§  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]