The Slat System

The slats are the leading-edge high-lift surfaces — seven per wing — that bend the wing's nose down at low speed, delay the stall and unlock the lift an approach needs. Like the flaps, they are driven from a single Power Control Unit (PCU) on the fuselage centreline, out along a torque-shaft transmission, to actuators on every panel. And like the flaps, the whole design is bent to one promise: the two wings may only move identically, and the instant they do not, brake the entire shaft to a stop. What makes the slat chapter worth its own article is that the slat transmission is physically separate from the flap transmission — its own PCU output, its own torque shafts, its own Wing Tip Brakes (WTB) — yet it is not independent of the flaps in the two ways that matter operationally: it shares the same two SFCCs, and it shares the Green hydraulic system.

That split — independent hardware, shared computers and shared Green — is the spine of everything below. Read it first as one warning, because it overturns the obvious conclusion:

[!warning]- "Slats are independent of flaps" is true of the transmission and false of the dependencies that bite you.

The mechanical chains genuinely are separate: a slat WTB latching does not touch the flaps, and a flap WTB latching does not touch the slats (FCOM says so explicitly). But the two systems share the two SFCCs and they share Green hydraulic. So lose one SFCC and both slats and flaps go to half speed; lose Green and both go to half speed, because Green is the one hydraulic system the slat side (Green + Blue) and the flap side (Yellow + Green) have in common. The slats are independent of the flaps for brakes and shafts, not for computers and Green. Hold that distinction through the whole article.

1. What this article lets you answer

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

1. How many motors drive the slat PCU, which two hydraulic systems feed them, and why does losing one system mean half speed, not a stop?
2. From the PCU to the wing-tip slat, what does the power pass through, and which parts are slat-only versus shared with the flaps?
3. Of Green, Blue and Yellow, which loss hurts the high-lift system most, and why?
4. When does a slat WTB latch the transmission, why can it not be released in flight, and which slat faults can you still clear in the air?
5. How do the SFCCs tell asymmetry from runaway — and do the two use the same sensors?

The scope is the slat drive chain and its monitoring (PCU → transmission → panel → WTB → pick-offs). The flap transmission is a separate article (Flap System); the lever, CSU and configuration framework are set up in High-Lift Overview; the automatic-retraction logic is in ARS and FLRS; the high-α anti-retraction is in Slat Alpha/Speed Lock; the full ECAM/QRH failure handling is in High-Lift Failures and QRH Jam and Loss of Control.

2. Architecture — one PCU, two wings, fourteen actuators each

FCOM compresses the whole slat-and-flap system into a single component list. The slat-relevant lines, 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.

and the core of the drive:

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. The POB locks the transmission when the slat or flap surfaces have reached the selected position or if hydraulic power fails.

then the sensors and brakes:

Two APPUs that measure the asymmetry between the left and right wings

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 FPPUs that feedback position information to the SFCCs An IPPU that sends position data to the ECAM.

Fix the four crew-facing data points from that: slats = Green + Blue, the slat WTBs cannot be released in flight, two APPUs watch left-right symmetry, and one IPPU feeds the ECAM. The maintenance source then opens up the hardware.

2.1 Inside the PCU — two motors summed into one shaft

The slat PCU is a hydromechanical box on the centreline. Per AMM 27-84-00:

The hydromechanical slat-Power Control-Unit (PCU) supplies mechanical power to the power transmission system. The slat PCU has two hydraulic motors, each with a Pressure-Off Brake (POB) and an electrically controlled valve block. Each valve block receives command signals from a Slat Flap Control Computer (SFCC) and controls its hydraulic motor and POB.

The two halves are symmetric but fed from different systems and owned by different computers. Per AMM 27-84-00, SFCC1 controls the valve block of the Green hydraulic system and the related motor on the PCU. SFCC2 controls the valve block of the Blue hydraulic system and the related motor.

