The Flap System

The flaps are the trailing-edge high-lift surfaces — two per wing, inboard and outboard — that let a 200-tonne aircraft cruise above 200 kt yet approach and land safely near 140 kt. But the flap mechanism is built on a design philosophy that is the opposite of the primary flight controls. The ailerons, elevators and rudder are distributed and independent: each surface has its own nearby servocontrols, and the loss of one does not affect the others. The flaps are centralised and serial: a single power unit at the fuselage centreline drives both wings through one continuous shaft, the two wings are forced to move in step, and the design's whole effort goes into one promise — keep the two wings exactly symmetric, and the instant they are not, brake the entire transmission to a stop.

The reason is that the flap's most dangerous failure is not "won't move" but asymmetric extension — one wing out, the other not. At approach speed that produces a large rolling moment that can take the aircraft beyond recovery. So the flap system is not engineered for speed; it is engineered so that the two wings can only move identically, and so that any departure from that is caught early and locked out. This article follows that one thread the whole way: from the lever, through the Slat and Flap Control Computers (SFCC), into the Power Control Unit (PCU) with its two hydraulic motors and differential gearbox, out along the torque-shaft transmission to the panels, and into the pick-off and disconnect sensors that watch for asymmetry and stop it.

[!warning]- A lost hydraulic system makes the flaps slow, not crooked. Asymmetry comes only from a mechanical break.

The intuitive fear is "lose a hydraulic system and one wing's flaps move while the other's don't." That cannot happen. There is one PCU on the centreline driving one torque shaft to both wings. Losing a hydraulic system only stops one of the two motors inside that single PCU; the differential gearbox lets the surviving motor drive the whole shaft at half speed but full torque, and both wings stay rigidly synchronised — just slower (FLAPS SLOW). Real asymmetry comes only from a mechanical disconnect or jam in the transmission, which the monitoring network is built to catch. Throughout this chapter, separate "slow" (hydraulic / SFCC, half-speed) from "crooked" (mechanical break, WTB lock-out).

1. What this article lets you answer

Five questions anchor the whole piece. If you can answer them from memory at the end, you have it:

1. The lever has five positions — what flap angle does each give, and why does position 1 map to two different flap angles (0° and 8.5°)?
2. How does the PCU turn two hydraulic motors into one output shaft through a differential gearbox, and what does "half speed" mean when one motor fails?
3. From the PCU to the flap panel, what does the power pass through (torque shafts, gearboxes, rotary actuators), and why is the chain so long?
4. When do the Wing Tip Brakes (WTB) apply, why can they not be released in flight, and how do they differ from the Pressure-Off Brakes (POB)?
5. How does the system know it has gone asymmetric or run away — what do the APPU, FPPU, interconnecting strut and track 4 sensor each watch?

The scope is the flap drive chain only (lever → CSU → SFCC → PCU → transmission → panel → position feedback → wing-tip brake). The slat chain is a separate transmission (Slat System); the automatic-retraction logic (ARS/FLRS) is in ARS and FLRS; the full ECAM/QRH failure handling is in High-Lift Failures; and the shared SFCC/configuration framework is set up in High-Lift Overview.

2. Architecture — one power unit, one shaft, ten drive stations

FCOM compresses the whole slat-and-flap system into one list of components. Per FCOM DSC-27-30-10:

The slat and flap system includes all of the following main components: Seven slats, two flaps surfaces and two ailerons (aileron droop function) per wing. These surfaces are electrically controlled and hydraulically operated. The flight crew extends the slats and flaps by moving the flaps lever on the center pedestal. It has five positions.

Note that FCOM counts aileron droop as part of the high-lift function (at large configurations the ailerons droop with the flaps to add lift — covered in Ailerons), and the seven slats belong to the slat article. This piece keeps to the two flap panels per wing.

The maintenance source lists what carries the power from the centre out to the tips. Per AMM 27-54-00:

The components of the power transmission are: the torque shafts and steady bearings in the fuselage and wings, a torque limiter in each transmission line, two right angle gearboxes in each wing, a kink gearbox in each wing, five drive stations in each wing, a wing tip brake in each wing, an Asymmetry Position Pick-off Unit (APPU) in each wing.

2.1 The spanwise drive train

The PCU sits on the aircraft centreline; an identical torque-shaft run goes out to each wing, so both wings always move from the same rotation of the same shaft. One wing, from root to tip (the other mirrors it):

 centreline                                                            wing tip
 ┌─────────┐  ┌────┐  ┌─────┐  ┌───┐ ┌───┐ ┌────┐ ┌───┐ ┌───┐ ┌───┐  ┌─────┐  ┌──────┐
 │   PCU   ├──┤ TL ├──┤ RAG ├──┤DS1├─┤DS2├─┤ KG ├─┤DS3├─┤DS4├─┤DS5├──┤ WTB ├──┤ APPU │
 └─────────┘  └────┘  └─────┘  └───┘ └───┘ └────┘ └───┘ └───┘ └───┘  └─────┘  └──────┘
  2 motors    system   right-  └inboard┘  kink  └──── outboard ────┘ wing-tip asymmetry
  + diff      torque    angle    flap     gear         flap           brake   pick-off
  gearbox    limiter   gearbox  DS1+DS2    box      DS3+DS4+DS5
       (the ── lines between boxes are the continuous torque shaft)

