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;

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    (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]