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CNS-80 VNAV
After upgrading to software version 2, the CNS-80 offers a new
display page, resembling an HSI with a VDI pointer at its left. The unit's computed VDI presentation is active for many GPS approaches, as well as for all LNAV and LPV approaches. However, I have not (yet) found in the pilot's manual any explanation of when VDI drive is withheld from an external HSI. By flying several different kinds of approaches, I seem to see that the external VDI will be active *only* when VNAV minima apply. Only on precision approaches having DH instead of MDA, my autopilot could arm and capture the glideslope, exactly the same as an ILS. On non-precision approaches my external VDI needle remained driven out of view, even though its glide-slope flag was pulled, and as a result I could not arm the glide slope for automatic capture. (Unexplainably, the CNS-80 annunciated LNAV/VNAV on approaches which actually were non-precision LNAV.) I'm guessing that the external VDI drive was withheld precisely to prevent glideslope capture on coupled non-precision approaches. I don't know the reason behind this, or even if that's the rule. Of course, LNAV coupling works for all approaches, as expected. ---JRC--- |
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