Airbus A350 High Lift System: Slats, Flaps & Differential Flap Setting (DFS)

01 · System Overview

The A350 High Lift System in One Mental Model

On commercial airliners prior to the A350, leading-edge slats and trailing-edge flaps were rigid, uniform mechanisms deployed solely for low-speed flight—takeoff, approach, and landing. Once retracted in cruise, the wing profile remained fixed regardless of fuel burn, altitude, or structural flex.

The Airbus A350 High Lift System (HLS) fundamentally rewrites this paradigm. By dividing the system into 7 leading-edge devices and 2 trailing-edge flap panels per wing—and inserting an Active Differential Gearbox (ADGB) between the inboard and outboard flap sections—the A350 enables autonomous, independent flap scheduling across all flight phases.

02 · Command & Computation

Cockpit Inputs, CSUs, and the Active/Active SFCCs

Crew commands originate at the pedestal-mounted FLAPS control lever (an LRU). The lever mechanism features five discrete gated positions: 0, 1, 2, 3, and FULL.

To move the lever, the pilot must lift a trigger to disengage the mechanical detent. To prevent unintended full-range lever sweep in a single movement, two mechanical baulks are positioned at gate 1 and gate 3.

Command Sensor Units (CSUs)

The lever arm drives two independent Command Sensor Units (CSU 1 and CSU 2):

• CSU 1 connects to SFCC 1; CSU 2 connects to SFCC 2.
• Each CSU contains 4 isolated sensors: 2 dedicated to the slat channel and 2 dedicated to the flap channel.
• If a single sensor fails inside a CSU, its associated SFCC channel is declared INOP, and the system drops into half-speed operation.

SFCC Architecture & Reset Provisions

High-lift computation is performed by two interchangeable Slat Flap Control Computers (SFCC 1 and SFCC 2) operating in an active/active concept. Both computers calculate surface commands simultaneously, exchanging data across a dedicated high-speed digital cross-talk link to compare results, execute monitoring functions, and validate sensor integrity.

In the cockpit overhead Integrated Control Panel (ICP), four reset switches allow individual channel recovery:

• CAPT RESET Panel: SLATS 1 and FLAPS 1 reset switches for SFCC 1.
• F/O RESET Panel: SLATS 2 and FLAPS 2 reset switches for SFCC 2.

Beyond crew lever inputs, the SFCCs interface via the AFDX network with the ADIRS (calibrated airspeed, altitude, angle of attack), PFCS PRIMs and SECs (sending flap positions for the Adaptive Droop Hinge Function and receiving cruise camber commands), the LGERS (weight on wheels), the Propulsion Control System (PCS) (which inhibits slat extension if engine cowls are unlatched/open), and the HMCA (monitoring Green and Yellow hydraulic pressure levels).

03 · Leading Edge System

Slat & Droop Nose Architecture: The Hybrid PCU

Each A350 wing carries 7 leading-edge devices:

1. Droop Nose Device (DND): Installed at the inboard wing root near the fuselage. Supported on 5 hinges, of which only 2 are driven (hinge arms 2 and 4) and 3 are non-driven. Driven by a plug-in geared rotary actuator (GRA) connected through an actuator lever and drive link.
2. Slats 2 through 7: Six outboard slats. Slats 2 through 6 are each supported and driven on 2 tracks using a rack-and-pinion drive mechanism. Outer Slat 7 is supported on two driven tracks plus a third non-driven track for additional aero-structural support.

The Hybrid Slat Power Control Unit (PCU)

Unlike traditional twin-hydraulic PCUs, the A350 Slat PCU employs a revolutionary hybrid electro-hydraulic drive:

• Electrical Motor (M): Controlled by the SFCC 1 slat channel. Powered by 230 VAC Emergency Bus 1 (EMER 1) through a Motor Control Electronic (MCE) module and a Remote Control Circuit Breaker (RCCB). Fitted with a solenoid-released Power-Off Brake (POB).
• Hydraulic Motor (VDHM): Controlled by the SFCC 2 slat channel. Powered by the Yellow hydraulic system. Equipped with a valve block, enable solenoid valve, brake solenoid valve, Electro-Hydraulic Servo Valve (EHSV), and Pressure-Off Brake (POB).
• Differential Gearbox: Both motors drive into a central speed-summing differential gearbox. In normal operation, both motors operate simultaneously at full speed.

