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I am an aviation layman. I am reading about PAPI lighting system and it seems like it provides vertical guidance, just like the glideslope component of an ILS.
Coupled with a LOC, both vertical and lateral guidance are available to the pilot.
Why isn't such an approach considered a precision approach?
Thank you.
I think you're overlooking the primary reason pilots fly instrument approaches: the conditions do not permit flight by visual reference.
The fact that a PAPI provides vertical guidance is more or less useless if you can't see it. Most ILS approaches involve a descent of at least 1500' from glideslope intercept.
If you break out from the clouds at ILS minimums, you can be as low as 200' above the runway surface; at 3°, that's around 3/4 miles from the touchdown zone and the PAPI. It's highly unlikely that the PAPI would be visible prior to that point, meaning the approach would be made with purely lateral guidance.
In addition to the issue of visibility, consider that a PAPI consisting of 4 red/white lights only lets you distinguish between five different states (ranging from all red to all white), whereas an ILS glideslope component will give you a continuous guidance. The latter allows you to spot small deviations or trends much earlier and with significantly higher precision.
A precision approach is one where vertical guidance is provided. ILS, (former) MLS, and other approaches where there is electronic vertical guidance may qualify as a precision approach (there are some other criteria). PAPI is not electronic guidance, and therefore would not meet the requirement of vertical electronic guidance on an instrument approach.
