In ITVV Concorde (documentary) as the flight's captain introduced the ADI (attitude director indicator) he explained the dotted line (circled above) as follows:
[...] and the little orange dotted line at the bottom there represents the maximum pitch down attitude in supersonic flight.
That marking is not in the copy of Concorde's flight manual I have (old copy from 1979), and the limitations section of that manual makes no mention of max pitch down attitude in supersonic flight, so apparently the limitation was added later on (something happened / was discovered).
I would have thought such limitation would have been based on an angle of attack (AOA), e.g. a certain negative AOA where the ogival wing would blanket the engine inlets:
- Each pilot had clear AOA indicators (attached here)
- There isn't a similar max pitch up marking on the ADI.
I've scoured the internet for that limitation but I couldn't find anything.