(a) When deactivation
of that equipment is directed by ATC;
(b) Unless,
as installed, that equipment was tested and calibrated to transmit
altitude data corresponding within 125 feet (on a 95 percent
probability basis) of the indicated or calibrated datum of the
altimeter normally used to maintain flight altitude, with that
altimeter referenced to 29.92 inches of mercury for altitudes
from sea level to the maximum operating altitude of the aircraft;
or