Zero-Hydraulic Slat Extension: If the Yellow hydraulic system is lost, the hydraulic motor POB applies, and the electrical motor drives the transmission alone at half speed. On the ground, operation of the electric motor is normally inhibited unless an engine is running, one hydraulic system is pressurized, or a dedicated OMS BITE menu is activated.

Slat Monitoring & Wing Tip Brakes (WTB)

Transmission torque shafts run spanwise through bevel gearboxes to drive 12 geared rotary actuators per wing (2 per surface). System health is tracked continuously:

• FPPU (Feedback Position Pick-off Unit): Mounted on the PCU differential gearbox to measure overall transmission position and speed.
• TSUs (Torque Sensor Units): Located on the LH and RH output shafts of the PCU to measure shaft torque and protect against transmission jamming or over-torque.
• APPUs (Asymmetry Position Pick Off Units): Mounted at each wing tip to measure end-of-line position and detect wing-to-wing asymmetry or transmission shaft rupture.
• WTBs (Wing Tip Brakes): Power-off brakes installed at each wing tip. Each WTB has two internal solenoids. Energizing either solenoid releases the brake; de-energizing both solenoids clamps the transmission in its current position to prevent runaway or uncommanded movement.
• Actuator Torque Limiters: Each geared rotary actuator includes an integrated mechanical torque limiter that slips if a surface jams, causing a visual pop-out indicator pin to trip. During reverse cycling, the limiter automatically re-engages once torque drops, but the mechanical indicator remains popped out for ground maintenance troubleshooting.

04 · Trailing Edge System

New Flap Architecture: Dual Hydraulic PCU & ADGBs

The A350 trailing edge is designed around an Adaptive Droop Hinge Flap (ADHF) geometry. Each wing contains an inboard flap and an outboard flap. The flap panels travel around drop hinges mounted on fixed support beams, numbered from Station 1 (fuselage junction) to Station 4 (outboard drive station), with Station 5 acting as an unpowered outboard guide track.

Dual Hydraulic Flap PCU

The Flap PCU mounted in the fuselage comprises two identical variable-displacement hydraulic motors connected through a speed-summing differential gearbox:

• Motor 1 (Yellow Hydraulic): Controlled by SFCC 1 flap channel.
• Motor 2 (Green Hydraulic): Controlled by SFCC 2 flap channel.
• Each motor has its own valve block, enable solenoid, brake solenoid, EHSV, and Pressure-Off Brake (POB). A single central FPPU monitors the PCU differential output shaft.

Active Differential Gearboxes (ADGBs): The Core Innovation

The breakthrough engineering of the A350 flap system is the insertion of an Active Differential Gearbox (ADGB) on each wing between the inboard flap and outboard flap:

• Each ADGB consists of an electric motor, a solenoid-operated power-off brake (POB), an electronic MCE, a differential gearbox, and a DFPPU (Differential Feedback Position Pick-off Unit).
• Powered electrically by 230 VAC Emergency Bus 2 (EMER 2), under the direct authority of the SFCC 2 flap channel.
• Uniform Flap Mode: When the crew selects a flap position, the Flap PCU drives the entire shaft line while the ADGB electric motors remain unpowered with their internal brakes applied. The ADGB acts purely as a mechanical passthrough gearbox.
• Differential Flap Mode: The SFCC 2 can release the ADGB brake and power the electric motor, adding or subtracting rotational speed to the outboard transmission shaft. This allows the outboard flap to extend or retract completely independently of the inboard flap!
• RAT Power Limitation: Under emergency electrical conditions powered by the Ram Air Turbine (RAT), the high-lift system power draw is limited to 6 kVA to avoid RAT generator stall. ADGB operating speed is automatically reduced accordingly.