Five things to read off it:

1. One power source, on the centreline. The entire flap system has a single PCU. Its output acts like a yoke: the same shaft drives both wings, so symmetry is mechanical — both sides take the same rotation, and the only way to go asymmetric is for a shaft or gear to break somewhere.
2. The shaft runs all the way to the tip, changing direction at the right-angle gearboxes (RAG — turning the drive from across the fuselage to along the span) and the kink gearbox (KG — following the wing's sweep). Most of the shaft is carbon fibre; a few highly-loaded sections are titanium (§7).
3. Five drive stations per wing feed the two panels: the inboard flap from DS1+DS2, the outboard flap from DS3+DS4+DS5. Each station gears the shaft's high-speed, low-torque rotation down to the low-speed, high-torque the flap needs.
4. The APPU sits at the far end of the shaft (near the tip). Asymmetry is judged by comparing the left and right APPU readings — the most distant point on each side, so any disconnect in between makes the two ends diverge.
5. The WTB sits just inboard of the tip end. It does nothing in normal operation; when the SFCC sees asymmetry, runaway or overspeed, it brakes the shaft mechanically, cutting power at the root of the fault before it can grow.

2.2 Component list

The contrast with the primary controls is the whole point: there, "control + actuation" sit together at each surface; here it is centralised power (one PCU) + distributed transmission (ten drive stations) + global monitoring (the SFCC comparing several sensors). Ten drive stations on one shaft are exactly why so much of the hardware is sensors that ask "are we still symmetric?" and a brake that can stop the shaft.

3. The lever — five positions, and why position 1 has two flap angles

The lever has five positions. Each maps to a flight phase, a flap angle and a slat angle. Per AMM 27-51-00 (flap angles shown; slat angles given for context):

That answer to anchor question 1 — why does lever 1 give two flap angles? — is that at lever position 1 the system chooses, by airspeed, between CONF 1 (flaps 0°, slats only, for holding) and CONF 1+F (flaps 8.5°, for take-off). AMM states the auto-command thresholds. Per AMM 27-51-00:

Two flap configurations, which are dependent on the airspeed, are possible with the slat and flap control lever at the position 1. The configurations are related to the computed airspeeds (CAS) and the lever position (0 or 2) before the lever is set to the position 1.

The CAS thresholds, per AMM 27-51-00: coming down from FULL/3/2 to 1 — at or above 200 kt → 0°, below 200 → 8.5°; going up from 0 to 1 — above 100 kt → 0°, at or below 100 → 8.5°. And once at 8.5°:

In the 8.5 degree configuration, the flaps retract automatically to 0 deg if the CAS is equal to or more than 200 knots.

In plain terms: slow gets you 8.5° (lift for take-off), fast gets you 0° (no flap load at high speed), and the direction of the change depends on whether you are coming down from a larger configuration or going up from clean. The complete automatic-retraction logic (ARS at 200 kt, flap load relief) is developed in ARS and FLRS; here we only fix why lever 1 carries two angles.

The lever mechanism itself has two detents — one at position 1, one at position 3. Per AMM 27-51-00:

The slat and flap control lever has two stops. One stop at position 1, and one stop at position 3. The stops prevent a one movement change of lever position from FULL to 2 and from 2 to 0.

Those stops are deliberate foolproofing: they stop a single careless pull from collapsing the configuration (for example FULL straight to 2 on approach, with a sudden loss of lift). You must lift past the detent to cross it.

4. From lever to computer — CSU and SFCC

4.1 The CSU — a sealed opto-electric translator

The lever is mechanical; the computers want an electrical signal. The translator is the Command Sensor Unit (CSU), which works optically. Per AMM 27-51-00:

The CSU 51CV is a sealed unit which changes the mechanical commands from the slat/flap control lever to electrical commands. The CSU has two sets of code wheels. Each code wheel moves between sets of Light Emitting Diodes (LED) and photo transistors. The LEDs transmit light to the photo transistors through openings in the code wheels.

Two sets of code wheels means dual redundancy: each SFCC reads its own set. For a command to be acted on, both reads must agree. Per AMM 27-51-00:

For a correct command, each channel must receive the same detent switch pattern from both of the opto-electric parts in the CSU.

The CSU also contains a friction brake with two jobs, per AMM 27-51-00: to hold the CSU in the last selected position if there is a failure of the input drive and to give a constant artificial feel — that constant damping in the lever is this brake.

The SFCC does not blindly obey; it compares the demand with the actual flap position and always follows the last valid lever movement. Per AMM 27-51-00:

Slat and flap movement always obeys the last correct control lever movement. For example, when the lever moves from position 3 to FULL, the flaps will extend in the direction of the FULL position. If the lever returns to position 3 before the flaps reach position FULL, the direction of flap movement changes and the flaps move back to position 3.

