@RWA
THS overview...
Pitch control is through 2 independent elevators and a trimmable hydraulic stabiliser (THS). These surfaces are normally driven by PRIM 1 by the green hydraulic jacks and THS motor 1. If all 3 PRIMs are lost, electrical control of the THS is lost; actuation is available through the manual pitch trim wheel control. 2 electrically controlled hydraulic servo jacks are fitted to each elevator having 3 modes: active in which the jack is positioned is electrically controlled, damping where the jack follows surface movement and centring where the jack is hydraulically maintained neutral. In normal operation, one jack is active, one is damping. If both servo-jacks fail, they default to the centring mode. Some manoeuvres cause the second jack to become active. The stabiliser is actuated by a screw jack driven by 2 hydraulic motors controlled by 1 of 3 electric motors or the mechanical trim wheel. The right elevator uses GRN & YLO hyd, left uses BLU & GRN. The THS uses BLU & YLO.
If a failure occurs in PRIM 1, assoc hyd system or hyd jacks, control is transferred to PRIM 2 for the elevator via the BLU and YLO jacks and the THS via motor 2. If both PRIM 1 & 2 are inop, control is transferred to SEC 1 for the elevator and PRIM 3, motor 3 for the THS. If all 3 PRIMs are inop, the elevator is controlled by SEC 1 and electrical control of the THS is lost.
See below BEA report quotes with my own bold and underscore for emphasis.
It has been stated by posters that in ALT LAW under some conditions the auto trim feature on THS can be inhibited and pilot should utilize manual trim as required.
At 2 h 10 min 51 , the stall warning was triggered again. The thrust levers were positioned
in the TO/GA detent and the PF maintained nose-up inputs. The recorded angle of attack, of
around 6 degrees at the triggering of the stall warning, continued to increase. The trimmable
horizontal stabilizer (THS) passed from 3 to 13 degrees nose-up in about 1 minute and
remained in the latter position until the end of the flight.
After the autopilot disengagement:
- the airplane climbed to 38,000 ft,
- the stall warning was triggered and the airplane stalled,
- the inputs made by the PF were mainly nose-up,
- the descent lasted 3 min 30, during which the airplane remained stalled. The angle of
- attack increased and remained above 35 degrees,
- the engines were operating and always responded to crew commands.