Station 2 Moving Damper & Safety Sensors

Because the A350 uses separate flap panels subject to immense aerodynamic loads, advanced monitoring and load-shedding hardware protects the wing structure:

• Station 2 Moving Damper: Installed parallel to the drive strut at Station 2, connecting the fixed support to the flap hinge lever. It acts as an active secondary load path. If the Station 2 drive strut fractures, the moving damper retracts at high velocity to absorb the load and prevent catastrophic overload onto Station 1. A Linear Variable Differential Transducer (LVDT) on the oil accumulator monitors position and fluid level, while a calibrated tension fuse protects against mechanical jamming.
• Load Sensing Drive Struts (LSDSs): Installed at Drive Stations 1 and 4 to measure the axial loads transmitted into the flap surfaces and detect strut disconnects.
• SPPUs (Station Position Pick-off Units): 8 units total (one at each drive station) driven by the flap hinge levers to detect panel skew and asymmetry.
• OTSSUs (Outboard Transmission Speed Sensor Units): Located at the wingtips to detect transmission overspeed or jamming during DFS operations.

05 · Aerodynamic Optimization

Differential Flap Setting (DFS) & Aerodynamic Optimization

The ability to articulate inboard and outboard flaps independently unlocks four distinct aerodynamic capabilities unique to the A350:

1. Take-off Configuration 1+F with DFS

When FLAPS lever 1 is selected on the ground prior to takeoff:

• The Flap PCU first drives both inboard and outboard flaps to a uniform 10.5°.
• The Flap PCU stops, and the SFCC 2 commands both ADGBs to drive the outboard flaps further to 13.5°.
• This tailored spanwise lift distribution produces superior takeoff climb gradients.
• Note: In flight at low speeds (or on maintenance jacks), selecting position 1 results in uniform 12° inboard and outboard flap extension without DFS.

2. Variable Camber in Cruise

During cruise with the FLAPS lever in position 0, the PFCS PRIM computers calculate optimal wing camber in real time based on Mach number, gross weight, center of gravity, altitude, and fuel burn. These small trimming commands are sent to the SFCCs:

• Inboard Flap Range: -2° (flaps up) to +4° (flaps down).
• Outboard Flap Range: -2° (flaps up) to +1° (flaps down).
• Outcome: Shifts the wing’s center of lift forward or aft, minimizing aerodynamic drag at high Mach numbers and directly cutting long-range fuel consumption.

3. Wing Load Control (Climb Phase)

During climb at heavy aircraft weights, the SFCCs command a symmetrical inboard DFS:

• The inboard flaps extend up to +4° while the outboard flaps remain in their clean profile.
• This shifts the center of lift spanwise inboard toward the wing root, significantly alleviating the wing root bending moment during maneuvers and atmospheric turbulence.

4. Lateral Compensation (Roll Trim)

The ADGBs can also command an asymmetrical outboard flap deployment of up to 3° on one wing:

• Generates an aerodynamic roll moment to balance lateral fuel imbalances, engine flameout (OEI) asymmetric drag, or servo-control actuator faults on roll spoilers/ailerons.
• Preserves maximum control authority on the primary ailerons and roll spoilers (spoilers 3 to 7).

Configuration 3 vs. 3+S (Vented Slats)

The High Lift System also alters slat kinematics based on ground versus flight status:

• Position 3 on ground (Takeoff): Slats command to Position 1, Flaps command to Position 3.
• Position 3 in flight (Approach/Landing): Automatically commands Configuration 3+S. The slats deploy to FULL (vented slat configuration with an aerodynamic gap) while flaps remain at Position 3, optimizing low-speed approach stability.

