High Lift — Architecture, Configurations and the SFCC/PCU System

The slat/flap system is a transmission-driven architecture: two SFCCs command a central PCU whose two hydraulic motors turn torque shafts out to the surfaces, with wing-tip brakes to stop a runaway. This article covers the components, the five-position lever and its configurations, and the half-speed degradations. The protections (ARS, FLRS, alpha/speed lock) and the WTB faults are in articles 28–30.

The slat and flap system includes... Seven slats, two flap surfaces and two ailerons (aileron droop function) per wing... Two SFCCs, each containing one slat channel and one flap channel. A PCU which is made of two independent hydraulic motors coupled by a differential gearbox. — FCOM DSC-27-30-10

1. The components

Per FCOM DSC-27-30-10, per wing there are seven slats, two flap surfaces, and two ailerons (aileron droop) — electrically controlled, hydraulically operated. The system architecture:

• two SFCCs (Slat Flap Control Computers), each with one slat channel and one flap channel;
• a PCU — two independent hydraulic motors coupled by a differential gearbox: green + blue for slats, yellow + green for flaps; a POB (Power-Off Brake) locks the transmission when the surfaces reach position or hydraulic power fails;
• two APPUs measuring left/right asymmetry;
• a flap disconnect detection system (excessive inner/outer flap differential movement → inhibits flap operation);
• WTBs (wing-tip brakes) that activate on runaway, asymmetry or overspeed — cannot be released in flight; blue + green for slats, green + yellow for flaps;
• FPPUs feeding position back to the SFCCs, and an IPPU sending position to the ECAM.

[!note]- It is one transmission per surface group, not independent actuators (integrative synthesis) Unlike the primary surfaces (each with its own servo), the slats and flaps are driven by a central PCU through torque shafts to all surfaces at once — so they always move symmetrically by design. The dangers of a shafted system are runaway and asymmetry, which is exactly what the APPUs, FPPUs and WTBs guard against (30). The POB holds the surfaces in place once stopped, so they don't creep under air load.

2. The five-position lever and configurations

Per FCOM DSC-27-30-20, the FLAPS lever has five positions commanding slats + flaps + aileron droop together:

[!warning]- Lever "1" gives CONF 1 or 1+F — slats move first, flaps follow Lever position 1 produces either CONF 1 (slats 16°, no flaps — "HOLD") or CONF 1+F (slats 16°, flaps 8°, droop 5° — takeoff), selected automatically by speed via the slat alpha/speed lock. The 1* and 2* configurations are reachable only by FLRS auto-retraction, not by the lever. Balks at positions 1 and 3 prevent a single action demanding excessive travel, and intermediate lever positions cannot be selected — the lever must be pulled out of detent before moving.

3. Degradations and interfaces

Per FCOM DSC-27-30-10:

• one SFCC inoperative → both slats and flaps run at half speed;
• one hydraulic system inoperative → the corresponding surfaces (slats or flaps) run at half speed;
• a flap WTB on still allows slat operation, and a slat WTB on still allows flap operation.

The SFCCs transmit flap/slat position to PRIM, SEC, FMGEC, ADIRU, EIU, CIDS and GPWS; the ECAM gets position directly from the IPPU for warnings and the E/WD indication.

[!note]- Slats and flaps degrade independently (integrative synthesis) Because the PCU uses different hydraulic pairs (slats green+blue, flaps yellow+green), losing one hydraulic system slows only the affected group to half speed (FCOM DSC-27-30-10) — slats and flaps are independent for hydraulics and for WTBs. Losing one SFCC slows both to half speed (the computers are shared). This is why high-lift failures are read as "slats vs flaps, half speed vs locked."

4. Counterintuitive points

[!warning]- One central PCU drives all the surfaces — symmetry is built in Two hydraulic motors + differential gearbox + torque shafts; the threats are runaway/asymmetry, guarded by APPU/FPPU/WTB (FCOM DSC-27-30-10).

[!warning]- Lever 1 is two configs, and 1*/2* are FLRS-only Slats-first (CONF 1 vs 1+F) by speed; CONF 1*/2* reachable only by FLRS (FCOM DSC-27-30-20).

Self-test

[!note]- Q1. The high-lift surfaces per wing, and what drives them? Seven slats, two flaps, two ailerons (droop); driven by a central PCU (two hydraulic motors + differential gearbox) through torque shafts; two SFCCs command it.

[!note]- Q2. Which hydraulics power slats vs flaps, in the PCU and the WTBs? PCU: slats green+blue, flaps yellow+green. WTBs: slats blue+green, flaps green+yellow.

[!note]- Q3. The two half-speed degradations? One SFCC inoperative → both half speed; one hydraulic system inoperative → the affected group (slats or flaps) half speed.

[!note]- Q4. What is special about lever position 1, and CONF 1*/2*? Lever 1 → CONF 1 or 1+F by speed (slats first); CONF 1*/2* are reachable only by FLRS.

Key takeaways

References

• FCOM DSC-27-30-10 (Flaps and Slats — Description) — seven slats, two flaps, two ailerons (droop) per wing, electrically controlled/hydraulically operated; two SFCCs (slat + flap channel each); PCU (two independent hydraulic motors coupled by differential gearbox, slats green+blue / flaps yellow+green, POB locks transmission at position or on hydraulic loss); two APPUs (asymmetry); flap disconnect detection (inner/outer differential movement); WTBs (runaway/asymmetry/overspeed, not releasable in flight, slats blue+green / flaps green+yellow); FPPUs (feedback to SFCC), IPPU (to ECAM); one SFCC inop → both half speed, one hydraulic inop → affected group half speed; flap WTB on still allows slats and vice versa; SFCC outputs to PRIM/SEC/FMGEC/ADIRU/EIU/CIDS/GPWS.
• FCOM DSC-27-30-20 (Flaps and Slats — Controls and Indicators) — FLAPS lever five positions (0/1/2/3/FULL) → CONF 0 (0/0/0), 1 (16/0/0), 1+F (16/8/5), 1* (20/8/10), 2 (20/14/10), 2* (23/14/10), 3 (23/22/10), FULL (23/32/10); balks at positions 1 and 3, no intermediate positions, lever out of detent.

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.