In response to APMR's comments there is no major technical reason why a GPS box couldn't give distance to MAPt. However, unfortunately these issues go back to the start of the product development cycle and require changes to international and industry standards to resolve. Mind you, that is not to say that making the changes would not be a worthwhile endeavour.
Many of the problems go back to DO-208 the industry standard for aviation GPS receivers. It is an engineers paradise, contains 214 pages of acronyms, test procedures and equations of which 1.25 pages are dedicated to 'operational characteristics' - in other words how the avionics should interface to the pilot.
Neither the FAA/EASA Technical Standard Order (TSO-C129) or the airworthiness requirements (e.g. FAA AC-20-138) required a GPS box to provide anything other than distance to the next waypoint to the pilot. Hence, when the manufacturers have produced their equipment they have complied with the standard. Also any assumptions made by IAP designers are based upon the functional performance outlined in the technical standards which are then reflected in PANS-OPS.
The problem is that even if we go back to sort out the equipment standards there will still be kit around for a long time that only displays distance to next waypoint. So what do you PPruners think we should do to make the best of the situation? In the Australian research mentioned above the specific problems related to workload/confusion seem to be:
1. No flight deck presentation of distance to MAPt (from the GPS),
2. Too many waypoints on the approach too close together with inconsistent leg lengths,
3. Presentation/interpretation of the charts themselves.
Given that (without investment in additional airfield infrastructure - e.g. a DME) the first may take a while to resolve how best should we address the latter 2 points in the IAP design? Also in the opinion of the PPruners would solving 2 & 3 be enough to provide requisite safety levels?
Apologies for the waffling post, but this is something of real interest right now.
Cheers, NM.
PS. As an aside I have sat through and participated in the international standardisation of these units for a number of years and its only recently that I have heard any mention of this distance to waypoint issue being a problem. There was flight crew participation in the standardisation process (albeit limited). I'm not saying that it isn't an issue - because it clearly is, I'm just wondering why no one spotted it before and could it be related specifically to the Australian IAP design?