Quote BOAC:
Here's a thought for the REAL 737 drivers out there. If, as it appears, we really do have to cater for lack of speed monitoring, why not push for an immediate change to min speed rev to allow it on a coupled GP, down, say to 100' (as I understand AB) or to FLARE active?
Of four airlines I have flown the 73 for (300-800), only one permitted the use of the AT during the approach after disconnecting the AP, and that was with manual inputs by leaving the AT on but deselecting SPEED on the MCP. The idea was that it would continue to give underspeed protection and the automatic GA thrust settings if required, but in the ARM mode would otherwise not interfere with manual thrust setting. The other three airlines forbid that use of the AT, rigidly decreeing that it must be completely off when manually flying. The reason I was given by the first company was an accident along the lines of this one, involving a stall and crash. I don't know any of the details, but it looks like a similar incident (though perhaps nothing to do with the RA) happened before.
As for redesigning the system as per your question, I suppose it's because that altitude would have to be input by the same RA that caused the glitch in the first place, or baro height above the threshold, which is awfully close to Cat III minima if there's any altimeter error (cold corrections too) and is at the Cat II minima. I'd want to see whether the RETARD mode was armed or not before getting so close to the tarmac. Another possible reason - I tried a simulated Cat II autoland at an airport carrying out Cat II trials, and the topography caused a bit of trouble: at 500'RA, we were only 180' above the threshold. Perhaps that could cause the AT to be unable to arm or engage RETARD in your scheme?