Originally Posted by
StudentInDebt
Don’t know if there is a different A320 EIS standard available but the FCOM DSC-31-40-K we use describes the Lateral Deviation Scale “dots” as representing 0.1Nm and does not vary with the selected RNP. For an RNP-AR 0.3 approach we would call LDEV at 1 dot (because that is reached before 1/2RNP) and go around at 2 dots (still less than RNP). For RNP 0.1 LDEV is called at 1/2 dot deflection and GA at 1 dot. Confusing eh?
There are pertinent quotes from FAA AC 90-101A(a):
The aircraft should have an appropriately-scaled, non-numeric deviation display (i.e., lateral deviation indicator (LDI) and vertical deviation indicator (VDI)) in the pilot’s primary optimum field of view (FOV). A fixed-scale course deviation indicator (CDI) is acceptable as long as the CDI demonstrates appropriate scaling and sensitivity for the intended RNP value and operation. With a scalable CDI, the scale should derive from the selection of the RNP value, and should not require the separate selection of a CDI scale. Alerting and annunciation limits must also match the scaling values. If the equipment uses default RNP values to describe the operational mode (e.g., en route, terminal area, and approach), then displaying the operational mode is an acceptable means from which the flightcrew may derive the CDI scale sensitivity. (b) In lieu of appropriately scaled LDIs and VDIs in the pilot’s primary optimum FOV, a numeric display of deviation may be acceptable depending on the flightcrew workload and the numeric display characteristics.
(13) Display of Deviation. The navigation system must provide a numeric display of the vertical deviation with a resolution of 10 feet or less, and the lateral deviation with a resolution of 0.01 NM or less.
5. Requirements for Using Lines of Minima Less Than RNP 0.3. The AFM or aircraft qualification documentation should identify if the aircraft is capable of using lines of minima associated with RNP less than 0.3, and the required equipment configuration to achieve this capability. For example, dual autopilots may achieve a smaller RNP capability than dual FDs.