        SFCC 2  (electrical)                 (electrical)  SFCC 1
           │                                              │
           ▼                                              ▼
   ┌───────────────┐                            ┌───────────────┐
   │ valve block   │   BLUE hyd      GREEN hyd  │ valve block   │
   │  (4 solenoids │◄════════          ════════►│  (4 solenoids │
   │   + POB)      │                            │   + POB)      │
   └───────┬───────┘                            └───────┬───────┘
           ▼                                            ▼
       ┌────────┐                                  ┌────────┐
       │ motor 2│                                  │ motor 1│
       └───┬────┘                                  └────┬───┘
           └──────────────┐              ┌──────────────┘
                          ▼              ▼
                  ┌────────────────────────┐
                  │  differential gearbox  │  two inputs → one output
                  └───────────┬────────────┘
                              ├──► FPPU  (feedback → both SFCCs)
                              ├──► IPPU  (position → ECAM)
                              ▼
                       output shaft ──► power transmission (both wings)

Read four things off it:

1. Symmetric halves, different supplies. Motor 1 takes Green and answers to SFCC1; motor 2 takes Blue and answers to SFCC2. Either motor can be lost — through its hydraulic system or its computer — and the other keeps driving.
2. The differential gearbox is the summing point. Both motor output shafts feed one differential gearbox that combines them onto one output shaft. Both motors running gives full speed; one motor held gives half speed (§3).
3. A POB sits behind each motor. The Pressure-Off Brake is spring-applied and pressure-released: pressure on releases it, pressure off (or a normal stop, or a failure) applies it. It is what "parks" the slats at the selected position and freezes them if a motor loses pressure.
4. Two pick-offs read the same gearbox. The FPPU feeds control (back to both SFCCs); the IPPU feeds display (to the ECAM). They are separate eyes on the same output — one for the loop, one for the screen.

One slat-only design point the AMM flags, easily confused with a part of the transmission below. Per AMM 27-84-00:

The slat PCU does not have a T-gearbox, its output shaft is connected directly to the differential gearbox.

This is about the inside of the PCU: unlike the flap PCU, the slat PCU has no internal T-gearbox — its differential output goes straight to the transmission. Do not confuse it with the Tee-gearbox in the fuselage (a separate transmission component, §2.2) that splits drive out to both wings. Two different "T"s.

2.2 The spanwise chain — centreline to wing tip

The output shaft drives a transmission that runs out to each wing. Per AMM 27-84-00:

The components of the power transmission system are: the torque shaft assemblies and steady bearings in the fuselage and the wings, a Tee-gearbox in the fuselage, a system torque limiter for each wing, a pair of right-angle gearboxes for each wing, a bevel gearbox for each wing, fourteen actuators for each wing, a Wing Tip Brake (WTB) in each wing ... a Asymmetry Position Pick-off Unit (APPU) in each wing.

One wing, from centre to tip (the other mirrors it):

 centreline                                                            wing tip
 ┌─────┐ ┌──────┐ ┌────┐ ┌─────┐ ┌───────┐ ┌─────────────────┐ ┌─────┐ ┌──────┐
 │ PCU ├─┤ Tee  ├─┤ TL ├─┤ RAG ├─┤ bevel ├─┤   14 actuators  ├─┤ WTB ├─┤ APPU │
 └─────┘ │ gbox │ └────┘ │ ×2  │ │ gbox  │ │ A×2 + B×12      │ └─────┘ └──────┘
   2     └──────┘ system  right- └───────┘ │ (slat1)  (slat  │ wing-tip asymmetry
 motors  splits   torque  angle   turns    │          2–7)   │ brake   pick-off
  + diff  to both  limiter gboxes  into     └─────────────────┘
  gearbox  wings   (1/wing)        L.E. line
        (the ── lines between boxes are the continuous torque shaft + steady bearings)

Read three things off it:

1. One PCU serves both wings. The Tee-gearbox is the splitter; both wings are driven from the same output rotation, so symmetry is mechanical — the two sides cannot diverge unless something breaks.
2. Torque limiters come at two levels. A system torque limiter per wing protects the whole line; every actuator has its own torque limiter as well (§4). Any jam that drives torque past the lock-out value locks that line out before the load reaches the structure.
3. The APPU sits at the very end of the shaft. Asymmetry and runaway are judged at the most distant point — where any disconnect upstream shows up as the largest divergence, giving the most sensitive detection.

2.3 Component list

[!warning]- FPPU, IPPU and APPU are the same physical unit in three different jobs — do not treat them as three different sensors.

Per AMM 27-84-00 the slat PCU's two pick-offs are the same as the Asymmetry PPU (APPU) ... All the PPU are interchangeable. The same synchro hardware becomes an FPPU (on the PCU, closing the control loop), an IPPU (on the PCU, driving the ECAM display), or an APPU (at the wing tip, measuring asymmetry) purely by where it sits and what it feeds. Same box, three roles — which is exactly why the monitoring logic can compare one against another.

