peekay4, thank you for your response and image. Even after your explanation I still don't know what the FAA are achieving by removing the VDA.
VDA only provides advisory obstacle protection until the MDA. Once below the MDA, the VDA is no longer valid -- you are now flying a visual segment and thus must (visually) fly clear of obstacles.
This is correct, but when the aircraft is visual don't we still expect that the VDA can be maintained to the threshold?. After all as procedure designers don't we build a VDA from the threshold for the aircraft to fly? I would not expect an aircraft to have to change the descent profile to get to the threshold. This is why the VSS was invented. If the VSS is not penetrated then we expect the aircraft can fly a constant profile to the threshold. If it is penetrated then you increase the VDA or remove the straight in minima. (ICAO criteria) Removing the straight in minima forces the aircraft to get visual at a higher altitude and thus have time to visual mitigate the offending VSS penetration.
Question: When the FAA remove the VDA are they also removing the straight in minima?
To try to explain better what I am saying, lets take the Birmingham example, and lets assume that there is no VDA published....
The example commonly cited is KBHM RNAV (GPS) RWY 36 (Birmingham, AL). If you fly the 3.04 degree VDA to the MDA, then transition to land visually, no problems. But if you continue flying the VDA below the MDA, you get perilously close to a house sitting on top of a ridge, 2nm from the runway. The roof of the house penetrates the 34:1 surface, providing just 190ft clearance below the VDA path (picture below):
With no VDA published the pilot still wants to fly as stable approach as possible and now has to guess a suitable descent profile. With no guidance from the approach plate...lets assume he guesses 3 degrees (cause thats a normal profile)...he now has much less than 190ft clearance from the house.
I would suggest the approach profile for this approach is too shallow. So as I see it you have 2 choices. Increase the profile 3.1 or 3.2 should do it...or publish a circling minima only ( I would put an aeronautical light in the roof as well). This forces the aircraft to have to visually mitigate the obstacle.
I just don't see how removing the VDA mitigates this problem..