Why did Concorde have a max pitch down attitude in supersonic flight?

Question

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:

1. Each pilot had clear AOA indicators (attached here)
2. 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.

Answer 1

I'm assembling this answer mostly based on Concorde operating manual, volume 2, in French.

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ADI

The Attitude & Director Indicator is (was) actually a superb instrument, only outdone by the Orbiter one. A lot of indications are crammed into the same window, prefiguring the EFIS to come many years later. Everything is driven by analog electronics, even the artificial horizon indicator which is not a gyro:

^{Source and more information on François S site}

From the manual: At Mach 2, 0.5° of pitch leads to 1,000 ft/min VS change. There is a fixed mark (13) at 5.5° pitch down, this is a limit for overspeed prevention.

Overspeed protection

To prevent overspeed, and structural damages, there is an audio alarm (cricket) if pitch goes below -5.5°. The same alarm is triggered when airspeed exceeds VMO + 6 kt or total temperature exceeds TMO + 7°C. In case of airspeed > VMO + 25 kt, the outer elevons are automatically reset to neutral to avoid control reversal.

There are similar alarms for high angle of attack values, at 16.5° and 19.5°. I'm adding this picture, even if indications are in French, I hope someone can come with the English version:

^{Concorde general limits, source}

Emergency descent

An emergency descent was conducted at the maximum allowed down pitch, -5.5°, until reaching FL500, then at pitch 0.