3. The PCU — two motors, one gearbox, half speed

3.1 The supply and the differential gearbox

The slat PCU is fed by two hydraulic systems. Per AMM 27-84-00, The Blue and the Green aircraft hydraulic systems supply hydraulic power to the slat PCU. Each system drives its own motor through its own valve block and POB. The differential gearbox is the heart of the redundancy. Per AMM 27-84-00:

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 the speed but with the full torque.

That single sentence is the mechanism behind the crew-facing rule "lose a hydraulic system → half speed". Lose Green (or Blue) and that motor's POB applies and holds it; the differential gearbox lets the surviving motor drive the whole transmission alone — half the speed, full torque. The clearest picture is two people carrying one pole: both walking, it moves fast; one stops and stands still, the other can still drag the pole along at half pace, with no loss of strength. Because torque is unchanged, the slats still reach the selected position — they just take twice as long.

FCOM gives the two crew-visible triggers. 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.

Note the asymmetry between the two triggers: lose an SFCC and both slats and flaps go half speed (each SFCC owns one slat motor and one flap motor); lose one hydraulic system and only the corresponding surfaces slow. For the crew the airmanship is identical either way — start extending earlier and give the slats time. Only the loss of both Green and Blue stops the slats entirely.

3.2 The valve block — direction and speed

Each motor is driven by a valve block of four solenoids. Per AMM 27-84-00:

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

Two of them set direction (extend / retract), one sets speed band (high-speed), one controls the brake. The POB solenoid is the fail-safe link. Per AMM 27-84-00:

The POB solenoid valve controls the POB: when energized, it lets the related hydraulic system pressure release the POB, when de-energized, it keeps the POB engaged.

So a normal extension runs a low → high → low speed profile. The motor starts in low speed (the control valve only partly open); once it is turning, the SFCC energises the high-speed solenoid. Per AMM 27-84-00:

when the high-speed solenoid valve is energized (and the extend or the retract solenoid valve is energized), the main control valve moves further away from its neutral position. The main control valve lets the full hydraulic flow to the motor, which then operates in the high-speed mode.

Near the target the SFCC drops the high-speed solenoid (back to low speed for a soft, accurate stop), then drops the POB and direction solenoids so the POB re-applies and locks the slats. The reasoning behind the three-step profile (integrative): a low-speed start avoids a shock load on the long shaft, high speed mid-travel saves time, and a low-speed finish lets the slats settle precisely on position without overshoot — the same instinct as easing a car into a parking space.

3.3 Follow-up control — slats obey the last lever movement

A point easily missed, and useful on a go-around: slat travel is follow-up, not "fire-and-forget". The SFCC compares the new demand with the actual position and chases the latest lever movement. Per AMM 27-81-00:

The SFCC memory stores the last correct command which controls the slat and flap position. The SFCC compare a new CSU command with the slat position signal which comes from the FPPU. Slat and flap movement always obeys the last correct control lever movement. For example, when the control lever moves from position 3 to FULL, the slats will extend in the direction of the FULL position. If the lever returns to position 3 before the slats reach the position FULL, the direction of the slat movement changes and the slats move back to position 3.

In other words, change your mind mid-stroke and the slats reverse at once — they do not have to finish the original travel first. On a go-around, bringing the lever back starts the slats retracting immediately toward the new selection; mechanically, the SFCC re-evaluates the FPPU position against the latest lever command on every change and drives toward the difference.

4. The transmission — fourteen actuators, two limiter levels

The PCU's output is high-speed, low-torque rotation; the slat needs low-speed, high-torque motion. The transmission does the reducing, the direction-changing and the spanwise distribution, and ends in fourteen actuators per wing.

4.1 Two actuators per slat, two actuator types

Seven slats, two actuators each, gives the fourteen. Per AMM 27-84-00, There are two actuators for each slat. They come in two kinds. Per AMM 27-84-00:

On each wing there are two types of actuators. Those installed on track 2 and 3 are referred to as type A actuators. The other actuators are referred to as type B actuators. Type A actuators have a larger diameter and a first-stage reduction gear.

The type A actuators drive slat 1 (the large inboard panel) through a lever-and-linkage gear; the type B actuators drive slats 2–7 through a pinion that engages a gear rack on the track. Type A is larger and carries an extra reduction stage because the inboard panel is the biggest and most heavily loaded — it needs more torque at lower speed.

