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On some non-precision approach plates, there is a VDA/TCH mark. My understanding of it is information-only. If you pass its starting fix at the charted altitude and maintain the given descent angle, the airplane will flying over the runway thresh hold at TCH.

However, I noticed one NOTAM mentions this information is not authorized, e.g. E16 RNAV (GPS) RWY 32 (on Nov 2019).

!FDC 9/9659 E16 IAP SAN MARTIN, San Martin, CA. RNAV (GPS) RWY 32, ORIG-C... GELVY TO RW32: 3.37/45 NA. 1903201729-2103201728EST - (From FAA website on 2019/11/22)

Given "GELVY TO RW32: 3.37/45" is still one spatial path for the non-precision approach, and the LNAV approach is still available. There are no airport elevation and re-defined fix height. So my questions align with,

1) Why am I not authorized to fly "GELVY TO RW32: 3.37/45", if my airplane is capable to maintain descent angle and have runway environment in sight right above MDA.

2) Are there any additional procedures to follow even for flying VDA/TCH at an airport when are authorized?

E16 RNAV (GPS) RWY 32 enter image description here

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This NOTAM is a Flight Data Center (FDC) NOTAM which is used to amend published Instrument Approach Procedures (IAP). That means this NOTAM is effectively amending the procedure as the FAA hasn't had the opportunity to change the procedure yet. This is probably due to a new obstruction that has been placed in the decent path. Perhaps a new cell phone tower has been erected. That is why it is always a good idea to check NOTAMs especially when flying under IFR.

EDIT: To clarify a comment I made to OP, the MDA usually requires 350 feet obstacle clearance along the final approach segment. However, when flying a glide path, the Required Obstacle Clearance (ROC) increases the farther away you are from the threshold (see graphic from Code 7700 Website).

The ROC is based off of the Obstacle Clearance Surface calculated for the approach. Notice that if a new obstacle is erected somewhere along the glide path it is possible for the MDA to still be satisfied but the glide path ROC may have to change. That is probably why the MDA is not changed in this NOTAM but the glide slope is Not Authorized.

Quote from 7700 website:

(1) If the OCS is penetrated, the OCS slope may be adjusted upward, thereby increasing the glide path angle. The glide path angle would increase because it is dependent on the required slope.

enter image description here

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  • $\begingroup$ However, the obstructions (if any) do not affect the MDA per NOTAMs, the VDA/TCH path is still protected until MDA. So I suspect there are additional obstruction clearance for VDA/TCH if specified? $\endgroup$
    – CrepuscularV
    Nov 22, 2019 at 21:04
  • $\begingroup$ On the final approach segment of an approach without vertical guidance like this one, 350 feet of obstacle clearance is needed on the final approach segment. It is possible that at that descent angle a new obstruction would have gotten too close to the glide path. See this site for more info: code7700.com/doa_required_obstacle_clearance.htm $\endgroup$
    – DLH
    Nov 22, 2019 at 21:36
  • $\begingroup$ This reminds me an article discussing a case when LNAV MDA is lower than LNAV/VNAV DA due to different obstacle clearance measurement: boldmethod.com/learn-to-fly/navigation/… $\endgroup$
    – CrepuscularV
    Nov 23, 2019 at 5:01

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