Gonzo wrote:
Bobcat, although I don't know if it is the case at SFO, usually when a DME is associated with a runway, it is 'zero-ranged' to read 0DME at the threshold, even though it is not sited there.
Yes, that’s also known as TDME (Terminal DME) and is co-located with the ILS. I know how DME works, and that it is often (always?) related to VOR or ILS.
Asiana 214 was cleared on a visual (TIPP TOE) to 28L. ILS (GS) was out of service. I believe the localiser was operational.
As far as I know, SFO VOR/DME (115.8) is not adjusted to threshold and being located almost exactly on the midpoint between 28L and 10R, approximately 5000 feet from both thresholds, would add a little less than one mile to DME compared to TDME.
Even though “180 to 5” means 5 miles to threshold, and when tuned to TDME would give the correct distance, it is possible that Asiana pilots had DME to SFO VOR in the display, right?
Of course, on a visual approach one would see the San Mateo Bridge (otherwise it wouldn’t be a visual).
Edit: My point is when “180 to 5” is ok “with no time to spare”, 180 to 4 would be a problem on a heavy jet. They would have to get rid of 40 knots, configured for landing and completed the landing checklist in one mile (less than 30 seconds) if required to be stable at 1000 feet. Sounds like a lot of work in a short time. But again, it’s just a matter of going around and try again.