4.2 The torque limiters — mechanical fuses at two levels

The transmission's protection against a jam is purely mechanical, at two scales. Per AMM 27-84-00:

There is a system torque limiter in the transmission system in each wing, and each actuator has a torque limiter. The torque limiters stop the transmission of too much torque to the actuators and the aircraft structure. The torque which causes a torque limiter to stop the transmission is referred to as lock-out torque. A mechanical indicator on each torque limiter shows when a lock-out torque has occurred.

A jam anywhere drives torque up; the nearest limiter reaches its lock-out torque and locks that line before the shaft is twisted or the structure is loaded — and pops a mechanical indicator so maintenance can find where. This is the mechanical last line; the electrical monitoring (§6) is layered on top of it.

4.3 The torque shafts — built to flex with the wing

The shaft is not one rigid rod; it is a string of stainless-steel and carbon-fibre (CFRP) sections jointed to follow the wing. Per AMM 27-84-00, the torque shafts are made partly of stainless steel and partly of carbon fibre reinforced plastic (CFRP), and:

Universal joints and splined articulating joints connect the torque shafts ... This permits small angular changes of alignment and wing flexing. Each shaft has at least one sliding end connection. The male part of the sliding end connection has an indicator groove. It shows when the engagement of the male and female part is below a minimum.

The lesson for the crew is not the materials but the principle: this is a continuous mechanical shaft running the full span, designed to absorb the wing's flexing through universal joints and to swallow thermal growth through sliding joints. The longer the mechanical run, the more places a jam or disconnect can hide — which is precisely why the electronic monitoring sits at the far tip end.

4.4 The slat tracks — and a fail-safe stop

Each panel rides on tracks. Per AMM 27-84-00, slat tracks 1 through 4 are I-section titanium beams, and tracks 1 and 4 are not driven; the rest carry the drive. Per AMM 27-84-00:

The slat tracks 5 thru 16 which are of inverted-U section, are machined from steel. They have a steel gear rack which bolts attach to the track channel. The gear rack engages with a pinion.

Two details matter to a pilot. First, the slat retracts into the wing in a way that touches the fuel system. Per AMM 27-84-00:

The track retracts through holes in the front spar into a sealed container which makes a projection into the fuel tank.

Second, the tracks carry fixed stops at each end, and the rear one is a quiet fail-safe. Per AMM 27-84-00, The forward stop (retract stop) prevents damage to the track container, the slat and the leading edge structure when the slat retracts, and:

The rear stop (extended stop) holds the slats when the transmission system is disconnected.

That rear stop is the fail-to-safe detail: if the transmission disconnects with the slats out, the airflow does not blow them back to retracted (a sudden loss of lift) — the extended stop holds them out (reasoning, from the cited stop function).

5. The Wing Tip Brake — the last bite

The WTB is the failure brake at the far end of each wing's shaft. 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 AMM 27-81-00:

An electrohydraulic pressure-off brake (referred to as a WTB) is located near the end of the transmission system in each wing. The WTB stop and hold the transmission if the SFCCs find some given types of failures. Each WTB has two solenoid valves. Each solenoid valve controls one section of the WTB. The Green and the Blue hydraulic systems supply the WTB. ... Each SFCC supplies one solenoid on each WTB. When the WTBs de-energize the transmission locks.

So the WTB is, like the POB, spring-applied and pressure-released, but split into two halves on two hydraulic systems and two SFCC solenoids. The redundancy is deliberate — it stops a single hydraulic loss from falsely braking the slats. Per AMM 27-81-00:

When the solenoids are energized, the fluid pressure moves the piston to release the spring force holding the brake on. ... 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.

One piston can hold the brake released; so a single Green or Blue loss does not latch the slats. The corollary is the opposite: latching needs both solenoid circuits de-energised — which is exactly what the SFCC does when it confirms a dangerous fault. And the latch is absolute in flight. Per FCOM DSC-27-30-10, the WTBs cannot be released in flight.

[!warning]- "The WTB cannot be released in flight" is a safety choice, not a hardware limit.

A latched WTB means the SFCCs have just caught something that could roll the aircraft or break the structure — asymmetry, runaway or overspeed. Re-energising a suspect transmission in the air, and letting a possibly-broken chain drive again, is far more dangerous than landing with the slats frozen where they are. So the latch is held until the ground, where it is reset by the proper procedure (§6). After a confirmed latch, you plan an approach for the configuration you have — the full procedure is in QRH Jam and Loss of Control.