Operationally this means flap travel is follow-up: change your mind mid-stroke and the flaps reverse immediately — they do not have to complete a stroke first. That matters on a go-around: pull the lever back and the flaps start retracting at once.

4.2 Inside the SFCC — dissimilar lanes against common-mode failure

The overview treats the SFCC as a black box of "two computers, each with a slat and a flap channel." The maintenance source opens the box on a subtle reliability design — each channel runs two lanes built from different chips and different languages. Per AMM 27-51-00:

ARINC 600 5 MCU cases contain the two SFCCs. ... Each channel has two lanes (lane A and lane B) ... The two lanes in each channel have different hardware and software: lane A has a MOTOROLA 68020 microprocessor (software programmed in Pascal language), lane B has a INTEL 80286 microprocessor (software programmed in C language). ... information exchange between lanes A and B through the dual-ported Random Access Memory (RAM).

This is dissimilar redundancy, and the reason for the effort is common-mode failure. Ordinary duplication runs two identical circuits, but a single chip flaw or a single software bug would corrupt both copies the same way, and they would even "confirm" each other. The fix is to make the two lanes genuinely different machines — different CPU vendors, different languages — so that one chip-level hardware defect, or one language/compiler-related software bug, cannot strike both lanes identically. One lane errs, the other almost certainly does not, and the disagreement is caught. This is a different layer from the cross-SFCC confirmation in §8: lane A/B dissimilarity defends a single computer against common-mode error; SFCC1/SFCC2 cross-confirmation defends against a single computer's false trip. You need not remember the chip numbers, but you should read a "microprocessor cross lane failure" as the two dissimilar lanes disagreed, after which the system distrusts the result and latches.

4.3 What the SFCC takes in over ARINC 429

The SFCC is not isolated; it reads air data and time. Per AMM 27-51-00:

The SFCCs receive data through the ARINC 429 busses from: the ADIRU 1, 2, 3, the CMC. (1) ADIRU Data — The SFCCs receives the computed airspeed (CAS) from all three ADIRUs. This data is used in the SFCCs for the flap load relief and the flap auto command function and the aircraft on ground/flight monitoring. (2) CMC Data ... This data is used in the SFCCs for the BITE and the WTB engagement test.

So the flap system depends on the ADIRS for CAS (to drive the auto-command and load-relief logic and to know ground vs flight) and on the CMC for time/date (to run the periodic WTB engagement self-test after landing). A loss of all three ADIRUs therefore reaches into the flap auto-retraction and ground/flight logic — an easily-missed cross-system dependency.

5. The PCU — two motors, one differential gearbox, half speed

5.1 Structure

All the flap power comes from one PCU. FCOM names its redundant core in a sentence. Per FCOM DSC-27-30-10:

A PCU which is made of two independent hydraulic motors coupled by a differential gearbox. The motors use green and blue hydraulic power for the slats, and yellow and green power for the flaps.

Fix this: flaps = Yellow + Green. The PCU is the shared physical unit for slats and flaps, but the slat side uses Green + Blue and the flap side uses Yellow + Green — Green is the common edge. AMM details the build. Per AMM 27-54-00:

The flap Power Control Unit (PCU) is a hydromechanical unit the main body of which is a case which contains a differential gearbox. The subsequent Line Replaceable Units (LRU) are attached to the casing: two hydraulic motors, two Pressure-Off Brakes (POB), two valve blocks, eight solenoid valves, two PCU pressure switches, two inlet filters, one Feedback Position Pick-Off Unit (FPPU), one Instrument Position Pick-off Unit (IPPU).

5.2 The differential gearbox and half speed

The differential gearbox is the heart of the design — it sums the two motors' output onto one shaft. Per AMM 27-54-00:

A differential gearbox connects the output shafts of the hydraulic motors and transmits their torque to the power transmission system. If a POB engages and holds its motor, the remaining motor moves the transmission system at half speed but at full torque.

That is where half speed comes from (anchor question 2). The clearest picture: two people carrying one pole between them. Both walking, the pole moves fast; one stops and stands still (a POB locks that motor), the other can still drag the pole along — half the speed, but no loss of strength (full torque). The differential gearbox's elegance is that one motor stopping does not jam the other — the surviving motor drives the whole transmission alone.

FCOM lists the two crew-visible triggers for half speed. Per FCOM DSC-27-30-10:

If one SFCC is inoperative, both slats and flaps operate at half speed.

If one hydraulic system is inoperative, the corresponding surfaces (slats or flaps) operate at half speed.

Half speed is not a fault, it is degraded operation — the flaps still reach FULL and still land, just more slowly. You see FLAPS SLOW on the STATUS page, and the airmanship is to start extending earlier to give it time. Note the difference between the two triggers: lose an SFCC and both slats and flaps go half speed; lose one hydraulic system and only the corresponding surfaces do.

