What is the purpose of the selected altitude and airspeed fields in ARINC 429?
DeltaLima very nicely covered the "what" of these labels. As to the "why", I have three examples of their uses:
- Autopilot setpoints (altitude, airspeed, heading) are displayed on the Primary Flight Display for the pilot's situational awareness (and so that she knows when to stop when twiddling the autopilot control knobs)
- They can be logged to a Flight Data Recorder for accident investigations, or a Quick Access Recorder for operational / maintenance purposes
- They're transmitted by certain types of Mode S transponder with "Enhanced Surveillance" capability. If ATC is also suitably equipped (e.g. a lot of Eurocontrol airspace) the preselected altitude is displayed on their scopes and helps to prevent altitude busts due to miscommunication or pilot error.