David Culp wrote:
Recently I added support for adjusting the DME readout based on an optional per transmitter bias that is part of Robin's nav data. For ILS approaches the DME transmitter could be at the opposite end of the runway, but transmitting a bias or "constant value" to subtract off the real value so it reads 0.00 at the touch down point.
I've never seen a negative DME, and I don't think it's possible to see one.
As far as I know (I'll confirm today at work) there is no requirement that the DME read zero at touchdown.
I've never heard of "DME bias" either -- that's not to say that it does not exist, but it's certainly not common.
As David Culp mentioned, there is no requirement at all for DME to read '0' at touchdown -- in fact, there's no requirement for DME at touchdown, period, because you have to have visual contact with the runway at that point. Typically, you will use DME to identify a step-down fix or the missed approach point in a non-precision approach, or to identify a crossing fix (and check the glidescope height) in a precision approach. DME can also be used to enter an approach using a DME arc. In none of these would a pilot expect the DME to read 0 or any other particular number.
To take a typical example, I have the LOC/DME 12 approach for Oshawa (CYOO) open on my desk because I was about to do a practice run through it. The approach uses three DME fixes:
1. The initial fix (IF) for a straight-in approach is at 15 DME.
2. A step-down fix from 2100 ft to 960 ft (minimum descent altitude, or MDA) is at AVRUL, which occurs at 5 DME.
3. The missed approach point (MAP) is over the runway threshold, which occurs at 0.8 DME.
All the best,
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