This is a facinationg subject and I remain somewhat clueless so can some -one explain what Vertical RNP is measuring whenit is utilized during an Integrated Navigation Approach. This is a term coined by Boeing for the IAN capabilty that is available, if not standard in the B737NG and B787 aircraft. Basically there is a 400' wide decent path created that looks like a glideslope and whether it be one mile or twenty-five miles from the MAP, it remains only 400' wide, unlike a ILS glideslope that expands in depth as it moves back from the MAP or runway TDZ. Sort of like a tube that descends to the MAP. This is different than say a VNAV approach and you utilize a glideslope just as you would in an ILS approach. During this approach a visual glidesople along a visual RNP and ANP pointers are presented for both lateral nav and vertical nav.
So the question remains in my mind just what are we looking at when we see vertical RNP values. Might menton that the vertical RNP/ANP scales do not appear until we wre excactly 1,024 feet above the MAP.
Sounds a tad different to our IAN equipped 737s (all those delivered from 2006 on). We allways have the lateral RNP/ANP scales displayed when not in approach mode (which is the case during IAN approaches) or VORLOC. The vertical ANP/RNP scales are in view from top of descent until interception of the final approach (ILS/IAN/GLS) when they will get replaced by the normal ILS deviation scales and pointer, regardless what type of approach is flown, well, actually never flew a VNAV/LNAV approach with those planes as it is not the recommended use of the autoflight system there so i cannot comment on that. When closing in onto the final approach the "ILS" deviation pointer will be displayed as ghost pointer until the approach is captured. The GP (GP vs GS, the latter is for an ILS/GLS) during an IAN approach has the same characteristic as an ILS or GLS approach and gets sensitive the closer you are to the runway. The only differences to the ILS/GLS approaches are that you will get an auto-callout "AUTOPILOT AUTOPILOT" at 100ft RA if the autopilot is still engaged and you cannot engage the second autopilot, the ROLLOUT mode won't be armed either.
The vertical path and the vertical RNP/ANP scales are calculated using baro altimetry and it is therefore very important to set the current QNH in the descent forecast page as the glidepath will be calculated using that value. Even 1 hPa difference will mean around 30ft difference above the runway which can become a problem in marginal weather or an annoyance in good weather when you deviate visually to the correct glidepath and get a "GLIDESLOPE GLIDESLOPE" callout from your EGPWS.