Windows 7 64bitMicrosoft Flight Simulator 2004PMDG 737 600/700 800/900 (CD-ROM)I am using PMDG737-800.When an airplane goes into a landing attitude, PAPI light shines certainly all white.Also at the time of low approach, PAPI light shines in white.What is a cause ?

Sometimes the visual slope is different from the electronic slope... you can look at the AFCAD file with ADE and check that out. Also, some airframes are long or big enough such that the pilot is visually above the ILS glide slope all the way to flare and there are three light vasi systems out there to address this.

And also, the PAPIs/VASIS are not always positioned alongside the touchdown (aiming) point. They can be corrected with ADE9X to the right position (or on older sceneries, with AFCAD).Colin B

And finally, you use either the PAPI or the ILS, you do not use both at the same time.If visual you use the PAPI/VASI and discard your instruments as you are visual i.e looking out the window.If not, you don't look out of the window and fly the instruments.For a good explanation refer to this place :http://www.pprune.org/archive/index.php/t-395854.html