5.3 The valve block and the speed profile

The valve block is the SFCC's hand on the motor. It has four solenoid valves. Per AMM 27-54-00:

The four solenoid valves are referred to as: extend solenoid valve, retract solenoid valve, high-speed solenoid valve, POB solenoid valve.

A normal extension therefore runs a low → high → low speed profile: it starts in low speed, accelerates to high speed mid-travel, slows again near the target, and stops. Per AMM 27-54-00:

When the motor operates with a certain speed, SFCC1 energizes the high-speed solenoid valve ... the motor operates at high speed. ... As the flaps get near to the specified position: SFCC1 de-energizes the high-speed solenoid valve ... the motor returns to the low-speed mode, the flaps operate at a lower speed.

But that low/high/low curve is the commanded speed band. The valve block also holds an easily-overlooked element — the pressure-maintaining valve — that ties speed to how much pressure the hydraulic system can actually supply right now. Per AMM 27-54-00:

The pressure maintaining valve controls and reduces system speed to agree with available hydraulic pressure.

This turns the two-step "half speed" picture into a continuous range (integrative reasoning, not a verbatim claim). Half speed is the discrete outcome of losing a whole motor or a whole hydraulic system. But real pressure is not all-or-nothing: when a system runs low but not zero — say a large flow demand elsewhere (gear, brakes, nose-wheel steering) draws it down — the pressure-maintaining valve scales the flap speed down in proportion to the available pressure. The flaps then extend slower than even the textbook half speed, varying with pressure. That is by design: it stops the motor from fighting for flow and dragging the system pressure further down, and it leaves flow for the higher-priority users while still keeping enough torque to drive the flaps home. So if you extend flaps with a low (but not lost) hydraulic pressure and see neither full speed nor a tidy half speed, that is normal — build the extra time into your approach planning.

Finally, the work is split across the two computers. Per AMM 27-54-00:

SFCC1 controls the valve block of the Yellow hydraulic system and the related motor on the PCU. SFCC2 controls the valve block of the Green hydraulic system and the related motor.

This unifies the two half-speed cases: whether you lose SFCC1 or lose Yellow, the result is the same — the Yellow motor is out and the differential gearbox runs the shaft on the Green motor alone, at half speed.

6. Two brakes — POB for parking, WTB for failure

The flap system has two completely different brakes, and they are the most-confused pair on type (anchor question 4).

6.1 POB — the everyday parking brake

The Pressure-Off Brake (POB), one per motor, lives in the PCU and is used every time the flaps stop. Per AMM 27-54-00:

A POB is attached to each hydraulic motor. It holds the output shaft of the hydraulic motor when: the hydraulic motors do not operate, the related hydraulic system does not supply sufficient hydraulic power, the WTB stops the flap transmission system because of certain system failures. ... Springs hold the friction disks together. When hydraulic pressure is applied to the POB, the friction disks are disengaged (against the pressure of the springs).

The name says everything: pressure on = released, pressure off = applied — a fail-safe arrangement. Lose hydraulic pressure and the springs lock the motor at once, freezing the flaps where they are. So when extension finishes and the SFCC de-energises the POB solenoid, the POB anchors the flaps — aerodynamic load cannot move them.

6.2 WTB — the failure latch

The Wing Tip Brake (WTB), one per wing near the shaft end, is the failure brake. Where the POB is routine, the WTB acts only on a serious fault, and once it acts it latches — and cannot be released in the air. Per FCOM DSC-27-30-10:

The WTBs which activate in the case of an uncommanded movement of the surfaces, such as runaway, asymmetry or over speed. They cannot be released in flight. They use blue and green hydraulic power for the slats, and green and yellow for the flaps.

The hardware shows why "cannot be released in flight" is absolute. Per AMM 27-51-00:

The Wing Tip Brakes ... are electrohydraulic pressure-off disc-brakes. Each WTB has: a central housing which has two hydraulic manifolds, a friction disc pack, a through torque shaft, two annular pistons, two solenoid valves, two electrical connectors, a proximity switch.

Two pistons, two solenoid valves — each SFCC controls one solenoid on each WTB. Per AMM 27-51-00, If hydraulic pressure is not available to one piston, the remaining piston gives sufficient force to act against the spring and let the brake off — so a single piston can hold the brake released. The corollary is that latching the brake needs both circuits removed. Per AMM 27-51-00:

The WTBs are latched when the power to both solenoids is turned off. If one SFCC does not operate, the connected WTB solenoid is not energized. Thus if the other SFCC subsequently detects a failure the related solenoid will be de-energized and the WTBs latched.

[!warning]- POB and WTB are not the same brake — FLAPS SLOW is the POB/half-speed layer, FLAPS LOCKED is the WTB latch.

The POB is routine parking braking: in the PCU, one per motor, applied at the target and released by pressure, every single stop. The WTB is an emergency latch: at the wing, one per wing, applied only on runaway / asymmetry / overspeed, and latched shut so it cannot be released in flight. Seeing FLAPS SLOW points you at the POB / half-speed layer (degraded but still moving). Seeing FLAPS LOCKED means a WTB has latched — the flaps are frozen at their current angle and you will land with that configuration. Once latched, a WTB resets only on the ground (and some hardware-latched faults need a dedicated ground reset); there is no in-flight reset, so a high-lift LOCKED is permanent for the rest of the flight — go straight to the jammed-configuration landing case (High-Lift Failures).

