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In civil transport aircrafts (featured with dual/triple redundancy), how does airplane's Airflow angle (AoA/AoSS) sensor measurements read by the pilot? By using voter mechanism as used for airspeeds or individual sensor measurements to pilot and co-pilot?

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  • $\begingroup$ AoA display is usually an option or not available at all. See also this question for the optional AoA display in the 737. $\endgroup$
    – Bianfable
    Aug 25 at 18:51
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    $\begingroup$ Airspeeds don't use voting for display in any aircraft I can think of. It is always one sensor to one display for all sensors. Voting is only used for excluding inputs from auto-flight systems and flagging the display as likely errorneous, but pilots generally still have to select which sensor they want to trust. $\endgroup$
    – Jan Hudec
    Aug 25 at 19:33
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The A320 is fitted with an air data inertial reference system (ADIRS) made of 3 ADIRUs. ADIRU 1 data (altitude, speed, etc) are shown on Capt side, ADIRU 2 on F/O side. ADIRU 3 can be selected manually.

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Each ADIRU contains an air data reference (ADR) and an inertial reference (IR). Each ADR receives the AoA value from a separate AoA probe.

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The AoA value is not displayed by default, but it is accessible on the MCDU after selecting the related page. The values are always monitored for validity by FAC/ELAC before being used.

The final value is the mean of valid inputs. If one ADR disagree it is silently discarded. If all ADR disagree, a message is displayed. Any ADR can be shut down using the overhead panel. From this report:

If one ADR has been detected as being faulty, or has been rejected by the flight control computers, and there is an airspeed or angle of attack disagreement between the remaining two ADRs, then a level 2 alert is raised. This activates the master caution and presents an amber NAV ADR DISAGREE message on the ECAM.

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The way the discrepancies are managed can lead to rejecting a valid value because the two other ADR are wrong, but agree.


All pictures are from the report.

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    $\begingroup$ Do you know if AoSS (sideslip angle) is shown anywhere in the A320? The sideslip indicator / beta target on the PFD is obviously related, but it doesn't show the angle in degrees. $\endgroup$
    – Bianfable
    Aug 26 at 12:57
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    $\begingroup$ @Bianfable: It's a measure in g. From the FCOM about the sideslip index: "in flight it shows sideslip (as furnished by ADIRS). One centimeter of displacement indicates 0.2g. The sideslip index is against its stop at 0.3g". So I assume the ADIRS uses the IRS to detect the slip, it's not a static pressure difference. $\endgroup$
    – mins
    Aug 26 at 15:20
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    $\begingroup$ I'd also like to note, that there are several ways that various plane manufacturers handle output. 1) Show them separately, as stated in this answer 2) Take the average (as long as the difference is within tolerance) 3) Output only one, using the other to validate and switch over as backup. Likewise, there are several methods for fault detection. $\endgroup$
    – MishaP
    Aug 27 at 6:26
  • $\begingroup$ @MishaP: That's a good addition, and you could make an answer, that would be more complete than this one. $\endgroup$
    – mins
    Aug 27 at 8:09

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