Mechanical Backup and the BCM — Pitch by Trim Wheel, Yaw by Backup Module
At the very bottom of the degradation ladder, with the flight-control computers or aircraft electrics lost, two independent last-resort controls keep the aircraft flyable: pitch via the mechanical THS trim wheel — the only mechanical reversion in the whole system — and yaw via the BCM, an electrical backup module that drives the rudder from the pedals. Because the rudder is fly-by-wire, the directional last resort is electrical, not a mechanical cable. This article covers both, plus the deep-electrical scenario where they appear.
Pitch mechanical control is achieved through the THS, using manual trim control. «MAN PITCH TRIM ONLY» is displayed in red on the PFDs. — FCOM DSC-27-20-20-50
1. Pitch mechanical backup — the THS trim wheel
Per FCOM DSC-27-20-20-50, pitch mechanical control is achieved through the THS, using the manual trim control, with MAN PITCH TRIM ONLY (red) on the PFDs.
[!warning]- The THS trim wheel is the only mechanical control on the aircraft Every surface is fly-by-wire except this one path: when the elevators are lost (or pitch electrical control is gone), pitch is flown purely by the mechanical THS trim wheel (FCOM DSC-27-20-20-50). The red MAN PITCH TRIM ONLY distinguishes it from the amber USE MAN PITCH TRIM of direct law: amber = elevators still work, just no auto-trim; red = elevators gone, the wheel is all you have in pitch. Pitch is then managed with trim-wheel + thrust changes and the aircraft's natural pitch/phugoid response — a coarse, slow control that demands smooth, anticipated inputs.
2. Yaw backup — the BCM (electrical, self-powered)
Per FCOM DSC-27-10-10, the BCM (Backup Control Module) provides backup yaw damping and direct rudder command from the pedals, via an independent unit, in case of total electrical failure or loss of rudder control due to a PRIM and SEC failure. It includes:
- its own generator — the Backup Power Supply (BPS) — driven by the B or Y hydraulic system;
- its own sensors (gyrometers and pedal-deflection);
- control of the B and Y hydraulic rudder actuators.
When active (as in yaw alternate law) there is no turn coordination.
[!warning]- The BCM replaces the mechanical rudder of older aircraft (integrative synthesis) Because there is no mechanical run from pedals to rudder, the BCM is what flies the rudder when the computers and electrics are gone. It is fully self-contained: it makes its own electrical power (BPS) from hydraulics and reads its own gyros and pedals (FCOM DSC-27-10-10) — so even a total aircraft electrical failure leaves the pedals able to move the rudder (24). This is the directional equivalent of the THS trim wheel, but achieved electrically, and is why "lateral mechanical backup" is the wrong frame for this aircraft.
3. The deep-electrical scenario — where the backups actually appear
Per FCTM PR-AEP-ELEC, the backups become relevant in the electrical emergency configuration. With the EMER GEN powered by the RAT (the deepest case), "the AP, pitch trim and rudder trim are not available. The aircraft will be out of trim in roll due to right outboard aileron up." The flight controls are in alternate law, reverting to direct law when the landing gear is extended (LAND RECOVERY ON).
[!note]- Mechanical/BCM backup is a deep-electrical endpoint, not just a computer failure (integrative synthesis) The mechanical THS and the BCM are the last rung reached by a cascade of computer or electrical losses. On RAT-only power you already lose AP, pitch trim and rudder trim, and fly out of trim in roll (FCTM PR-AEP-ELEC) — and a further loss of the flight-control computers themselves or all aircraft electrics is what brings in the THS wheel (pitch) and the BCM (yaw). So this level is reached gradually: alternate → direct → mechanical/BCM, each step shedding more (reconfiguration chain).