FCOM adds a reassuring note: the slat and flap brakes are independent — locking one leaves the other working. Per FCOM DSC-27-30-10:

If the flap WTB is on, the flight crew can still operate the slats, and if the slat WTB is on, the flight crew can still operate the flaps.

Because the slat and flap chains are separate transmissions (separate motor sides of the PCU, separate WTBs), "flaps locked" does not mean "slats lost" — which matters when you choose a landing configuration.

7. From PCU to panel — a reduction transmission

The PCU outputs high speed and relatively low torque; the flap needs low speed and high torque (to hold against large aerodynamic loads). The transmission does the reducing, the direction-changing and the spanwise distribution.

7.1 The torque shaft

Most of the shaft is carbon fibre for stiffness and lightness; the highly-loaded sections are titanium. Per AMM 27-54-00:

The torque shafts, except for the shafts 0RH, 2RH and 3 are made of carbon fibre reinforced plastic (CFRP).

and per AMM 27-54-00, The compliant shafts 0RH and 2RH are machined from titanium. Universal joints allow the run to follow the wing's geometry, and each shaft has at least one sliding end connection.

7.2 The drive station

Each drive station is the gear set that brings the shaft's power "down" onto the flap. Per AMM 27-54-00:

Each drive station has: a down-drive gearbox, a down-drive shaft, an input-bevel gearbox, which has a torque limiter, a cross shaft, an actuator.

The reduction is three stages, per AMM 27-54-00: The down drive has a gear ratio of approximately 1:2, The input-bevel gearbox has a gear ratio of approximately 1:2, and Both types of actuator have a gear ratio of approximately 1:220:

 torque shaft
   ──►┌──────────────┐   ┌────────────────────┐   ┌─────────────────┐
      │ down-drive   ├──►│ input-bevel gearbox ├──►│ rotary actuator ├──► flap panel
      │ gearbox ~1:2 │   │ ~1:2 + torque limit │   │      ~1:220     │
      └──────────────┘   └────────────────────┘   └─────────────────┘
        cumulative reduction ≈ 1:2 × 1:2 × 1:220 ≈ 1:880  (integrative; AMM gives each stage)

Multiplying the three gives roughly 1:880 — the AMM gives each stage but not the product, so the total is a calculated figure. It turns the PCU's fast, low-torque rotation into the slow, high-torque motion the flap needs.

The rotary actuators come in two sizes, the larger where loads are highest. Per AMM 27-54-00:

There are two types of rotary actuators: type A is installed on drive stations 1, 4 and 5, type B is installed on drive stations 2, and 3. The type B actuator is larger than the type A and is installed on drive stations 2 and 3 because of the high aerodynamic loads at these points.

7.3 The torque limiters — mechanical fuses

The transmission has bidirectional torque limiters at two levels: a system torque limiter per wing (between PCU and DS1) and a station limiter inside each input-bevel gearbox. Per AMM 27-54-00:

The torque limiter installed in the transmission system of each wing operates in both directions to stop the transmission of too much torque to the drive stations in the wings. A 'pop-out' type indicator shows when a lock-out torque has occurred.

and per AMM 27-54-00, Each input gearbox has a torque limiter which operates in both directions. Each torque limiter is set with its own individual lock-out torque values for extension and retraction. The limiters are the mechanical last line of defence: if a station jams or a mechanism fault generates abnormal torque, the limiter locks that line out before the shaft is twisted off or the wing structure is damaged. A lock-out is detected by the SFCC as a system jam, which presents as F/CTL FLAPS FAULT with a FLAPS LEVER RECYCLE prompt.

8. How the system knows it is in trouble — pick-offs and disconnect sensors

This is what most distinguishes the flap system from the primary controls: a monitoring network watching for asymmetry, runaway and disconnect. The SFCC does not just open the motor — it continuously compares position data. Per AMM 27-51-00:

The SFCC monitor the power transmission system for these failure conditions: asymmetry (a position difference between the two APPUs), runaway (a position difference between the APPUs and the FPPU), uncommanded movement (a movement in the wrong direction, or movement away from the last commanded position), overspeed, flap disconnect, flap track 4 sensor displacement, system jam, half speed, low hydraulic pressure.

8.1 The three position pick-offs

The three pick-offs are the same physical unit, interchangeable, with the same scaling. Per AMM 27-51-00:

The turn of the input shaft through 360 revolutions gives 360 degrees synchro transmitter output. For full travel of the flaps, the angular output of each synchro transmitter is 338 degrees.

Their jobs differ (anchor question 5):

• FPPU (Feedback PPU) — on the PCU, measuring how far the differential-gearbox output has turned: the commanded / source position. It has two independent synchros, one to each SFCC.
• APPU (Asymmetry PPU) — one per wing, at the far end of the shaft: the actual position reached at the tip end.
• IPPU (Instrument PPU) — also on the PCU, but only for display and overspeed, outside the control loop (§9).

