PAPI guidance mislead when using external view

[F737NG]

@MissTschai

I noticed today that the PAPI lights guidance is not based on the aircraft height but the current viewpoint. This means that if you are in external view mode the papi lights change depending on the angle you look at the aircraft and how far you are zoomed out.

That’s exactly the desired behaviour.

Don’t think of the PAPI as guidance based on “aircraft height”, rather the desired approach angle.

For instance, the 747 has a much higher flightdeck than most airliners.
If it’s on an ILS approach, the flightcrew would see three whites and one red on the PAPI when descending the glideslope per the ILS beam.

See in the video below how this 747 is on profile, yet the PAPI shows exactly what I describe:

https://youtu.be/8-vOExi_X0I?feature=shared&t=706

This is why the three-bar VASI exists:

It’s all just geometry.