 feet or less above the    1 statute mile....  Clear of clouds.
surface (regardless of MSL
altitude).
More than 1,200 feet above the  1 statute mile....  500 feet below.
surface but less than 10,000                       1,000 feet above.
feet MSL.                                          2,000 feet
horizontal.
More than 1,200 feet above the  5 statute miles...  1,000 feet below.
surface and at or above                            1,000 feet above.
10,000 feet MSL.                                   1 statute mile
horizontal.
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[Amdt. 103-17, 56 FR 65662, Dec. 17, 1991]