“At SFO, runway 29L has a 4-light PAPI system on the left side of the runway.”
It’s called a VASI (Visual Approach Slope Indicator) There are two groups of four lights spaced some distance apart on the approach end of the runway. If both sets of lights show “white” to the pilot, he’s high, if both show “red” he’s below the standard glides lope ( which is, at most airports, 3 degrees down). He’s on the glide slope when the far set of lights are white and the near set are red.
I have read in some local accounts that the plane had an unusually high rate of descent. The VASI would have been all red in this condition.
No, a VASI is two lights (or sets of lights), arranged along the side of a runway, in a "column".
PAPI (Precision Approach Path Indicator) is a set of 4 lights, arranged in a "row" on one side of the runway.
There is also a tri-color, single-light VASI. But, it is not used much in the US.