Flaps — Two per Wing, the Differential-Gearbox PCU and Half-Speed/Full-Torque

Each wing has two single-element flaps (inboard and outboard) on tracks, driven by a central PCU through torque shafts to five drive stations. The PCU's two hydraulic motors join at a differential gearbox — so losing one motor gives half speed at full torque, not a stoppage. This article covers the flap-specific mechanism; the slats are in 27, the protections in 28–30.

Two single-element flaps are installed on tracks on the trailing edge of each wing... A hydraulic Power Control Unit (PCU) supplies the power... A mechanical transmission system transmits the power to the rotary actuators, which move the flaps on the tracks. — AMM 27-50-00

1. The surfaces and the drive chain

Per AMM 27-50-00 / 27-54-00, each wing has an inboard flap (on two tracks) and an outboard flap (on three tracks), single-element, hydro-mechanically operated. The drive chain:

PCU (fuselage) → torque shafts → 5 drive stations per wing → gearboxes + rotary actuators → carriages → flaps.

The PCU has two hydraulic motors, each with a POB (Pressure-Off Brake) and an electrically-controlled valve block, supplied by the Yellow and Green hydraulic systems; their output shafts join at a differential gearbox. Torque limiters in the drive-station input gearboxes prevent excess torque reaching the actuators and structure.

[!warning]- Lose one motor and you get half speed at full torque — not a stop The differential gearbox is the key: "If a POB engages and holds its motor, the remaining motor moves the transmission at half speed but at full torque" (AMM 27-54-00). So a single hydraulic/motor loss slows the flaps to half speed (25) but keeps full torque — they still reach position, just slower. The POB holds a stopped motor's shaft so the running motor can drive against it through the differential.

2. Electrical control — CSU, SFCC, valve blocks, two speeds

Per AMM 27-51-00, the slat/flap lever → CSU (Command Sensor Unit) converts the mechanical lever command to electrical signals for SFCC1 and SFCC2. Each SFCC's flap channel controls one of the two valve blocks on the PCU (each valve block on its own hydraulic supply). Each valve block has four solenoid valves + a pressure switch:

• low-speed mode: the retract or extend solenoid is energised;
• high-speed mode: the extend (or retract) plus the high-speed solenoids are energised;
• the POB solenoid releases the POB to allow movement.

[!note]- The flaps have a high- and low-speed mode — separate from the half-speed degradation (integrative synthesis) Don't confuse two things: the high/low-speed mode is a normal transmission-speed selection (more solenoids = faster), used to optimise flap travel time; the half-speed degradation is a failure result of running on one motor through the differential gearbox (AMM 27-51-00 / 27-54-00). One is by design, the other is a fallback — both can be in play.

3. POB, WTB and disconnect protection

Per AMM 27-54-00, each POB stops and holds its motor's output shaft when the valve-block solenoids de-energise (normal shutdown), hydraulic pressure is insufficient, or a defined failure occurs — so the flaps hold position rather than creep. Near the end of each wing's transmission, a WTB (wing-tip brake) — electrohydraulic, Green + Yellow supplied, one solenoid per SFCC — stops and locks the transmission on certain failures (30). The flap system also has disconnect proximity switches on the interconnecting struts (detecting inner/outer flap separation) and track-4 sensors.

[!note]- POB holds, WTB locks — two different "stops" (integrative synthesis) The POB is the normal holding brake at each motor (engages on shutdown/low pressure); the WTB is the protective brake out at the wing tip that locks the whole transmission on a runaway/asymmetry (30, AMM 27-54-00). A POB-held flap can still be driven by the other motor; a WTB-locked transmission cannot move until reset.

4. Monitoring

Per AMM 27-51-00, the two SFCCs continuously monitor the transmission, LRUs, signals and power supplies. Flap failures give no level-3 warnings — only level-2 cautions on the EIS (via SDAC/FWC/CMC/DMC); flap position appears on the EWD (from the IPPU) with the slat position and movement direction.

[!warning]- Flap faults are cautions, not warnings The flap system raises level-2 cautions, never a level-3 (red) warning (AMM 27-51-00) — a flap problem degrades a capability but is not an immediate-action emergency. Read it on the EWD alongside the slat indication.

5. Counterintuitive points

[!warning]- One motor lost = half speed but full torque The differential gearbox lets the remaining motor drive at half speed/full torque against the POB-held one (AMM 27-54-00).

[!warning]- High/low-speed mode ≠ the half-speed failure case High/low speed is a normal solenoid selection; half speed is the one-motor fallback (AMM 27-51-00).

Self-test

[!note]- Q1. How many flaps per wing, on how many tracks, and the drive chain? Two single-element flaps — inboard on 2 tracks, outboard on 3 — driven PCU → torque shafts → 5 drive stations → rotary actuators → carriages.

[!note]- Q2. What happens if one PCU motor is held by its POB? The other motor drives the transmission at half speed but full torque through the differential gearbox.

[!note]- Q3. Which hydraulics power the flap PCU and the WTB? PCU: Yellow + Green; WTB: Green + Yellow (one solenoid per SFCC).

[!note]- Q4. What warning level do flap failures reach? Level-2 cautions only — never a level-3 warning; shown on the EWD.

Key takeaways

References

• AMM 27-50-00 (Flaps — Description and Operation) — two single-element flaps per wing (inboard on two tracks, outboard on three), hydro-mechanical; PCU + mechanical transmission → rotary actuators on tracks; two SFCCs monitor/control; IPPU → EIS position.
• AMM 27-51-00 (Flaps Electrical Control and Monitoring) — lever → CSU → SFCC1/2 flap channels → two valve blocks (each own hydraulic supply, four solenoids + pressure switch); low-speed (retract/extend solenoid), high-speed (extend/retract + high-speed solenoids), POB solenoid; WTB control (Green+Yellow, one SFCC solenoid each, locks on failure, reset via CMS/lever cycle/SFCC reset); disconnect proximity switches + track-4 sensors; level-2 cautions only (SDAC/FWC/CMC/DMC).
• AMM 27-54-00 (Flaps Hydraulic Actuation and Power Transmission) — PCU two hydraulic motors (Yellow + Green) each with POB + valve block, differential gearbox; POB holds on shutdown/low pressure/failure; one motor held → other drives at half speed/full torque; torque shafts → five drive stations; torque limiters prevent excess torque.

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.