^ When the Rwy is selected to the top with inbnd course selected (and within 25NM track miles of the RWY waypoint) and executed the a/c FMS will draw a 3 degree glide path to the the runway. This works for ANY coded ILS approach irrespective of ground equipment status and provides continuous omnidirectional pitch guidance to the runway in VNAV from the a/c FMS. Once the a/c intercepts the g/s it will track and follow it if inbound, or will level off if not (or initially below a 3 degree GP).
The videos I've seen animating the accident have shown the accident a/c as going from high on profile and somewhat fast to low and very slow.
The aircraft will descend in HOLD/VNAV SPD until intercepting the glidepath and then transition to SPD/VNAV PTH. This happens irrespective of where the MCP is set (so long as it is initially set below the aircraft altitude and VNAV ALT isn't captured on the way down before setting the MAP alt--i.e. even if a carrier in violation of most SOPs and good operating practice would set MCP at zero or field elevation (instead of 1000 or 1500 AAL) the autothrottles would still engage in SPD mode and the pitch would maintain the glide path once the aircraft intercepted the glide path from above--the same thing will NOT happen in FLCH SPD--although in either mode if the pitch flight director is followed either manually or on autopilot the aircraft won't get slow because the F/D is commanding speed on elevator--in the case of FLCH it would get very low in the case of VNAV it won't because the path will be intercepted).
Is this foolproof ? No. Nothing is foolproof because fools are so damn clever. It won't work if the pilots manually override the autothrottles or if they leave the autothrottle arm switches off--and there are probably other ways one can crash an airplane if one is dedicated toward disregarding all aspects of basic airmanship. But it is a useful tool to provide a margin of safety on a visual approach and works whether or not the ILS is operational--so long as a coded approach for an ILS exists.
In any case, visual approaches and a regulatory agency banning visual approaches has nothing to do with enhancing safety or even this particular accident--if anything it's a training/SOP/CRM issue (to include why wasn't a G/A promptly initiated and called for by the other crew members when the approach became significantly destabilized at low altitude--and may never have been stable to begin with). Additionally, if the airline believes for some reason visual approaches present a unique hazard at a particular airfield or under certain conditions given their training level and equipment it is free to provide training toward this--or tell their pilots not to accept them.
Last edited by Shep69; 11th Aug 2013 at 04:29.