Originally Posted by
pineteam
Thank you guys!
I found the reference in the FCOM and indeed the scale is fixed regardless of the RNP value for all A320.
So I guess we go around for RNP 0.2 and RNP 0.3 at 2 dots. That’s weird but oh well.
.... Our SOP follows exactly what's written in the FCOM:
Quote:Go around in case of L/DEV reaches 2 dot or XTK reaches 1 RNP.
But for some guys it's whatever comes first but I don't agree with that since it's not written that way. I think like you said in case of RNP 0.3 the limiting factor should be the XTK and not the L/DEV scale.
Hi
My take on this is: because you have an LDEV (even though alternative means of monitoring lateral deviation can be approved), you cannot exceed the FSD of the display. In other words you must maintain a 'live needle'. So you would have to go around on reaching 2 dots anyway (assuming that is FSD as seems usual for the bus).
I can see that your scale might be fixed (allowed with caveats) but, I cannot see why you need to use the fixed scale of 2 dots = 0.2 when flying RNP of 0.3. Your avionics must surely be capable of scaling to 0.3 because it is the RNP of every bog standard LNAV and LNAV/VNAV final segment.
I suspect it is only
below RNP 0.3 you are stuck with a fixed scale of 0.2. When that is equal
or larger than the RNP in use it would be logical to go around on reaching "2 dots or 1
x RNP", whichever comes first (as "some guys" at your company think). Or another SOP like StudentInDebt's, eg go around at
x dots even if that is less than RNP because it provides an equal or greater level of safety.