A “low altitude awareness display”, which is a brown strip along the right side of the DU-1080 primary
flight display, is used as a visual annunciation of the airplane’s nearness to the ground. The low
altitude awareness display is inside the bottom part of the altitude display and begins to appear when
a radio altitude of less than 550 feet is reached. At touchdown, the low altitude awareness display
reaches the horizon line. The yellow line, which divides the brown area from the rest of the display
will disappear at a radio altitude below 60 feet.
If radio altimeter information is invalid, the radio display will be amber dashes, and the low
awareness display will not appear.
Functional testing of the radio altimeter system and the ADI display digital readout is accomplished
on the ground by depressing the TEST button on the DC-550 Display Controller. The following
displays will occur: a radio altitude of 50, +5, -5 feet will be indicated until the button is released, at
which time the actual altitude will be displayed. The radio altitude decision height display shows
dashes when the TEST button is held down, and then displays the current set altitude for the
remainder of the test. The radio altimeter TEST cannot be accomplished when APR CAP function of
the flight director is selected. The radio altitude decision height tone check will depend on the radio
altitude selection (RA) set on the pilot's ADI display. Testing the radio altimeter system with the TEST
button on the DC-550 display controller will also test the EFIS failure flags and annunciators (lamps)
in the MS-560 Flight Director Mode Selector.
NOTE
The test function activated by the TEST button is disabled after the glideslope has
been captured during an ILS approach using the autopilot or flight director.
While taxiing over ice or snow, the radio altimeter may fluctuate by as much as fifty feet. Outputs
from the radio altimeter system are used to desensitize the flight director and autopilot as the
airplane passes 1100 feet AGL with the glideslope engaged during an ILS approach. If the radio
altitude is invalid, gain programming becomes a function of glideslope capture, time, and airspeed.
Cessna Citation XLS - Instrumentation & Avionics
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