Now not knowing the Airbus It would perhaps be better if not only a visual warning appeared but also an audio warning like "Autotrim Fail" or "Use/Revert Manual Trim".
When you go into Direct Law:
- USE MAN PITCH TRIM on PFDs
- ECAM message "F/CTL DIRECT LAW"
- Above message includes "MAN PITCH TRIM" message
- ECAM message will have associated audio alert
However, as with any ECAM message, it is prioritised according to what other messages / alerts are being triggered.
One of the first things I was taught in primary training is, "Don't trim into a stall." Apparently, the AB system will do just that in some circumstances.
An Airbus is slightly different to your primary trainer. Firstly, it is not "trimming" as such, but aligning THS and Elevator to retain full elevator control. There are circumstances you need to fly, and retain control at low speed: landing / GPWS / Windshear, and retain control to manoeuvre and counter rapid trim changes (e.g. full power).
So if the nose is more or less level and thrust has not been increased, why on earth should it think it has started to overspeed in the space of 2 or 3 seconds?
Because in the cases where it has "pitched up",
AFAIK they were
genuine overspeeds. The aircraft does not need to accelerate to go from cruise speed, to significant overspeed.
A body in space cannot change in any direction without some sort of acceleration/deceleration vector as measured by precision accelerometer /gyroscopes
As above, you are misunderstanding basic aerodynamics. Overspeed refers to IAS/M number - nothing to do with KE / Ground Speed.