Originally Posted by
CousinEddie
Regarding the VD scale on RNAV approaches, I brief that a centered VD on final will not guarantee 2 red / 2 white on the PAPI as you would normally expect on a centered ILS glide slope. Be prepared to make a slight correction visually using the PAPI despite the nicely centered VD indication. Don’t just lock onto the VD scale all the way down. Crosscheck and correct. A recent example was SAN. Centered VD indication on the RNAV was giving a steady 3 whites on the PAPI (737 fleet). That glide path is 3.5 degrees, so correcting for being high on the PAPI can be more of an issue below 1000 feet.
I generally agree with you with one caveat. A ground based glide path (ie ILS GS) is completely different from a GPS based glide path (without WAAS) and will not give you the same thing every time since it’s not baro compensated. As another poster said, you follow the VD path provided by the GPS approach and then adjust accordingly when within the PAPI usable distance, which is 3.4 NM from the threshold, which is about 1000’ on a 3 degree GP. So if the GPS glide path is giving me three white and one red on the PAPI, simply adjust to two red/two white. Be careful of blindly following the GPS based VD pointer.