Sec.  90.423   Operation on board aircraft.

   (a) Except as provided in paragraphs (b), (c), and (d) of this section, and
   except as may be provided in other sections of this part with respect to
   operation on specific frequencies, mobile stations first authorized after
   September 14, 1973, under this part may be operated aboard aircraft for
   air-to-mobile,  air-to-base, air-to-air and air-to-ship communications
   subject to the following:

   (1) Operations are limited to aircraft that are regularly flown at altitudes
   below 1.6 km (1 mi) above the earth's surface;

   (2) Transmitters are to operate with an output power not to exceed ten
   watts;

   (3) Operations are secondary to land-based systems;

   (4) Such other conditions, including additonal reductions of altitude and
   power limitations, as may be required to minimize the interference potential
   to land-based systems.

   (b) Exceptions to the altitude and power limitations set forth in paragraph
   (a) of this section may be authorized upon a showing of unusual operational
   requirements which justify departure from those standards, provided that the
   interference  potential  to regular land-based operations would not be
   increased.

   (c) Mobile operations aboard aircraft in the services governed by this part,
   under licenses in effect September 14, 1973, may be continued without regard
   to provisions of paragraph (a) of this section, as follows:

   (1) Operations may be continued only for the balance of the term of such
   licenses if aircraft involved are regularly flown at altitudes greater than
   1.6 km (1 mi) above the earth's surface.

   (2) Operations may be continued for one additional renewal license term if
   the aircraft involved are regularly flown at altitudes below 1.6 km (1 mi)
   above the earth's surface.

   (d) Operation of radiolocation mobile stations may be authorized without
   regard to limitations and conditions set forth in paragraphs (a), (b), and
   (c) of this section.

   [ 43 FR 54791 , Nov. 22, 1978, as amended at  58 FR 44960 , Aug. 25, 1993]