“Common sense” would tell you that if you are visual with the approach lights, then you are good to fly down them.
However I’ve just read that they are valid only 0 to 4nm out. Beyond 4nm you could be seeing them through a power cable, trees, etc.
Beyond 4nm you could be seeing them through a power cable, trees, etc.
That doesn’t make sense. To see them from more than 4nm out through trees or cables, you’d have to be flying too low to see them at all.
That said, I do recall a 4nm limit from training, here’s the FAA text (link to full page here).
4.?The VASI is a system of lights so arranged to provide visual descent guidance information during the approach to a runway. These lights are visible from 3-5 miles during the day and up to 20 miles or more at night. The visual glide path of the VASI provides safe obstruction clearance within plus or minus 10 degrees of the extended runway centerline and to 4 NM from the runway threshold. Descent, using the VASI, should not be initiated until the aircraft is visually aligned with the runway. Lateral course guidance is provided by the runway or runway lights. In certain circumstances, the safe obstruction clearance area may be reduced due to local limitations, or the VASI may be offset from the extended runway centerline. This will be noted in the Airport/ Facility Directory.
Why that would be the case, I don’t really know. Atmospheric phenomena (haze, heat shimmer), perhaps?
That doesn’t make sense. To see them from more than 4nm out through trees or cables, you’d have to be flying too low to see them at all.
VASIS could theoretically illuminate a spot on a mountain if the terrain is like that. Very unlikely though.
A related issue is the danger associated with a “Black Hole Approach”:
Interestingly, the Pro Tip video says that the VASI is only guaranteed out to 4 NM.
This link sets out some common optical illusions that can affect pilots:
Nothing to do with anything visual. Basically the VASI is set up to give minimum obstacle clearance, if flown within its limits (+-10 degrees laterally, no idea what allowance is made for glide path).
If you KNOW there is no obstacle between your 20 mile point when you start using the VASI (and taking into account its precision), no problem with using it – but you are on your own, the approach designers only checked it to 4 NM out.
I think one obvious scenario is that there is a mountain say 20nm on the extended runway centreline, and the instrument approach procedure is well inside that 20nm, but if somebody decided to fly a long final from 30nm out, they might hit some cables on top of the mountain. The mountain would have to be just the right height (about 6000ft, assuming a 3 degree VASI glideslope).
If you can see that VASIs from that range you’re a better man than me. I struggle to discern PAPIs clearly until within 10nm.
OK; a ~3000ft mountain at 10nm would do the same job 
In the US, the VASI are evaluated for obstacles out to 4 NM. However, the VASI system must be more expensive because most of the VGSI installations in the US use the PAPI system. The PAPI is only evaluated out to 4 SM and the point from which this is measured is not the threshold, but at the PAPI location which is often offset another 1000 feet from the threshold. Putting all this together, if you have DME or GPS to the threshold, it will read 3.3 at the end of the obstacle evaluation area. ((5280 X 4)-1000)/6076 = 3.31. At airports that have rising terrain the further you are from the threshold along the final approach course, you can’t depend on the PAPI for more than 3.3 NM.
However, the VASI system must be more expensive because most of the VGSI installations in the US use the PAPI system.
More likely because VASI is an outdated system and PAPI offers better precision.