6. Monitoring — asymmetry, runaway, and the two-vote latch

6.1 What the SFCCs watch, and the level ceiling

The two SFCCs continuously compare the slat sensors. Per AMM 27-81-00:

The SFCCs monitor the power transmission system for these failure conditions: asymmetry (a position difference between the two APPU), 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, system jam, half speed, low hydraulic pressure.

The two most-confused conditions use different comparisons (anchor question 5):

• Asymmetry = left APPU vs right APPU. The two wing tips have reached different positions — the most dangerous case, because unequal lift across the span produces a rolling moment.
• Runaway = APPU vs FPPU. The actual tip position disagrees with the commanded/source position at the PCU — the slats are moving the wrong way, or not stopping.

Both are derived from the same three eyes (two APPUs at the tips, one FPPU at the PCU), but compared against different references. There is also a hard ceiling on how loud a slat fault can get. Per AMM 27-81-00:

Failures in the slat system will not give any level 3 warnings. The Electronic Instrumentation System (EIS) system shows level 2 cautions to the flight crew.

Slat-system faults top out at a level 2 caution — never a red level 3 master warning. (The one red high-lift item, CONFIG SLATS NOT IN T.O CONFIG, is a take-off configuration warning, not a slat-system fault — §7.)

6.2 The two-vote latch

A single SFCC's suspicion does not lock the slats. It acts on itself, then asks the other SFCC to agree. Per AMM 27-81-00:

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

That is the key design: the detecting SFCC throws an F/CTL SLAT SYS 1(2) FAULT with SLATS SLOW, but the WTBs only latch — escalating to SLATS LOCKED — when the other SFCC confirms. If the second SFCC does not confirm, the system records a PPU fault instead and does not lock the slats. The two-vote rule is what stops one computer's misread from welding healthy slats in place — and it is the dividing line between a recoverable SLAT SYS FAULT (half speed, carry on) and a frozen SLATS LOCKED.

7. Confirmed versus unconfirmed — what you can still clear in the air

The single most useful correction in this chapter: a slat fault is not automatically "land with it". The SFCCs split faults into unconfirmed and confirmed, and the unconfirmed ones can be cleared in flight by a slat/flap lever recycle. Per AMM 27-81-00:

When you move the slat/flap control lever opposite to the commanded position, you can reset the subsequent failures: a CSU fault, a CSU out of detent, a system jam, a system half speed fault, an unconfirmed PPU wiring fault, an unconfirmed asymmetry, an unconfirmed runaway, an unconfirmed uncommanded movement, PCU pressure switch logic fault, PCU solenoid electrical fault, an unconfirmed WTB solenoid electrical fault, a PCU high speed solenoid valve jam, a PCU overspeed.

Read that list carefully: a system jam and a PCU overspeed are both clearable by a lever recycle — until they are confirmed and escalated. So "jam" and "overspeed" do not automatically mean a frozen configuration; whether they do depends on whether the two SFCCs confirmed them. When a slat fault appears, the first read is whether the ECAM offers FLAP LEVER RECYCLE — if it does, the fault is in the recoverable class and a single recycle may restore normal operation.

Once a fault is confirmed, the WTB latches in one of two ways, and the way decides which ground reset is needed. Per AMM 27-81-00:

The WTB is de-energized and hardware latched when the SFCCs find: a CSU disadjustment, a confirmed overspeed, a microprocessor cross lane failure, a WTB manual release failure, a fault in the WTB, detected by the WTB engagement test.

The WTB is de-energized and software latched when the SFCCs find: a WTB solenoid valve fault, a confirmed PPU wiring failure, a confirmed asymmetry, a confirmed runaway, a confirmed uncommanded movement, an installation coding failure, an operation coding failure, an unsuccessful AIT.

Note the split that catches people out: a confirmed overspeed is hardware latched, while a confirmed asymmetry is software latched — they are not the same latch. That distinction sets the ground reset. Per AMM 27-81-00:

You can start the SFCC/WTB RESET from the MCDU when the aircraft is on ground ... It resets the hardware and software latches of the WTBs, the SFCC and all start-up inhibitions.

The SFCC resets when you open and close the related circuit breakers of the SFCC. The SFCC reset also resets the software latches of the WTBs and the start-up inhibitions of the SFCC when the automatic integrity test was successful.

