Can't remember who I got the info from but flying a FMS GS on a non precision app below MDA does not provide terrain clearance to the runway. First thing that came to mind when I read the a/p was on at the time of the accident.
Here are a couple of cites from the FAA Airman's Information Manual:
The VDA [vertical descent angle] provides the pilot with information not previously available on nonprecision approaches. It provides a means for the pilot to establish a stabilized descent from the FAF or stepdown fix to the MDA. Stabilized descent is a key factor in the reduction of controlled flight into terrain (CFIT) incidents. However, pilots should be aware that the published angle is for information only - it is strictly advisory in nature. There is no implicit additional obstacle protection below the MDA. Pilots must still respect the published minimum descent altitude (MDA) unless the visual cues stated 14 CFR Section 91.175 are present and they can visually acquire and avoid obstacles once below the MDA. The presence of a VDA does not guarantee obstacle protection in the visual segment and does not change any of the requirements for flying a nonprecision approach.
Visual descent points (VDPs) provide pilots with a reference for the optimal location to begin descent from the MDA, based on the designed vertical descent angle (VDA) for the approach procedure, assuming required visual references are available. Approaches without VDPs have not been assessed for terrain clearance below the MDA, and may not provide a clear vertical path to the runway at the normally expected descent angle. Therefore, pilots must be especially vigilant when descending below the MDA at locations without VDPs. This does not necessarily prevent flying the normal angle; it only means that obstacle clearance in the visual segment could be less and greater care should be exercised in looking for obstacles in the visual segment. Use of visual glide slope indicator (VGSI) systems can aid the pilot in determining if the aircraft is in a position to make the descent from the MDA. However, when the visibility is close to minimums, the VGSI may not be visible at the start descent point for a “normal” glidepath, due to its location down the runway.
https://www.faa.gov/air_traffic/publ...M/aim0504.html
The LOC Rwy 18 at BHM has a published VDA (3.28 degrees) but no VDP so the AIM says 'pilots must be especially vigilant when descending below the MDA'.
According to the NTSB briefing the autopilot was on until one second before the end of the DFDR data. As others have noted, it would normally automatically disconnect at 50 feet below the D-DA of 1250 feet (assuming they could identify IMTOY). Technique I was taught in Toulouse years ago was to manually disconnect the autopilot ahead of time with the runway in sight so you don't get the aural warning (is it the 'cavalry charge'?).
Somehow, the plane thought it was still above 50 feet below the D-DA entered on the approach page of the FMS.
Possibly BHM is on the list of airports with database errors, requiring the egpws to be inhibited...or it was MEL'd but i suspect they descended into terrain at a high rate of descent from above , then descending through and below the desired flight path, and...my 2 cents worth,
Yes, I agree, there are a couple of times when you might inhibit the EGPWS but not when there is an obstacle or because you are visual, I think that guy is either trolling or just making stuff up.
A gross altimeter setting error on descent could sure make the crew think they were high on path. And, the controllers might not see it since at a quiet time of the night the UPS plane might get lower altitude clearances without ever leveling off.
An approach speed of 140 knots (Vref+5) would be about max landing weight for an A300 as PSJ earlier observed. However, for a freighter on a short leg, this would not seem unusual.
Configured, on speed, but thinking they were high on path, maybe chasing a bad profile downward with V/S in a futile attempt to get P.DES to go green?