WillowRun 6-3, A-Squared, Capn Bloggs & All those who believe GoogleEarth Elevation/Hgt data:
GoogleEarth provides good approximations of real-earth elevations and is good for relative differences, but it cannot be considered accurate at any single location within confidence levels that you'd want to consider whether you have appropriate clearance or not for approach/dep procedure design. Sometimes it's spot on to the nearest metre, other times not. Depending on where the location is and also often where the location is in relation to nearby terrain and buildings, the elevations can easily be out by up to say ±8 mtrs / ±26.5ft. This is due to the various sources of the data and how the data is modelled. The positional referencing of the image data to WGS84 lat/longs however is far more accurate.
In a sense it's a bit like the FlightAware data, in that it's sampled from actual observations using available data sources. The data is not certified to a particular integrity level though and thus needs to be used and interpreted with care and qualifications, depending on the purpose.
If Capt Bloggs and ASquared calcs are actually more or less correct, then that will come out in the investigation as result of FLT inspection of the PAPIs and resurvey of the terrain and remaining obstacles near the crash site, as well as review of the procedure flown (as selected in the FMS and as flown). In any case, if as it seems to be that the aircraft was simply way too low at that point, the issues you raise may, or may not, in fact be relevant or significant.
ironbutt57:
Yes, you're right: obstacle clearance for an NPA (with a FAF) in the primary area of a final approach segment is 250' (246' actually, or 75m). Take the highest obstacle between the FAF and the MAPt, add the obstacle clearance margin (and any comfort buffer that the designers feel appropriate for whatever reason, including taking into account potential inaccuracy tolerances in the obstacle and terrain data they're using) and round up to the nearest 10' = MDA.
Use of stepdown fixes allow interim descent points in the final segment, obviously.
However, when flying on the VDA (or even dive & drive), once you get to the MDA, that's it: can't continue descent unless RWY in sight and intending to land, and from then on you're in the visual segment and obstacle clearance is up to the pilot.
See also the next bit.
For info: re Obstacle Clearance in the Visual Segment (of Charted APCHs):
Some time back ICAO mandated that all States should assess new and old procedures (when reviewed/amended) for obstacles in the visual segment - ie, between the MDA/DA and the THR (actually, 60m prior usually).
Basically this is done by assessing a pseudo surface at 1.12
° less than the promulgated APCH angle. Safety assessments, mitigations (which could include removal or cutting down of obstacle) &/or amendments to the procedure would be required if an obstacle penetrated the visual segment surface. Safety assessment might also indicate additional mitigations, such as additional marking/lighting of obstacles / terrain in the visual segment - eg, like the hill area 1/2-1 mile prior to the runway - ref
skysign's post #380).
As I posted before the last 1/2 to 1 mile you litteraly buze the hill all the
ways down to the RWY. It is like doing a low flyby over a downhill slope to the
rwy.
It has been several years since I have done that approach, so I can
not really say how high above the terrain you are. But the first night I did
that approach the only think that came to my mind was " Only sh.. that is close
!!! and that is being on the Papi.
In my opinion, on a "normal approach"
the ground appear that close when you are really close to crossing the threshold
and not being 1/2 to 1 mile away from the rwy.
Aterpster can elaborate on current FAA implementations of this ruling if required (in addition to the analysis that they do of 34:1 surfaces).
In any cases, statements regarding obstacles in the visual segment and implications for pilots are contained in the FAA AIM (relevant extracts are in this thread).
Note also previous posts on the 1° clearance for the PAPIs when installed and calibrated as additional mitigation.
Missed Approach:
To those who have discussed the relevant techniques when reaching the MDA whether flying dive & drive (heavies seriously still doing that?) or CDFA (or even DDA = MDA+50 for CANPA technique):
Unless the operator has a specific approval to do otherwise, the MDA is a legal minimum and busting it is a serious no-no.
With regard to this thread though, they'd gone well past the MDA and info provided by NTSB indicates that they were proceeding to land, so further discussion of handling the missed in this case is irrelevant.
ATerpster:
Thx for trying to correct the various misunderstandings of the different types of APCHs with regard to LNAV, LNAV/VNAV, RNAV GNSS APV, LPV, RNAV RNP AR, etc. Saved me some time. Well done on providing all of the other relevant AFM and FAA material for reference too.
Ian W: If, after reading Aterpster's reply and other material from qualified sites, then you could continue the discussion on the TechLog, but not in this thread as those issues are irrelevant here.