So three resets, in a clear hierarchy:

[!warning]- A slat fault is not a sentence to land with it — until the two SFCCs have confirmed it.

The instinct "any SLAT SYS FAULT means I land with the slats I have" is wrong for the unconfirmed class. An unconfirmed asymmetry or runaway, a system jam, or a PCU overspeed can often be cleared in flight by a single lever recycle, and the slats return to normal. Only a fault the second SFCC confirms latches the WTB into SLATS LOCKED, and that is the one you cannot release in flight. The presence of a FLAP LEVER RECYCLE prompt is your signal that self-recovery is on the table.

8. SLAT TIP BRK FAULT is not SLATS LOCKED

Two messages with "tip brake" in their meaning are constantly confused, and they are at different layers. SLATS LOCKED (above) means a WTB has actively braked the transmission — the slats are frozen. SLAT TIP BRK FAULT is a watchdog on the WTB hardware itself — the slats are not braked and still extend and retract normally. It has two sources.

The first is a daily self-test of the brake. Per AMM 27-81-00:

The SFCC 1 automatically starts daily the slat WTB engagement test during the flight phase 9 (after landing). ... If the WTB engagement test was not done or unsuccessful for 10 consecutive days: on the EWD the warning SLAT TIP BRK FAULT comes on, the LAST LEG REPORT, GROUND REPORT or GROUND SCANNING shows the maintenance message PERFORM WTB ENGAGEMENT TEST.

The second is the maintenance manual-release detection: with the WTB's maintenance device in the 'M' position, a low Green or Blue pressure sensed, and the proximity switch indicating target FAR, SFCC 1 reads a manual-release condition, hardware-latches the WTBs and, per AMM 27-81-00, it shows on the EWD the warning SLAT TIP BRK FAULT.

The takeaway: SLAT TIP BRK FAULT reports a problem with the brake device or its self-test (ten days without a successful test, or a manual release), not a braked transmission. Seeing it does not mean "land with a frozen configuration" — the slats are still operable. SLATS LOCKED, by contrast, is the real lock-out. The full ECAM handling of both lives in High-Lift Failures; the point here is to keep the two firmly apart.

9. The hydraulic dependency matrix — why Green hurts most

Combine two FCOM facts — slats use Green + Blue, flaps use Yellow + Green, and a lost hydraulic system means the corresponding surfaces run at half speed — and a matrix every pilot should carry falls out (integrative reasoning from FCOM DSC-27-30-10, not a verbatim table):

                 GREEN     BLUE     YELLOW
   SLATS           ●         ●         —        slats use Green + Blue
   FLAPS           ●         —         ●        flaps use Yellow + Green

   Loss of one system:
   ┌────────────┬──────────────────────────────────────────────┐
   │ GREEN lost │ slats half speed AND flaps half speed (shared)│
   │ BLUE  lost │ slats half speed; flaps unaffected            │
   │ YELLOW lost│ flaps half speed; slats unaffected            │
   ├────────────┼──────────────────────────────────────────────┤
   │ GREEN+BLUE │ slats stop (both motors dead); flaps still    │
   │            │   have Yellow → half speed                    │
   │ YELLOW+GRN │ flaps stop; slats still have Blue → half speed│
   └────────────┴──────────────────────────────────────────────┘

Green is the worst to lose because it is the only system the slat side and the flap side share: lose Green and both high-lift surfaces drop to half speed at once. Blue touches only the slats, Yellow only the flaps, so losing either degrades just one. In a double-hydraulic case the matrix decides what survives — a Blue + Yellow loss, for instance, leaves both slats and flaps running on Green alone, both at half speed. The operational consequence is always the same shape: when Green is involved, plan to configure earlier, because the time margin to get the slats and flaps out shrinks. The detailed double-failure procedures are in the QRH and High-Lift Failures.

10. Slat angles and a PFD trap

The lever's five positions set the slats to fixed angles. Per FCOM DSC-27-30-20, the slat angles by lever position:

Two points: the slats reach 16° at lever 1 regardless of whether the flaps select CONF 1 or CONF 1+F (the airspeed-driven flap choice does not change the slats), and the slats do not increase from lever 3 to FULL — both give 23°; FULL adds only flap angle.

[!note]- The EWD shows slightly larger slat angles than the FCOM controls table — which is right?

