THS — the Ball-Screw Trim Surface, Electrical Auto-Trim and Mechanical Override
The Trimmable Horizontal Stabilizer carries the elevators on its trailing edge and provides the powerful, sustained pitch trim that the auto-trim continuously sets. It is moved by a ball screw and two hydraulic motors, controlled electrically (auto-trim) or mechanically (the trim wheels) — and turning a wheel cancels the electronic command. This is the surface behind both auto-trim and the MAN PITCH TRIM reversion.
The THS is actuated by a ball screw, powered by two hydraulic motors or held by brakes... The hydraulic motor servoloops are controlled either electrically or mechanically. The electrical control achieves the autotrim function of the pitch normal and alternate laws. The mechanical control achieves the pre-takeoff trim or normal trim of the pitch direct law. — AMM 27-40-00
1. What the THS is
Per AMM 27-40-00, the THS is attached to the rear fuselage and moves about an axis for pitch trim, with the two elevators on its trailing edge. It is moved by a ball screw powered by two hydraulic motors (or held by brakes) — a fail-safe ball-screw assembly. Control is both electrical (FCPC) and mechanical:
- electrical control = the auto-trim of pitch normal and alternate laws;
- mechanical control = pre-takeoff trim and the normal trim of pitch direct law (and the standby).
[!note]- THS = slow/strong trim; elevators = fast/limited control (integrative synthesis) The pitch system is a division of labour: the elevators give the quick response but limited authority, while the THS provides large, sustained trim. Auto-trim constantly drives the THS to offload the elevators toward neutral. The ball screw held by brakes is irreversible — it holds its position with no power and cannot be back-driven by aerodynamic load, which is why the THS doesn't run away on a failure.
2. The PTA — three motors, electrical control
Per AMM 27-44-00, the THS actuator is an electrohydraulic unit: hydraulic pressure drives a gear system that turns the ball screw-jack on the THS, controlled by the Pitch Trim Actuator (PTA). The PTA has three brushless DC motors, each with its own controller — a Digital Electronic Module (DEM).
[!note]- The three PTA motors are the electrical redundancy (integrative synthesis) The PTA's three motors map to the computer chain: PRIM 1 drives THS motor N°1, PRIM 2 → motor N°2, PRIM 3 → motor N°3. So electrical THS control survives two PRIM failures; only with all PRIMs lost is electrical THS gone and the THS reverts to the mechanical wheel (01, 19). Each DEM independently controls its motor.
3. Mechanical control — the wheels that override
Per AMM 27-41-00, the mechanical control is a standby system: two control wheels (trim wheels) in the centre pedestal, each with a graduated scale, drive (via toothed belts, a sprocket, chains and cables through a tension regulator) the mechanical input shaft of the THS actuator. It is used on the ground when electrical control is not possible, and in flight as standby when automatic control is unavailable.
An override mechanism makes sure that the mechanical pitch trim commands cancel the electronic pitch trim commands.
[!warning]- Turning the trim wheel cancels the electronic (auto-)trim The override mechanism means a mechanical wheel input overrides and cancels the electronic command (AMM 27-40-00). This is exactly how MAN PITCH TRIM works — the wheel drives the actuator's mechanical input shaft directly, regardless of the computers. It is also why an inadvertent wheel input in normal flight will fight the auto-trim: the wheel always wins. The graduated scale shows the trim setting (e.g. the green-band takeoff value, 05).
4. Position indication
Per AMM 27-40-00/45-00, the THS position is indicated mechanically (the wheel scales) and electronically. Two transducer packs: the command pack monitors the PTA output for the DEMs, and the monitor pack sends THS position to the FCPCs → FCDC → ECAM, where it appears as the pitch-trim indication.
5. Counterintuitive points
[!warning]- The ball screw is irreversible — the THS holds, it doesn't run away Powered by two hydraulic motors or held by brakes, fail-safe (AMM 27-40-00) — it stays put with no power.
[!warning]- The trim wheel always overrides the electronic trim Mechanical commands cancel electronic ones (AMM 27-40-00) — the wheel wins over auto-trim.
Self-test
[!note]- Q1. How is the THS moved, and what makes it fail-safe? A ball screw powered by two hydraulic motors (or held by brakes) — irreversible/fail-safe, holds position with no power.
[!note]- Q2. Electrical vs mechanical control of the THS? Electrical = auto-trim (normal/alternate laws), via the PTA's three DC motors (PRIM 1/2/3). Mechanical = trim wheels for direct-law/pre-takeoff trim and standby.
[!note]- Q3. What is the PTA and how many motors? The Pitch Trim Actuator — three brushless DC motors, each with its own controller (DEM).
[!note]- Q4. What does the override mechanism do? Mechanical (wheel) commands cancel the electronic pitch-trim commands — the basis of MAN PITCH TRIM.
Key takeaways
| Point | Detail |
|---|---|
| What | rear-fuselage trim stabilizer carrying the elevators; ball screw + two hydraulic motors (or brakes), fail-safe/irreversible |
| Electrical | auto-trim (normal/alternate) via PTA's 3 brushless DC motors + DEMs (PRIM 1/2/3) |
| Mechanical | trim wheels (pedestal, graduated scales) → actuator input shaft; direct-law/pre-takeoff/standby |
| Override | mechanical commands cancel electronic — wheel wins (MAN PITCH TRIM) |
| Position | command + monitor transducers; monitor → FCPC → FCDC → ECAM |
References
- AMM 27-40-00 (THS — Description and Operation) — THS on rear fuselage, moves about an axis for pitch trim, carries the two elevators; ball screw powered by two hydraulic motors or held by brakes, fail-safe; controlled electrically (FCPC) and mechanically; electrical control = auto-trim of pitch normal/alternate laws, mechanical control = pre-takeoff trim / normal trim of pitch direct law / standby; override mechanism — mechanical commands cancel electronic commands; PTA with three brushless DC motors each with a DEM controller; position indicated mechanically + electronically (command + monitor transducer packs, monitor → FCPC → FCDC → ECAM).
- AMM 27-41-00 (THS Mechanical Control) — standby system, two control wheels in centre pedestal with graduated scales, toothed belts/sprocket/chains/cables/tension regulator to the actuator mechanical input shaft; ground trim when electrical not possible, in-flight standby when automatic control unavailable.
- AMM 27-44-00 (THS Hydraulic Actuation) — electrohydraulic THS actuator, hydraulic pressure → gear system → ball screw-jack; controlled by PTA (three brushless DC motors + DEM controllers).
Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.