Hi rudderrudderrat,
Thanks for that very interesting link, and I note Owain Glyndwr's quick response that reversion to Pitch Alternate may not have been necessary during the certification flights. Experience suggests he is usually right, in which case I infer that Normal Law and Alpha-Protection would have been retained, but alpha-max modified to a figure at or above Cz-max. Alpha-Floor would also be inhibited. If so, this would have the advantage that THS (pitch) up-trimming would cease at alpha-prot. That would make the exercise much safer than using Pitch Alternate, in which - as you have pointed out - the THS would continue trimming to the stall and beyond (as it did later on AF447).
I am told that there have traditionally been three alternative scenarios for establishing figures of Vs during the certification of aeroplanes. In each case the a/c is flown straight and level (Nz = 1.0g) for as long as possible with a deceleration of 1 kt/sec. The following are very rough, layman's descriptions for OG, HN39 and others to correct/amend/expand:
(1) If full back stick/column is reached before the stall, Vs is defined as the lowest speed achieved. (Normally, that definition applies only if elevator authority is insufficient to achieve a classic stall. However, it would have applied to the A320 in the Alpha-Protection mode of Normal Law for artificial reasons.)
(2) The a/c is maintained at 1g until the stall is denoted by a marked "break" (nose-drop), at which point the speed is nominated as the Vs. (This was the traditional British method for BCAR certification.)
(3) As for (2), but the a/c was allowed to execute a mild bunt, probably starting momentarily before the 1-g break, with continuing deceleration, until the Nz fell to a certain level. At that point the resulting speed was nominated as the Vs, sometimes described as the "minimum speed in the stall", provided it was not more than a certain percentage below the Vs1g. (This was the traditional American method for FAR certification.)
The FAR method (3), with its lower values of Vs, gave American a/c a commercial advantage over British a/c when field performance was being considered, because the different regulators applied the same factors to Vs for the calculations of V2 (1.2 Vs) and Vref (1.3 Vs). I always wondered why - on a calm day when the numbers were right - one could confidently close the throttles at 50 ft on a VC10 or BAC1-11, but wouldn't even consider it on a B707.
Sometime in the 1980s, there is said to have been a levelling of the playing field by agreement, but I notice that my stall graphs for the A310 - the last Airbus type to be certificated before the A320 - are headed "VS MINIMUM". So did the DGAC use FAR criteria for the A310 certification?
So to the A320. The version of the cryptic, ubiquitous curve of CL-versus-alpha that you
posted 4 days ago is annotated "1G stall" over the apogee associated with CL max (unlike the versions of Cz-versus-alpha in the early FCOMs, which omit it). Although John Farley pointed out (on seeing it earlier) that the caption was redundant and potentially misleading, AI was presumably trying to send a message that their Vs is taken (a) at CLmax and (b) at an Nz of 1G, not a lower figure.