Originally Posted by Air Rabbit
I wouldn't put too much credence in the fact that VASI or PAPI lights can miss-lead a pilot to the degree that he/she could correctly use those visual aids and still wind up in the position indicated by the aircraft with the tail low in this photo. This pilot obviously had other, rather distinct areas where his/her attention, recognition, and/or piloting response was severely lacking! Aviation is not everything in one basket - just like its not just airspeed or not just altitude or not just power setting ... there are a multitude of factors that all have to be taken into consideration - all have to be managed - and they all have to be monitored and corrected when, where, and HOW ... as it becomes necessary to do so.
The real story of Fox River:
Originally Posted by Underfire
The diagram is coincident, both the ILS and PAPI are set at 3°. The diagram shows where the points meet, when and how they diverge.
The centre of that PAPI beam (ie half way between angle C and angle B) is
not parallel with centre of the ILS GS. Angle C is parallel with the GS; that diagram indicates that that PAPI is set at 2.75°. They are neither coincident in angle or GPI.
All this talk of changing visual slope indications on short final depending on what type one is in is ironic; we had a T-VASI, where one could accurately fly a constant picture, 3° slope to the runway to a GPI of your choice. 747 could fly 2 light high to match the pilots eyes with the ILS GS, a bugsmasher could fly 2 lights low and land on the piano keys. Alas, it fell out of favour for the cheaper (and nastier) 4-box PAPI.
Originally Posted by Aterpster
It means the glidepath angles are different. The GPIs could be the same, or different, as well.
What would be the point of having different angles? There is obviously a logical reason for different GPIs and the same angle, but I cannot see why you'd want different angles.
I had a look at SFO 28L on Airnav: both the ILS and the VASI are 2.85°, and the note says they are not co-incident. Why? because the MEHT is...64ft vs the ILS TCH of 53ft, around 200ft different on the ground.
I'm ready for a look at an airport that has PAPIs set to a different
angle to the ILS...