A320 Slats and Flaps

The A320's high-lift system lets a swept, supercritical wing designed for cruise at Mach 0.78 take off and land at the speeds of a much slower aeroplane. Each wing carries five leading-edge slats and two trailing-edge Fowler flaps. The pilot moves one lever, and two computers extend slats and flaps together to a set combination of angles called a configuration, or CONF.

Because the computers stand between lever and surfaces, the system can do more than follow orders. It refuses to retract the slats when the wing is close to the stall, retracts the flaps by itself before they are overspeeded, and locks the slats or flaps if a surface moves when it should not. Each of these functions explains an indication or a behaviour that A320 crews meet on the line and that type rating examiners like to ask about. How slats and flaps raise lift is covered in high-lift devices, and the machinery common to transport aeroplanes in flap systems.

High-lift system overview

The A320 wing is swept at 25° and fitted with leading-edge slats and trailing-edge Fowler flaps: five slat surfaces and two flap surfaces per wing. Like the flight control surfaces, they are electrically controlled and hydraulically operated. The flaps lever on the centre pedestal is the only crew control, and every lever position commands slats and flaps together.

View full schematic

The high-lift system works with other systems. When the flaps are extended, the ailerons droop 5° to add lift at low speed. Slats 3, 4 and 5 on each wing, the three outboard ones, are heated by bleed air when wing anti-ice is selected; slats 1 and 2 and the flaps are not. The slat/flap position is shown permanently on the upper ECAM display, next to the engine parameters.

SFCCs and power control units

Two slat flap control computers (SFCCs) run the system. Each contains a slat channel and a flap channel, so either computer can control both sets of surfaces. They read the lever, command the drives, monitor the surface positions and run the protections described below, using corrected angle of attack and airspeed from the air data and inertial reference units (ADIRUs).

The slats and the flaps each have a power control unit (PCU): two independent hydraulic motors coupled by a differential gearbox, which drives the surfaces of both wings. The two motors are fed by different hydraulic systems:

Because the two motors share the work through the differential, losing one of them does not stop the surfaces. If one hydraulic system is lost, the slats or flaps it powers move at half speed. If one SFCC is lost, slats and flaps both move at half speed. The architecture of the three hydraulic systems is described in A320 hydraulic system.

The slat computers also matter to the flight controls. The failure of the slat channels of both SFCCs is one of the failures that cause a reversion to pitch alternate law without protection, with roll direct law (see A320 flight control laws and protections).

Configurations and the flaps lever

The flaps lever has five positions: 0, 1, 2, 3 and FULL. Before moving it, the pilot must pull it out of its detent. Balks at positions 1 and 3 prevent the pilot from calling for too much travel in a single movement, and no intermediate position can be selected. Position 1 gives two different configurations, depending on where it is selected.

Selecting position 1 on the ground before take-off gives CONF 1+F. Moving the lever from 0 to 1 in flight gives CONF 1, slats only. The angles are those of the A320 with CFM56 engines. The EASA type certificate data sheet gives A320s with IAE or CFM LEAP-1A engines 27°/40° in CONF FULL at the same 177 kt VFE. The A321's angles and VFEs differ again: CONF FULL is 27°/25° at 190 kt on the A321ceo and 27°/34° at 186 kt on the A321neo. A pilot in a mixed fleet therefore flies the VFE shown on the PFD rather than a remembered figure. With slats or flaps extended, the maximum operating altitude is 20,000 ft.

Some functions depend on the configuration. Speedbrake extension is inhibited in CONF FULL, and the crew retract the speedbrakes before selecting FULL, because their automatic retraction at that moment would cause a pitch-down. On approach the crew select flaps 1 at green dot speed and extend each further step when the speed is below the VFE of the next configuration, as described in A320 characteristic speeds. In a go-around the flaps are retracted one step.

