It’s not unusual IRL for the PAPI/VASI (collectively called VGSI) lights to not reflect the same approach as an ILS indicates. It will be noted on the approach chart. As an example, at KIAD, ILS runway 19L, you’d find the note: "VGSI and ILS glidepath not coincident. VGSI angle 3.0/TCH 75, with another note: “GS 3.00//TCH 55.” TCH is threshold crossing height. Basic geometry would tell you that using the same descent angle but crossing the threshold at different heights means you will arrive at different touchdown points. Thus, you’ll get a different view of the VGSI lights depending on whether your ILS crossing height is above or below the visual crossing height.
So there is no error in those indications in all cases. That’s not to say that there may not be errors in some of the thousands of approaches in MSFS. The basic rule is: if you’re flying visually, use the lights to help guide you down. If you’re flying an ILS approach, fly the glideslope indicator to touchdown, even if you make a visual sighting of the runway early.
Regards
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