4. Putting the last resort together
With computers/electrics lost, the crew has:
| Axis | Last-resort control | Type |
|---|---|---|
| Pitch | THS manual trim wheel | Mechanical |
| Yaw | BCM (rudder from pedals) | Electrical (BPS-powered) |
| Roll | — (no dedicated mechanical/backup reversion) | uses rudder/yaw + bank coupling |
[!note]- It is a survival mode to buy time, not a way to fly the trip (integrative synthesis) The purpose of this level is to keep the aircraft controllable in pitch and yaw while the crew works to recover a flight-control computer or electrical power (which restores at least direct law). Roll has no separate reversion, so heading is managed through the rudder (BCM) and the resulting bank (and any roll authority still electrically available). As soon as a computer/electrical source returns, the aircraft climbs back up the reconfiguration ladder — typically into direct or alternate law. Flying technique is coarse and smooth, anticipated — the THS wheel is slow and there is no auto-trim.
5. Counterintuitive points
[!warning]- Two last-resort controls, two different technologies Pitch backup is mechanical (THS wheel); yaw backup is electrical (BCM with its own BPS) (FCOM DSC-27-20-20-50 / DSC-27-10-10) — there is no mechanical rudder.
[!warning]- Red MAN PITCH TRIM ONLY ≠ amber USE MAN PITCH TRIM Red = elevators lost, THS wheel only; amber = no auto-trim but elevators still work (FCOM DSC-27-20-20-50 / -30).
[!warning]- On RAT-only power you are already out of trim in roll AP, pitch trim and rudder trim are unavailable, right outboard aileron up (FCTM PR-AEP-ELEC) — before even reaching the mechanical/BCM endpoint.
6. Self-test
[!note]- Q1. What is the only mechanical control on the aircraft, and its PFD cue? The THS manual pitch trim wheel; MAN PITCH TRIM ONLY (red) on the PFD.
[!note]- Q2. How is the rudder flown when the computers/electrics are lost? By the BCM — direct rudder from the pedals, electrically, powered by its own BPS (B/Y hydraulic-driven), with its own gyros/pedal sensors; no turn coordination.
[!note]- Q3. When does the BCM activate? On total electrical failure or loss of rudder control due to PRIM + SEC failure.
[!note]- Q4. In the RAT-only electrical emergency configuration, what is lost and how does it fly? AP, pitch trim, rudder trim unavailable, out of trim in roll (right outboard aileron up); alternate law, direct on gear extension.
7. Key takeaways
| Point | Detail |
|---|---|
| Pitch backup | THS manual trim wheel (only mechanical control); MAN PITCH TRIM ONLY red; slow/coarse, smooth inputs |
| Yaw backup | BCM: direct rudder from pedals, electrical, BPS from B/Y hydraulics, own gyros/pedals; no turn coordination |
| BCM triggers | total electrical failure, or rudder-control loss from PRIM+SEC failure |
| Roll | no dedicated reversion — manage via rudder/bank |
| Scenario | deep electrical loss (RAT-only: no AP/pitch/rudder trim, out of trim in roll) → mechanical/BCM endpoint |
| Purpose | survival mode to recover a computer / electrical power |
References
- FCOM DSC-27-20-20-50 (Reconfiguration Control Laws — Mechanical Back Up) — pitch mechanical control via the THS using manual trim control; MAN PITCH TRIM ONLY displayed red on the PFDs.
- FCOM DSC-27-10-10 (Flight Controls — General) — BCM provides backup yaw damping and direct rudder command from the pedals via an independent unit, for total electrical failure or loss of rudder control due to PRIM+SEC failure; own generator (BPS) supplied by B or Y hydraulic system, own gyrometers and pedal-deflection sensors, control of B and Y hydraulic actuators; when active (yaw alternate law) no turn coordination.
- FCTM PR-AEP-ELEC (Electrical Emergency Configuration) — RAT-only EMER GEN: AP, pitch trim, rudder trim not available, out of trim in roll (right outboard aileron up); alternate law, direct law on landing-gear extension (LAND RECOVERY ON).
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.