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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.

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    $\begingroup$ My guess is a structural limitation, I wouldn't imagine supersonic flight is very fun with high pitch angles. $\endgroup$ Sep 3, 2021 at 22:41
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    $\begingroup$ @stevederekson555: Structural limitations would be a reason for a negative-AoA limit, not a negative-pitch-angle limit. $\endgroup$
    – Vikki
    Feb 23, 2023 at 0:16

2 Answers 2

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


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:

Concorde ADI

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

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.

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    $\begingroup$ @ymb1, I know you know but for other persons crossing a French documentation, incidence or alpha (like in Airbus "alpha prot") in French is the angle of attack in English, not the mounting angle. $\endgroup$
    – mins
    Sep 4, 2021 at 13:49
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    $\begingroup$ Alpha is AoA even in English. $\endgroup$
    – Vikki
    Sep 5, 2021 at 17:06
  • $\begingroup$ "In case of airspeed > VMO + 25 kt, the outer elevons are automatically reset to neutral to avoid control reversal" - That's surprising; overspeed-induced control reversal is generally due to control-input-induced wing twist (the control surface acting as a servo tab for the wing), and one would expect the Concorde's long-chord deltas (a much stiffer shape against chordwise twist than a typical subsonic wing shape) to be practically immune to this sort of effect. $\endgroup$
    – Vikki
    Sep 5, 2021 at 17:08
  • $\begingroup$ Are there alarms for high negative alpha as well, or just for high positive alpha? (I know that most airliners don't have alarms for excessive negative AoA - but the Concorde was hardly a typical airliner.) $\endgroup$
    – Vikki
    Sep 5, 2021 at 17:10
  • $\begingroup$ @Vikki: 1/ Control reversal, see page 5 of this document. 2/ Alarms on low AoA: There are not. Alarms on high AoA are particular, they protect the longitudinal stability at low speed, and allow "autostabilization* computers to prevent a stall. $\endgroup$
    – mins
    Sep 5, 2021 at 21:46
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The fixed mark at 5.5 degrees of pitchdown on the Concorde ADI was a reminder for the pilots that, flying at Mach 2, the corresponding vertical speed would be 18000 ft/ mn.

Even flying at 55000 ft, this means only 3 minutes to reach the ground!

During a flight test for certification of the newly installed TCAS, all the flight envelope alarms had to be checked. As we manually tested the 5.5 degrees pitch down audio alarm, my eyes read for a few seconds 18000 ft/mn on the Vertical Speed Indicator.

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