The comparison logic: asymmetry = left APPU vs right APPU (the two tip ends turning by different amounts → a wing's chain has disconnected or jammed); runaway = APPU vs FPPU (the source has not moved but the end has, or the two disagree → the transmission is running away).

8.2 The two disconnect sensors

APPU comparison alone is not enough — sitting at the shaft end, it can miss a disconnect between the inner and outer flap. Two local "disconnect" sensors fill the gaps.

The flap interconnecting strut links the inner and outer flaps and watches DS1/2/3. Per AMM 27-51-00:

The flap interconnecting strut has these functions: it permits limited differential movement between the inner and outer flaps, it gives an alternative load path for the flap drive in the event of a disconnect in the drive stations 1, 2 or 3, it gives information to the SFCC if the limit of differential movement is more than permitted, it absorbs energy if a flap drive disconnect occurs.

The strut is dual-purpose: a sensor (over-limit differential → reports a disconnect) and a mechanical backup (it carries the outer flap on the inner one if a drive disconnects, stopping the panel being torn away by the airflow).

The track 4 sensor strut watches DS4/DS5, but only below 18° flap. Per AMM 27-51-00:

The flap track 4 sensor strut performs these functions in a range <18° flap position: it allows limited movement between the flap and the intermediate station, it provides information of the relative position of the outer flap to the SFCC.

Per AMM 27-51-00, If the drive at drive station 4 or drive station 5 becomes disconnected the beam assembly is displaced relative to the main flap — and the strut senses that displacement.

Read as three lines of defence (integrative framing): the APPU watches overall symmetry of the two tip ends; the interconnecting strut watches the inner/outer break (DS1/2/3); the track 4 sensor watches the outer-flap far end (DS4/5). Any one tripping makes the SFCC shut that chain down and, if confirmed, latch the WTB — which is why a flap disconnect almost never grows quietly into a large asymmetry.

8.3 The shutdown — and the cross-confirmation

When asymmetry is confirmed, the SFCC actions chain together. Per AMM 27-51-00:

If an SFCC detects an asymmetry it: de-energizes its related WTB circuits, performs an abnormal shutdown of the related PCU valve block ... transmits a failure message to the other SFCC flap channel. If the flap channel of the other SFCC confirms the failure it: software latches the WTBs ... it shows on the EWD the warning FLAPS LOCKED.

Note the cross-confirmation: a single SFCC's suspicion is not enough — the other SFCC must agree before the WTBs latch, which stops one computer's misread from locking healthy flaps. Per AMM 27-51-00, if the second SFCC does not confirm, the system records a PPU failure message and does not latch. This is the balance between being sensitive and avoiding false trips.

8.4 The flap-only third latch — disconnect locks out the start-up

The slats and flaps share the hard-latch / soft-latch / lever-recycle structure (Slat System). But the flap has one extra latch class the slat lacks, triggered by exactly the two disconnect sensors above (the slat has neither sensor). Per AMM 27-51-00:

Start-up Inhibitions (Reset only on the Ground or SFCC Power-up) — The SFCC do a start-up inhibition for the subsequent faults: a flap disconnect fault, a flap disconnect switch fault, a flap track 4 fault, a flap track 4 switch fault.

This is why a flap disconnect is harder to self-recover than a slat fault. The "recycle the lever to clear an unconfirmed fault" trick does not work on a disconnect: once reported, the SFCC inhibits start-up of that channel and it resets only on the ground or by powering the SFCC down and up — no in-flight recovery. The reasoning (integrative): a disconnect means a real mechanical break between flap panels or at the outer-flap end, so the system will not let you "try again to see if it cleared" in the air — it locks out and keeps the current configuration stable to landing. So in-flight self-recovery of a high-lift fault is really three tiers: unconfirmed electrical faults can be cleared by a lever recycle; confirmed asymmetry/overspeed latches the WTB and needs a ground reset; a flap disconnect inhibits start-up entirely — no recycle, certain landing with the frozen configuration.

9. Position indication and distribution

9.1 The display path — IPPU to EWD

How the crew sees flap position runs through the IPPU, independent of the control loop. Per AMM 27-55-00:

The IPPU 3CF is attached to the flap Power Control Unit (PCU) ... The IPPU supplies the flap position data directly to the Flight Warning Computers (FWC1, FWC2) ... The FWCs transmit the data to the Electronic Instrument System (EIS). The EIS shows the position of the flaps on the EWD in the cockpit.

On the EWD a wing-shaped symbol shows a green index tracking actual position. If a WTB latches, the flap side turns amber and shows F-LOCKED. Per AMM 27-55-00:

the white F indication changes to amber, the flap index changes to amber, an amber F-LOCKED message comes on above the set position indication.

The maximum-flap speeds (VFE) and the related PFD markings are the crew's everyday flap limit; the exact figures live in Controls and Indications.

