Originally Posted by West Coast
Quote:
VASI lights were never intended to be, and are not an acceptable source of glide path indication all the way to touchdown.
Can you provide supporting documentation? I've not seen this explained quite like that.
As I said, I wasn’t there when the concept and the design stages were envisioned; commission, and completed – and I can’t provide any “supporting documentation,” other than what can be logically understood by how those systems are defined and described in official documents published by the FAA and their Airports Division – with particular focus on these specific systems. That language goes into some minor detail regarding what a pilot may expect from the use of and the dependence upon the use of, these systems – whether discussing the Visual Approach Slope Indictor (VASI) system or the Precision Approach Path Indicator System (PAPI).
Those descriptions are as follows:
Visual Glide Slope Indicator (VGSI) Systems
VGSI(s) provide vertical guidance for a VFR approach or for the visual portion of an instrument approach. The angle established by the VGSI(s) is referred to as the visual glidepath angle. The signal formats used to establish the visual glidepath angle can vary from a single light source, two or three light sources in a longitudinal array, and four or more light sources in a lateral and/or longitudinal array. Setting the required visual angle is a function of ground installation personnel.
VASI
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.
PAPI
The precision approach path indicator (PAPI) uses light units similar to the VASI but are installed in a single row of either two or four light units. These lights are visible from about 5 miles during the day and up to 20 miles at night. The visual glide path of the PAPI typically provides safe obstruction clearance within plus or minus 10 degrees of the extended runway centerline and to 4 SM from the runway threshold. Descent, using the PAPI, should not be initiated until the aircraft is visually aligned with the runway. The row of light units is normally installed on the left side of the runway and the glide path indications are as depicted. 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 PAPI may be offset from the extended runway centerline. This will be noted in the Airport/ Facility Directory.
Nowhere in any of these descriptions is there any reference to any Visual Glide Slope Indicator (VGSI) system being able to, or that it should be relied upon for, accurate glide path indication to the actual touchdown – and, to the contrary, describes these systems providing vertical guidance for a VFR approach or for the visual portion of an instrument approach, neither of which relies upon or references accuracy to touchdown or touchdown position.
Additionally, it should be noted that even the most accurate precision Instrument Landing System does not provide accurate vertical guidance all the way to a safe touchdown. All ILS approaches to and including touchdown are predicated on an autopilot interface with the airplane’s radar altimeter – and is not related to the ILS glide slope information – as touchdown would occur some distance
past the glide slope transmitter - and a visual touchdown would also be well beyond the VASI or PAPI indicators.