Air carrier pilot conducting an RNAV 28 approach to HDN airport reported the PAPI system is not aligned correctly and is difficult to see due to blending in with snow covering the ground.

Narrative

The PAPI for Rwy 28 at HDN is out of adjustment. The final approach course is more than 19 degrees off of the runway heading. The PAPI glideslope is 3.5 degrees. After maneuvering off the RNAV final approach course to visually align with the runway; VNAV guidance is lost. At this point one cannot see the PAPI; it is too dim compared to the bright sunlight glare off the snow. Descending without vertical guidance; we eventually picked up the PAPI with a three-red indication. I shallowed the glide path to intercept two-white/two-red; but THE PAPI WENT TO A WHITE-RED-WHITE-RED INDICATION !!! I kept the rate of descent constant until we had an unequivocal indication which of course would be W-W-W-R that quickly went all-white. I had to descend at a greater-than-1000 fpm rate to make the touch-down zone. We had previously briefed that 1300 fpm would be our stable limit due to the published 3.5-degree glideslope.Poor maintenance/certification of the PAPI such that the second light at a minimum is not properly aligned vertically. Poor FMS VNAV design such that vertical guidance is lost during off-axis approaches. Poor design of standard PAPI facilities which make them invisible when surrounded by brightly-lit snow. Suggestion immediate NOTAM that the HDN Rwy 28 PAPI is unusable. Conduct a calibration and vertical re-alignment of all lights on the HDN Rwy 28 PAPI. Design and install a black background panel extending 3 ft beyond the PAPI both horizontally and vertically behind the PAPIs at all airports where the PAPI is commonly seen in a field of snow. Modify FMS software such that vertical guidance is maintained up to 30-deg off of the final approach course until the missed approach point.