The Airbus A320’s Slat and Flap System: Part 1-Technical Background

Understanding how the A320’s Flaps and Slats work is exactly what will help you understand them when they don’t.

Join us in this training and finally develop a solid understanding of the A320’s Flap and Slat system.

Flap and slat problems whilst seemingly innocuous, can present the crew with a number of different problems. 

As such, they require a solid understanding of the system function and design.

And when you understand the system in this manner, you’ll find that the operational procedures required will make a lot more sense. 

In this deep dive, we’re going to start at the very beginning because understanding just how the system is designed to work is exactly what will help you understand just how it behaves when it doesn’t.

Trust me.

Let’s get inside…

The Three Types of Slat and Flap Problems:

There are basically three types of problems that can affect the slats and flaps.

The first is a computer fault within an SFCC (Slat Flap Control Computer).

The second is due to activation of the Wing Tip Brakes (WTB), which will lock the slats, flaps or both, at a particular position.

The third are slat and flap problems that result from failures within the relevant hydraulic system(s).

How it all fits together.

Let’s start with a bit of tech review.

The A320 has:

  • 5 slats on the leading edge of each wing.
  • 2 flaps on the trailing edge of each wing, one outer flap and one inner flap.

Let’s meet SFCC 1 and 2:

The slat and flap surfaces are controlled by two Slat and Flap Control Computers which are also known as SFCC 1 and SFCC 2.

Each SFCC has two channels.

One channel controls the slats and one channel controls the flaps.

Let’s look at the Slat channel first.

I’ve greyed out the Flap channel to make it less distracting.

The A320's SFCC Slat Channels use Blue and Green Hydraulics.

Each SFCC Slat Channel controls a motor.

  • The SFCC #1’s Slat Channel controls a motor which uses Blue Hydraulic fluid.
  • The SFCC #2’s Slat Channel controls a motor which uses Green Hydraulic fluid. 

Both of these motors are connected to a single Differential Gear Box (DGB).

The Differential Gearbox takes the mechanical power from the motors and uses it to move the slat surfaces out on the wing.

So, the takeaway here is:

  • Each SFCC has a Slat Channel.
  • Each SFCC Slat Channel is connected to a Motor.
  • The Slat Channel on SFCC#1 uses Blue Hydraulic pressure to drive its motor.
  • The Slat Channel on SFCC#2 uses Green Hydraulic pressure to drive its motor.
  • The two motors drive the Differential Gear Box which in turn moves the slat surfaces.

This design allows either SFCC to use its motor and through the DGB, drive all slat surfaces, using its respective hydraulic pressure.

For example, imagine that SFCC 2 fails or if there is a loss of Green Hydraulic pressure.

In this case, SFCC 1 can drive all slat surfaces through the DGB using its motor and Blue Hydraulic pressure.

On the other hand, if SFCC 1 fails or Blue hydraulic pressure is lost, SFCC 2 can use its motor and through the DGB, drive all slat surfaces using Green Hydraulic pressure.

Look at the diagram to make sure you understand this.

With both SFCC 1 and SFCC 2 systems working properly everything is well-powered and the slat surfaces extend and retract at their normal speeds.

With one SFCC or one hydraulic system inop (Blue or Green), the Slats will operate at half speed and you will see a SLATS SLOW message on the STATUS page.

Now let’s take a look at the SFCCs’ Flap Channel.

It’s built in pretty much the same way as the Slat Channel, but I’ll go through it in the same way just to make sure you get it.

Each SFCC Flap Channel controls a motor which drives the flaps.

  • The SFCC #1’s Flap Channel controls a motor which uses Green Hydraulic pressure.
  • The SFCC #2’s Flap Channel controls a motor which uses Yellow Hydraulic pressure. 

Just like the Slats, each of these flap channel motors are connected to a single Differential Gear Box (DGB). 

The DGB allows either motor to drive all flaps using its respective hydraulic fluid source.

That means SFCC 1 can use its motor and through the DGB, drive all flaps using Green Hydraulic pressure. 

This comes in handy if SFCC 2 fails or if there is a loss of Yellow Hydraulic pressure.

And it also means that SFCC 2 can use its motor and through the DGB, drive all flaps using Yellow Hydraulic pressure.

As you may have guessed, this comes in handy if SFCC 1 fails or if there is a loss of Green Hydraulic pressure.

With both Flap Channels of SFCC 1 and SFCC 2 systems working properly, the flap surfaces will extend and retract at their normal speeds.

With one SFCC or one hydraulic system inop (Green or Yellow), the Flaps will operate at half speed and you will see a FLAPS SLOW message on the STATUS page.

In the graphic above, we can see the entire system as discussed thus far.

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The Wingtip Brakes:

I’m not sure why Airbus decided to call them wingtip brakes, but anyway, I’ll chalk that up to another translation issue.

These are essentially brake units that lock the slat and flap surfaces and prevent them from moving any further.

They are activated when:

  • An asymmetry is detected in the slats or flaps.
  • A mechanism overspeed has been detected in the slats or flaps.
  • A symmetrical runaway has been detected in the slats or flaps.
  • An uncommanded surface movement has been detected in the slats or flaps.

Once activated, they cannot be released in flight, which means that the affected surfaces will be locked.

It’s important to understand that:

  • There’s a separate set of wing tip brakes for the flaps, and if the flaps have been locked (Flap WTB have activated), the slats can still be operated.
  • There’s a separate set of wing tip brakes for the slats, and if the slats have been locked (Slat WTB have activated), the flaps can still be operated.
  • There is also the somewhat unlikely scenario in which both the slat and flap wingtip brakes activate, leaving them both locked.

Wingtip Brakes are provided for Slat and Flap surfaces on each wing and are hydraulically powered.

  • Blue and Green hydraulics power the Slat WTBs.
  • Blue and Green hydraulics power the RH Flap WTBs.
  • Blue and Yellow hydraulics power the LH Flap WTBs.

I hope this helped put some of the technical background of the Slat/Flap system into perspective.

This is a necessary evil because understanding the system is critical to understanding the differences in the way the Slat/Flap system can fail.

All too often, I see that pilots do not correctly understand slat and flap failures at the technical level which creates unnecessary problems when they attempt to deal with them inflight.

In Part 2 of this lesson, we’ll take a look at a wide variety of Slat/Flap failures and you’ll quickly realise just how useful the knowledge we’ve developed here, is.

Good on’ya for sticking it out.

Be kind, be smart, fly well.

Until next time…

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Meryem

Very happy to read and thanks for the clear expilanation

Louis

This is awesome!!!

Mike

do you know what material used for A320 flap guide rail

CharlieB1997

Your content is incredible. PLEASE keep it coming!

flyberg

Loved the schematics, so much better than the FCOM…
Thanks!

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G'day Mate! I'd love to hear your comments!x
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