The position-indication source gives a different set. Per AMM 27-85-00, the EWD reads Slats in 0° ... Slats in 17° ... Slats in 21° ... Slats in 24° for positions 0/1/2/3-FULL — about one degree more than the FCOM controls table's 0/16/20/23. They are two different conventions: FCOM 16/20/23 is the pilot/controls figure, AMM 17/21/24 is the EWD indication figure. For flying and teaching, use the FCOM 16/20/23; the AMM figures are only for reading against the EWD scale, not a correction to FCOM.

There is also a transient PFD warning worth pre-empting. Per FCTM PR-NP-SOP-120:

OVERSPEED WARNING is based on the actual Slats/Flaps surface position. Therefore, during Slats/Flaps transition, the dynamic acceleration of the airplane may lead to a temporary OVERSPEED WARNING even if the current speed is out of the red and black strip displayed on the PFD. In this situation, there are no operational consequences.

Because the overspeed warning watches the actual surface position while the PFD speed scale follows the selected (lever) position, a brief overspeed warning can appear during a slat/flap transition with no real exceedance — and with no operational consequence.

11. Flying the slats — a few scenes

1. Normal extension to CONF 1. Lever to 1; the CSU signals both SFCCs; both valve blocks drive their motors (Green and Blue) full speed; the differential gearbox outputs full speed; the shaft drives all seven slats per wing to 16°; the FPPU confirms, the SFCCs drop the POBs and the slats lock. The S index on the EWD tracks to position. A few seconds, a small nose-down change, and more available lift.
2. Configuring with Green lost. The Green motor's POB holds; the slats run on the Blue motor alone → half speed. And because the flaps share Green, they are also at half speed. Expect SLATS SLOW, configure earlier, and do not mistake the slow travel for a jam.
3. Configuring with Blue lost. The slats run on the Green motor alone → half speed; the flaps are unaffected (Yellow + Green). "Slats slow, flaps normal" is the signature that Blue is the missing system.
4. Asymmetry caught. One side's chain binds; the left and right APPUs diverge; the detecting SFCC de-energises its WTB circuit and shuts its valve block down, posting F/CTL SLAT SYS 1(2) FAULT with SLATS SLOW. If the other SFCC confirms, the WTBs software-latch: SLATS LOCKED, an amber S LOCKED on the EWD, a placarded max speed. The slats freeze at their current angle — no in-flight release — and you plan the approach for that configuration (QRH Jam and Loss of Control).
5. Slats not in take-off configuration. Advancing the thrust levers with the slats outside an allowed take-off setting triggers CONFIG SLATS NOT IN T.O CONFIG — a red level 3 configuration warning (not a slat-system fault), which the take-off configuration test is there to catch on the ground beforehand.
6. One SFCC lost. Both slats and flaps go to half speed (each SFCC owns one slat motor and one flap motor). Same symptom as a single hydraulic loss — half speed — but a different root cause: a computer, not a hydraulic system.

Self-test

[!note]- Q1. How many motors drive the slat PCU, which systems feed them, and why is a single hydraulic loss "half speed, not stop"?

Two independent hydraulic motors, fed by Green and Blue, coupled by a differential gearbox. If one system fails, that motor's POB applies and holds it, and the differential gearbox lets the remaining motor drive the whole transmission at half the speed but full torque — so the slats still reach the selected position, just twice as slowly. Only losing both Green and Blue stops the slats. (Losing one SFCC also gives half speed, but to both slats and flaps.)

[!note]- Q2. From the PCU to the wing-tip slat, what does the power pass through, and what is slat-only versus shared?

PCU (two motors → differential gearbox) → Tee-gearbox (splits to both wings) → system torque limiter (per wing) → right-angle gearboxes (wing root) → bevel gearbox (into the leading-edge line) → torque shafts/steady bearings → type A actuators ×2 (slat 1) + type B actuators ×12 (slats 2–7) → WTB and APPU at the tip. Fourteen actuators per wing. The slat transmission — its PCU output, shafts, WTBs — is separate from the flaps; what is shared is the two SFCCs and the Green hydraulic system.

[!note]- Q3. Of Green, Blue and Yellow, which loss hurts the high-lift system most, and why?

