Originally Posted by
EagleA25
I would feel a lot safer with an established LDEV attached to the VDEV, or is the Yo-Yo sufficient?
Sort of, this is my take on it:
The FMS generated lateral track, FD steering commands to follow it, and the AP control inputs are - on the modern bus - all just elcetronic signals and digital values generated by one multipart computer. Thus their targets cannot be different one from another as long as the individual but not-so-separate elements are actually working.
n.b. The B's design is most likely operationally superior. And A. will follow suit with newer designs for the more high-level RNP installations
a) There is no lateral deviation scale on PFD
b) If the aircraft does really fly off track, the A/C symbol on ND graphically shows displacement from the target (green line) path , and a numerical value of the cross-track deviation is displayed (in NM). E.g. 0,02L. This label was re-designed years ago to show that second decimal digit, which was a certification requirement.
As a pilot, during my first (and the only proper
) RNAV APCH course I was trained to know by heart what is the max permissible deviation before self-declaring unable to continue.
c) If the aircraft loses certainty of where it actually is -> the value of EPE = Estimated Position Error (= ANP in Languedoc dialect) looses beyond the accuracy required, then
. there is a MCDU message NAV ACCUR DOWNGRAD
. there is a TRIPLE-CLICK audio warning
. there is a ND warning flag NAV ACCUR DOWNGRAD
This
google picture search shows what a PROG pg of MCDU looks like. The blue REQUIRED accuracy value is pilot-modifiable.
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Hence from a certification point of view, the aircraft is capable of
- both monitoring and alerting the ANP v.s. RNP in forward field of vision (on ND)
- displaying the actual cross-track error to the pilot.
Functionally equivalent to CDI.