Home' RTCA Documents for Review : ACAS X MOPS DRAFT Vol.1 Contents 239
ACAS X MOPS
11. The annunciation of “Maintain Rate” RAs is changed from prior versions of TCAS, to directly
annunciate the sense of the action including annunciation of “Reversal” or “Crossing”, e.g., “Climb,
Climb” or “Climb, Climb NOW” or “Climb, Crossing Climb”, instead of the ambiguous “Maintain
Vertical Speed.” The annunciation and Label 270 Contents are the same as for their standard rate
equivalents, except for the “Rate to Maintain” given in Bits 11-17.
12. Rows 17, 18, 19, and 20 apply when the logic determines that a positive RA can safely “Weaken” to a
• If the positive RA was a Climb (Row 17), the new RA is displayed with a green arc from 0 – 300
fpm and annunciated “Level-off.” If the positive RA was a Descend (Row 19), the new RA is
displayed with a green arc from -300 – 0 fpm and annunciated as “Level-off” EXCEPT for the
special case described in row 21 and Note 13. Use of the green arc helps to reduce altitude
The advisories described in Rows 17 and/or 19 are issued withen the aircraft is climbing or
descending. If the aircraft is already level, the advisories described in Rows 18 and/or 20 are
• If the positive RA was a Maintain Climb Rate (Row 18), the new RA is displayed only by a red arc
for all rates less than zero fpm, i.e. do not descend. If the positive RA was a Maintain Descent
Rate (Row 20), the new RA is displayed only by a red arc for all rates greater than zero fpm, i.e.,
do not climb. For both cases, the new RA is annunciated as “Monitor Vertical Speed.”
• “Increase Climb” RA does not “weaken” to “Climb” and “Increase Descent”does not weaken to
13. When an aircraft with an active “Descend” RA reaches the low altitude inhibit, the RA will weaken to
“Do Not Climb”, indicated by a red arc for all rates greater than zero fpm, and be annunciated as
“Monitor Vertical Speed.” A green arc / target is not displayed.
14. The combination of the Up Advisory bits and the Down Advisory bits in the ARINC Label 270
contents indicate “Do Not Descend” and “Do Not Climb”, respectively (a dual VSL 0 RA),
annunciated as “Level-off.” A green arc / target shall (1754) be displayed between ±250 fpm when a
VSI or VSI tape display is used. When pitch guidance is used for the display of RAs, it is recommended
that a green “fly-to” target be displayed between the two RA trapezoids; the display shall (1755) leave
sufficient room between the two RA “avoidance” trapezoids for the ownship reference symbol.
15. A Multi-threat Level-off issued while ownship is already level indicates that the action is to maintain
level flight. It is annunciated “Level-off” and a Green Arc is displayed. This advisory has neither
“Up” nor “Down” sense. By convention the “Up/Down” bit will be set to “0” in this case.
16. In general, RA reversals are from a positive RA to the opposite sense positive RA. The RAs described
in lines 28 and 29, i.e., a reversal from “Climb” to “Do Not Climb” have been observed to occur in
two circumstances: multi-threat encounters; and, at low altitudes when descend RAs are inhibited.
The advisories described in line 27 and line 28 involve a reversal of a previous VRC or RA intent.
These RAs are annunciated as “Level-off” and a green arc / target is displayed.
The advisory described in line 28 can also be issued when ownship is climbing to emphasize the
Level-off. It is annunciated as “Level-off” and a green arc / target is displayed.
The advisory described in line 29 is issued when ownship is level or descending to emphasize the
climb prohibition while not prescribing a Level-off. It is annunciated as “Monitor Vertical
Speed” and a green arc / target is not displayed.
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