06 · Flight Envelope Integrity

SFCC High Lift Protection Laws & Speed Thresholds

To guarantee flight envelope protection, the SFCCs continuously process calibrated airspeed (CAS), angle of attack (AOA), and altitude from the ADIRS. When critical thresholds are breached, the SFCCs override pilot lever selections:

07 · Failure Modes & Indications

Abnormal Operations, Jamming & Reconfiguration

The A350 HLS features extensive fault-tolerant redundancy designed to maintain safe handling even under multiple electrical, hydraulic, or mechanical losses:

Half-Speed Operation

The system automatically transitions to half-speed drive when redundancy is reduced:

• Slats at Half Speed: Occurs upon loss of one SFCC slat channel, loss of the Yellow hydraulic system, or loss of the 230 VAC electrical supply. The inoperative motor brake applies, and the operational channel drives the speed-summing differential at half rate.
• Flaps at Half Speed: Occurs upon loss of one SFCC flap channel, or loss of either Green or Yellow hydraulic systems.

Total Hydraulic Power Loss

In the catastrophic event of a total dual hydraulic failure (Green and Yellow zero pressure):

• Slats: Continue to operate! Driven at half speed by the electric motor powered by 230 VAC EMER 1 via SFCC 1.
• Flaps: The dual-hydraulic Flap PCU cannot operate, locking the inboard flaps. However, the electric ADGBs (230 VAC EMER 2) can still drive the outboard flaps! The cockpit SFLM displays INNER FLAP LOCKED.

Mechanical Asymmetry vs. Jamming

• Transmission Rupture / Asymmetry: Detected via comparison between LH and RH APPUs (slats) or SPPUs (flaps). The SFCCs command an immediate PCU shutdown and trip the Wing Tip Brakes (WTB) on both wings. The cockpit displays SLATS LOCKED or FLAPS LOCKED.
• Transmission Jamming: Detected by the TSUs (slats) or FPPU/OTSSUs (flaps). The SFCCs command a PCU shutdown with POBs applied (WTBs are not tripped for a pure jam). Flight and maintenance procedures require a recycle operation.
• ADGB Electrical Fault: If an ADGB motor or MCE fails, the system locks the ADGB brake and displays DIFF FLAP in amber on the SFLM. The flaps continue to operate in uniform setting mode.
• Spoiler Droop Interference: If a roll spoiler fails to execute its coordinated droop angle (up to 10° controlled by PFCS PRIM/SEC), the system posts FLAP RETRACT INHIBIT on the SFLM to prevent mechanical surface clash.

08 · Line & Base Maintenance

Maintenance Best Practices: Rigging, Tools & OMT

Maintaining the A350 High Lift System requires adherence to strict safety protocols, specialized ground support tooling, and Onboard Maintenance Terminal (OMT) procedures:

High-Voltage Safety Precautions

The electric motors of the Slat PCU, Flap ADGBs, and THSA receive high-voltage power (230 VAC converted up to 540 VDC via MCEs). Technicians must observe:

• Warning: High Voltage & Hot Surface. The Joule effect generates significant heat in the motor casings and electronic modules. Allow components to cool before handling.
• Hydraulic Depressurization: Before removing hydraulic lines from any actuator, depressurize both Green and Yellow hydraulic systems and system reservoirs. Depressurize the actuator using its manual depress release valve or maintenance key.

Specialized Maintenance Tooling

OMT BITE & Configuration Tables Loading

The PRIMs and SECs latch all flight control faults in Non-Volatile Memory (NVM). When replacing high-lift components, technicians must follow this exact three-step sequence:

1. Fault Confirmation: Un-latch the fault via OMT using the PRIM and SEC NVM ACTUATOR FAULT ERASURE menu, then run a BITE test to verify if the fault reproduces.
2. Component Replacement & Configuration: After installing a new actuator or electronic module (FCRM, EM, or MCE), load the Configuration Tables from the SECs (via REMOTE MODULE CONFIGURATION TEST) before attempting mechanical rigging.
3. PPU Electrical Zero Calibration: For APPUs, FPPUs, DFPPUs, and SPPUs, align mechanical markings using the locking plate and shaft window, then validate and store the electrical zero calibration through the dedicated OMT interactive BITE menu. Finish with a final NVM fault erasure and operational test.

09 · Technical Q&A

Frequently Asked Questions