What is the purpose of the selected altitude and airspeed fields in ARINC 429?
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2 Answers
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Both labels are used to transmit the altitude and airspeed that are dialed into the Mode Control Panel of the autopilot. They are not necessarily the current altitude/airspeed, instead they are the target values that the autopilot is steering to.
answered May 9, 2019 at 7:35
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$\begingroup$ so what is the altitude selected based on altitude? ground or sea level? $\endgroup$ May 9, 2019 at 7:39
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$\begingroup$ It is based on the barometric altitude of the pilot flying. So pressure altitude corrected for the altimeter pressure setting. $\endgroup$– DeltaLima ♦May 9, 2019 at 7:58
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$\begingroup$ @LeoAnggaraKrisna The current and selected altitude can be pressure (FL), true (MSL) or absolute (AGL). The autopilot doesn't care which; it will just fly the plane up or down as needed to make current equal selected. $\endgroup$– StephenSMay 9, 2019 at 21:23
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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.
