THS - Trimmable Horizontal Stabiliser
In Normal and Alternate Law the autotrim moves the THS to keep the a/c in trim longitudinally. This ensures elevator authority in both directions.
In Direct Law the autotrim does nothing - the pilot moves the THS using the trim wheel.
In Abnormal Attitude Law (>50° NU pitch or <60 KT CAS in this accident) the pitch law is Alternate but without autotrim. Once the pitch is <50° and CAS above 60kt the autotrim should function again.
However at 15:45:57 the pitch attitude exceeds 30° nose DOWN and the Abnormal Attitude Law again applies - so no autotrim.
Thus from Direct Law (15:45:15) to approx 15:45:53 the trim wheel is the only way to move the THS, then there was about 3 or 4 seconds with autotrim enabled then back to the trim wheel only.
When the a/c goes into Direct law an ECAM warning informs the crew, and on both PFD's the message "USE MAN PITCH TRIM" appears in amber.
TP