Here we go again...
Pardon me for not taking any side. In one post I agree with you, in the next I go like this:
MU3001A wrote:
HarryMann Yes, a one way check valve. IF (x=27', RETARD, otherwise don't)
Well, first you have to be 100% sure that the output sample rate is quick enough so it's not jumping from 28' to 26'. And clear every runways outer edges for 2' high objects. No fences more than 2 feet high. Theoretically RA could bounce from a fence when at 27' and in the next reading measure 26'. The result would be no RETARD and the problems that would imply. Although I have to admit this would be a rather low-angle approach, if you're at flare height right above the airport fence.
Now, there is also a question about accuracy. Is the RA really accurate to one feet?
Another spooky thing about this design is that a RA could output a faulty 27' reading at 2000' just like the -8' in this accident. We are not sure what cased the -8' reading, are we? Could as well have been 27' with the same outcome.
As you cleverly put it:
Well, it's debatable whether the under-speed/over-speed protection available is worth the risk [...]
Debatable. Yes indeed! That what it is! And that's what we're doing.