9.2 The other half — the SFCC feeds six systems

The IPPU→FWC→EWD chain is only the display half. The other half — the more critical one — is that the SFCC broadcasts the flap/slat position it computes to six downstream systems, for law computation, speed computation and terrain warning. Per FCOM DSC-27-30-10:

The SFCCs transmit flaps/slats positions to the following systems: PRIM and SEC; FMGEC; ADIRU; EIU; CIDS; GPWS. Note: The ECAM system receives the position information directly from the Instrumentation Position Pick-Off Unit (IPPU). This information is used for warnings and position indications on the E/WD.

That note joins the two halves: the display chain (warnings + EWD index) runs IPPU→FWC→ECAM, while the computing systems (PRIM/SEC/FMGEC/…) take the SFCC's direct broadcast. The two paths are deliberately separate, so corrupt display data cannot poison the position the laws use, and a corrupt computing feed still leaves the EWD an independent channel to show real flap position.

What the six recipients do with configuration (integrative reasoning — FCOM lists the recipients but does not itemise each use):

• PRIM and SEC (flight-control computers): configuration is a key law input — at large configurations the PRIM commands aileron droop from flap position (Ailerons), and law gains and protection availability shift with configuration.
• FMGEC (flight management/guidance): computes VFE / VFE NEXT and the characteristic speeds for each configuration, and drives the PFD speed scale — those markings originate in the configuration the SFCC feeds it.
• ADIRU: receives configuration for its air-data-related logic (FCOM lists it as a recipient; the specific use is not extrapolated here).
• EIU (Engine Interface Unit): brings configuration into the engine/thrust-management side.
• CIDS (Cabin Intercommunication Data System): links configuration / flight phase to cabin logic.
• GPWS (ground-proximity warning): selects the flap mode for its terrain envelopes and TOO LOW FLAPS logic — which is why some operators' procedures require GPWS FLAP MODE OFF when the flap system is unserviceable, so an untrusted flap position cannot drive a wrong-mode warning.

The takeaway for the crew: flap position is far more than an EWD index — it is the common upstream of laws, speeds and terrain warning. A flap fault (untrustworthy position) can therefore ripple into the flight-control laws, the PFD speed scale and GPWS, not just the FLAPS indication. That is precisely why the ECAM display is wired to the IPPU directly — to leave one independent channel showing true flap position even if the SFCC distribution is suspect.

10. Flying the flaps — a few scenes

1. Selecting take-off configuration (lever 0 → 1+F → 2). Lever to 1, the CSU opto-codes it to both SFCCs; low speed on the ground, so the auto flap command gives CONF 1+F (8.5°); lever to 2 gives 14.5°. Both motors (Yellow + Green) drive the shaft at full speed, both wings extend symmetrically, the green EWD index follows. With only one hydraulic system available it runs half speed (FLAPS SLOW) — select earlier.
2. Cleaning up after take-off. Lever to 1 above 200 kt, so the auto command gives CONF 1 (0°); continue to 0.
3. An approach at half speed. Yellow fails, so the flaps run on the Green motor alone, half speed; you see FLAPS SLOW and start extension noticeably earlier to reach FULL in time. Slats are unaffected (slats use Green + Blue).
4. Asymmetry detected during extension. A drive station disconnects → the left and right APPU diverge → SFCC1 detects asymmetry → de-energises its WTB circuit and shuts down its valve block → messages SFCC2 → SFCC2 confirms → the WTBs software-latch, the flaps freeze, and the EWD shows FLAPS LOCKED with an amber F-LOCKED. No in-flight recovery — go to the jammed-configuration landing case (High-Lift Failures).
5. A system jam. A mechanism jams → the combined motor speed drops below the jam threshold → the SFCC detects a system jam → abnormal shutdown → the EWD shows F/CTL FLAPS FAULT with a FLAPS LEVER RECYCLE prompt. Recycle the lever as prompted: a clearable jam recovers; one that does not means landing with the jam.
6. Changing your mind on a go-around. Flaps at FULL for landing, you go around and bring the lever back to 3 → the flaps immediately reverse toward 22.5° ("always obeys the last correct control lever movement") — follow-up response, no stroke to complete first.

Self-test

[!note]- Q1. At lever position 1, is the flap angle 0° or 8.5°? What decides?

Either — decided automatically by airspeed (the auto flap command). Coming down from FULL/3/2 to 1: at or above 200 kt → 0° (CONF 1), below 200 → 8.5° (CONF 1+F). Going up from clean (0) to 1: above 100 kt → 0°, at or below 100 → 8.5°. And in the 8.5° configuration the flaps retract automatically to 0° if CAS is at or above 200 kt. The logic is "slow gives lift, fast keeps load off the flaps." The full retraction logic (ARS/FLRS) is in the ARS and FLRS article.

[!note]- Q2. How does the PCU drive one output shaft from two motors, and what happens if one motor fails?

