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I'm wondering which of the two Radio Altimeters: Captain (left) or F/O (right), that the 737 does the RA callouts based on.

For instance, if the F/O's RA is slightly off, and passes 500ft before the captain's RA, does the aircraft call "500" when the first (in this case the F/O's RA) RA passes that altitude, or is it always based on the captain's RA?

Also, while on this topic, could someone confirm if the "Minimums" callout is always based on whatever is set on the Captain side, because to my knowledge, that is the case.

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The FCOM (volume 2, 15.20.17) states:

The GPWS provides height callouts based on altitude set by the Captain's Minimums selector.

According to the investigation of Turkish Airlines 1951, the captain's and FO's radio altimeter are feed independently, and the autopilot is wired to react to whatever is happening on the captain's side. This includes the aural warnings.

Curiously, I cannot locate anywhere in the FCOM, FCTM nor QRH what happens when the RAs disagree. The only related information I find is, to use the autopilot for landing you'd need both RAs to be functional. If one of them fails while the autopilot is in landing mode (i.e. glideslope and/or localizer mode), the autopilot disconnects.

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  • $\begingroup$ It provides height callouts based on Captain's minimums selector, but does it use left RA? I suspect it actually uses built-in GPS (because EGPWS is GPS-based) $\endgroup$
    – Jan Hudec
    Commented May 26, 2018 at 22:15
  • $\begingroup$ What happens when the RAs disagree is embedded in your last statement. The safety assessment considers two failure types. The first is 'loss of function' which means you're getting no data from the RA or the RA data is set to Fail Warn by the RA's internal Continuous BIT. The other failure type is 'Hazardously Misleading Information' or HMI. In this case you receive what looks like good data, but it's wrong. The protection for this is the use of two RAs. The FCC will compare the data from the two and if they differ by some set amount, the FCC declares a failure. $\endgroup$
    – Gerry
    Commented May 26, 2018 at 22:18
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    $\begingroup$ @JanHudec It uses the RA. EGPWS uses GPS for Forward Looking Terrain Avoidance. It still uses the RAs and Air Data for GPWS Modes 1 through 5 and altitude callouts. $\endgroup$
    – Gerry
    Commented May 26, 2018 at 22:21
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    $\begingroup$ @JanHudec - The minima call out is selectable via a knob, either baro or radio, and then the value. $\endgroup$
    – user14897
    Commented May 26, 2018 at 23:28
  • $\begingroup$ the autopilot is wired to react to whatever is happening on the captain's side. This includes the aural warnings. This is not correct. There are two autopilots, each fed by the corresponding RA. There is a selector switch to use left, right or both autopilots. The autothrottle system is seperate and only uses the captain’s RA. If he captain’s RA reports unreliable data, it will automatically switch to the FO’s RA. Only the later systems on the 738 have a comparator that prevents retard mode if the RA’s disagree. Nothing about the callouts is indicated in the TK1951 report $\endgroup$
    – TomMcW
    Commented May 28, 2018 at 0:23

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