Green. Slats use Green + Blue and flaps use Yellow + Green, so Green is the only system the two sides share: lose Green and both slats and flaps drop to half speed at once. Blue affects only the slats, Yellow only the flaps. Extremes: Green + Blue lost stops the slats (flaps still have Yellow → half speed); Yellow + Green lost stops the flaps (slats still have Blue → half speed). When Green is involved, configure earlier.

[!note]- Q4. When does a slat WTB latch, why can it not be released in flight, and what can you still clear in the air?

A WTB latches when the SFCCs confirm a dangerous condition — asymmetry, runaway, overspeed (and CSU disadjustment, etc.). It is spring-applied / pressure-released, and latching needs both SFCC solenoid circuits de-energised; a single Green or Blue loss does not latch it. It cannot be released in flight because a latch means a possibly roll-inducing or structurally-threatening fault was just caught — re-driving a suspect chain is more dangerous than landing frozen. But unconfirmed asymmetry/runaway, a system jam, and a PCU overspeed are clearable in flight by a lever recycle — confirmation by the second SFCC is what makes the latch permanent until a ground reset.

[!note]- Q5. How do the SFCCs tell asymmetry from runaway — same sensors?

Same three eyes (two wing-tip APPUs + one PCU FPPU), different comparisons. Asymmetry = left APPU vs right APPU (the two tips reached different positions → a rolling moment, the most dangerous case). Runaway = APPU vs FPPU (the tip disagrees with the commanded/source position → moving wrongly or not stopping). Either trips the detecting SFCC to shut down and post SLAT SYS FAULT; only a second-SFCC confirmation software-latches the WTBs and escalates to SLATS LOCKED.

[!note]- Q6. SLAT TIP BRK FAULT appears — must you land with the slats you have?

No. SLAT TIP BRK FAULT is a watchdog on the WTB hardware/self-test — ten consecutive days without a successful WTB engagement test (run automatically after landing), or a maintenance manual-release detection. The transmission is not braked; the slats still extend and retract normally. It is a different layer from SLATS LOCKED, which is an actual WTB lock-out freezing the slats. Do not confuse the brake's health watchdog with a braked transmission.

Key takeaways

The slats reduce to the same idea as the flaps, driven from the same SFCCs and the same Green: make both wings move from one rotation, watch the far tips for any divergence, and brake the shaft the moment they disagree — while leaving an in-flight escape (the lever recycle) for the faults that were never truly confirmed.

References

Per FCOM DSC-27-30-10 (Flaps and slats — system components; seven slats, electrically-controlled/hydraulically-operated; two-motor differential-gearbox PCU; slats use Green + Blue; POB locks the transmission; APPUs measure asymmetry; WTB triggers and "cannot be released in flight"; FPPU/IPPU; slat/flap WTB independence; half-speed notes for one SFCC and one hydraulic system). Per FCOM DSC-27-30-20 (Flaps and slats controls and indicators — slat angles by lever position 0/16/20/23/23; S LOCKED indication). Per AMM 27-84-00 (Slats hydraulic actuation and power transmission — hydromechanical slat PCU; Green + Blue supply; differential gearbox half speed/full torque; slat PCU has no internal T-gearbox; four solenoid valves and low/high/low speed; power-transmission component list; fourteen actuators, type A/B; system and actuator torque limiters and lock-out; torque-shaft steel/CFRP, universal/sliding joints; slat tracks 1–4 titanium/5–16 steel, track container into the fuel tank, forward/rear stops; SFCC1-Green / SFCC2-Blue split). Per AMM 27-81-00 (Slats electrical control and monitoring — WTB control, Green + Blue supply, both-solenoid latching; WTB pressure-off disc brake and single-piston release; transmission monitoring conditions; level 2 ceiling; asymmetry abnormal shutdown and two-SFCC confirmation → SLATS LOCKED; "always obeys the last correct control lever movement"; flap-lever-recycle clearable list; hardware vs software latch; SFCC reset and MCDU SFCC/WTB RESET; daily WTB engagement test → SLAT TIP BRK FAULT; manual-release detection → SLAT TIP BRK FAULT). Per AMM 27-85-00 (EWD slat/flap position angles 0/17/21/24/24). Per FCTM PR-NP-SOP-120 (transient OVERSPEED WARNING during slats/flaps transition, no operational consequence). Hydraulic-source dependency (Green + Blue) per ATA-29. Items flagged as reasoning — the Green-is-worst hydraulic dependency matrix, the three-step speed-profile rationale, and the rear-stop fail-safe inference — 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.