David,
You are right, however there are some DME's that do transmit a bias value. From Robin's data format specification:
DME bias in nautical miles – often used for DMEs associated with an ILS that are not located near the runway threshold. This bias must be subtracted from the calculated distance to the DME to give the desired cockpit reading. This is typically used for a DME located in the centre of an airport that serves multiple l ILS approaches, so that the DME will read zero at the threshold of a runway. This is a common set up in Europe.
All the approaches at KLAX have a 1 or 2 nm bias for the DME. Like you say, there is no *requirement* for this, but there are installations that have a bias set up.
If no one knows any better, then for now I will clamp the DME display to 0.0 when we fly inside the bias range so it never goes negative.
Regards,
Curt.
David Megginson wrote:
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,
David
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Curtis Olson http://www.flightgear.org/~curt HumanFIRST Program http://www.humanfirst.umn.edu/
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