Alpha/speed lock function

Retracting the slats lowers the stalling angle of attack. The alpha/speed lock therefore stops the slats retracting when the wing is already at a high angle of attack or a low speed, for example if flaps 0 is selected after take-off before the aircraft has accelerated. The SFCCs use corrected angle of attack or airspeed from the ADIRUs.

When the flaps lever is moved from 1 to 0, slat retraction is inhibited if:

• the angle of attack is above 8.5°, or
• the speed is below 148 kt.

Once the lock is active, the slats retract only when the angle of attack is below 7.6° and the speed is above 154 kt. Both conditions must be met, and the gap between the two sets of thresholds stops the function cycling. The lock does not activate on the ground below 60 kt. While it is active, A-LOCK pulses in green above the slat indication on the upper ECAM display. No crew action is needed: the slats retract by themselves once angle of attack and speed are back within normal values.

Automatic flap retraction

The automatic retraction system (ARS) protects the flaps from an overspeed after a take-off in CONF 1+F, whose VFE is 215 kt. In CONF 1+F, when the speed reaches 210 kt, the flaps retract automatically to 0, leaving the aircraft in CONF 1 with the slats still extended, and the VFE shown on the PFD changes from that of CONF 1+F to that of CONF 1. On some A321s the VFE of CONF 1+F is 225 kt.

The function also works the other way. In CONF 1, the flaps extend automatically to 10°, giving CONF 1+F again, when the speed falls to 100 kt. After an automatic retraction, CONF 1+F is not available again until the speed is 100 kt or less, unless CONF 2, 3 or FULL has been selected in the meantime.

Wing tip brakes and monitoring

The SFCCs watch for any movement of the surfaces that they did not command. In case of a runaway, an asymmetry between the two wings, an overspeed or another uncommanded movement, they apply the wing tip brakes (WTBs), which lock the slats or the flaps in position. The WTBs are hydraulically powered: green and blue for the slats, green and yellow for the flaps. They cannot be released in flight, and their circuit breakers have red caps to prevent anyone from resetting them.

Slats and flaps are independent. If the flap WTBs are on, the crew can still operate the slats, and if the slat WTBs are on, the flaps still work. The aircraft then lands in whatever configuration the remaining surfaces allow.

The flap attachments are monitored too, by flap attachment failure detection. Four flap disconnect proximity switches feed the two landing gear control and interface units (LGCIUs), which process the signals and pass them to the SFCCs; the LGCIUs do not monitor SFCC failures themselves. Normal operation of the left or right flaps is reported as long as at least one SFCC detects it, and the FLAPS LOCKED warning is triggered when both SFCCs detect an attachment failure.

Indications and slat/flap faults

On the upper ECAM display the slat and flap position appears permanently, with A-LOCK when the alpha/speed lock is active. Before take-off, the configuration warnings CONFIG SLATS NOT IN T.O CONFIG and CONFIG FLAPS NOT IN T.O CONFIG are triggered both by the T.O CONFIG test and when take-off power is set. The F/CTL FLAP/MCDU DISAGREE caution, a disagreement between the flap setting and the one entered in the MCDU, is inhibited when the CONFIG FLAPS warning is present.

Most slat and flap failures leave a controllable aeroplane that must land faster and needs more runway. A slow surface after one SFCC or one hydraulic failure costs only time; a locked surface means an approach in a reduced configuration, with the speed and landing distance corrections given by the procedure (see landing gear, flap and trim malfunctions). For a landing with reduced flap, the GPWS FLAP MODE pushbutton set to OFF inhibits the TOO LOW FLAPS alert, and the LDG FLAP 3 pushbutton inhibits it for planned CONF 3 landings.

Some procedures protect the surfaces on the ground. After an approach in icing conditions, or on a runway contaminated with slush or snow, the flaps are not retracted until after engine shutdown, once the ground crew have confirmed that slats and flaps are clear of ice. In hot weather on the ground, with an OAT above 30 °C, keeping the slats in CONF 1 during a transit avoids the AIR L(R) WING LEAK alerts that overheating around the wing bleed ducts can cause.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.