Two independent hydraulic motors (flaps use Yellow + Green) feed a differential gearbox that sums their torque onto one output shaft. If one motor's POB locks it, the remaining motor drives the whole transmission at half speed but full torque — like two people carrying a pole, one stops, the other drags it along at half pace with no loss of strength. Half speed is triggered either by losing an SFCC (both slats and flaps slow) or by losing one hydraulic system (the corresponding surfaces slow); you see FLAPS SLOW.

[!note]- Q3. From the PCU to the flap panel, what does the power pass through, and why so long a chain?

PCU → system torque limiter → right-angle gearbox (changes direction) → kink gearbox (follows the wing sweep) → five spanwise drive stations. Inside each drive station: down-drive gearbox (1:2) → input-bevel gearbox (1:2, with a torque limiter) → rotary actuator (~1:220) → flap panel. The three stages multiply to roughly 1:880, converting the PCU's fast low-torque rotation into the slow high-torque the flap needs. The chain is long because it must distribute symmetrically from the fuselage centre to both tips and feed each panel through several drive stations.

[!note]- Q4. POB and WTB are both brakes — how do they differ, and why can the WTB not be released in flight?

The POB is routine parking braking: in the PCU, one per motor, applied at the target position and released by hydraulic pressure (pressure-off = applied, pressure-on = released, fail-safe), used at every stop. The WTB is the failure latch: one per wing near the shaft end, applied only on runaway/asymmetry/overspeed and only when both SFCC solenoid circuits are de-energised. Once latched it resets only on the ground (some hardware-latched faults need a dedicated ground reset), and there is no in-flight reset path — hence "cannot be released in flight."

[!note]- Q5. How does the system know the flaps have gone asymmetric or run away, and what does each sensor watch?

By comparing three position pick-offs. The FPPU (on the PCU) gives the source/commanded position; an APPU per wing (at the shaft end) gives the actual tip-end position. Asymmetry = left APPU vs right APPU; runaway = APPU vs FPPU. Two local disconnect sensors fill the gaps: the interconnecting strut watches DS1/2/3 (and provides a backup load path), the track 4 sensor strut watches DS4/DS5 (valid below 18°). Any trip → SFCC shuts the chain down, and on cross-confirmation by the other SFCC the WTBs latch → FLAPS LOCKED. The IPPU only feeds the EWD display and overspeed, outside the control loop.

[!note]- Q6. The flaps lose a hydraulic system. Will one wing move while the other stops, giving asymmetry?

No. There is one PCU on the centreline driving one torque shaft to both wings, so the two sides always take the same rotation. Losing a hydraulic system only stops one of the PCU's two motors; the differential gearbox runs the shaft on the other motor at half speed, full torque, and both wings stay rigidly synchronised — just slow (FLAPS SLOW). Hydraulic loss makes the flaps slow, not crooked. True asymmetry comes only from a mechanical disconnect or jam, which the APPU/strut/track-4 network is built to catch.

Key takeaways

The flaps reduce, in the end, to one idea repeated at every level: make the two wings move from the same rotation, and brake everything the moment they do not. The lever, the PCU, the long symmetric shaft and the pick-off network are all that one idea, built in hardware.

References

Per FCOM DSC-27-30-10 (Flaps and slats — system components; electrically-controlled/hydraulically-operated surfaces; five lever positions; PCU two-motor differential gearbox; flaps use Yellow + Green; WTB triggers and "cannot be released in flight"; half-speed notes; slat/flap WTB independence; SFCC distribution of position to PRIM/SEC, FMGEC, ADIRU, EIU, CIDS, GPWS, ECAM via IPPU). Per AMM 27-51-00 (Flaps electrical control and monitoring — lever angle table; auto flap command CAS thresholds; two lever stops; CSU opto-electric dual code wheels and friction brake; "always obeys the last correct control lever movement"; ARINC 600 dissimilar dual lanes — MOTOROLA 68020/Pascal and INTEL 80286/C, dual-ported RAM; ARINC 429 inputs from ADIRU and CMC; WTB structure and both-solenoid latching; transmission monitoring conditions; PPU 338° scaling; interconnecting strut and track 4 sensor strut; start-up inhibition for disconnect faults; asymmetry abnormal shutdown and cross-confirmation → FLAPS LOCKED). Per AMM 27-54-00 (Flap PCU — hydromechanical build; differential gearbox half speed/full torque; four solenoid valves and low/high/low speed sequence; pressure-maintaining valve; SFCC1/Yellow and SFCC2/Green split; POB; power-transmission components; torque shaft CFRP/titanium; drive station down-drive/input-bevel/actuator ratios 1:2/1:2/~1:220; rotary actuator types A/B; system and station torque limiters with pop-out). Per AMM 27-55-00 (Flap position indicating — IPPU to FWC to EWD; amber F-LOCKED indication). Hydraulic-source dependency (Yellow + Green) per ATA-29. Items flagged as reasoning — the ~1:880 total reduction, the "three lines of defence" framing, the pressure-maintaining-valve continuous-speed reading, and the per-recipient use of the SFCC position broadcast — are integrative synthesis from the cited passages, not verbatim manual statements.

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.