Flap/Slat Components And Working Principle

22 January 2021, 12:25

Flaps and slats are secondary control surfaces that contribute to the lift force on the aircraft by increasing wing area and wing camber. These control surfaces used during take-off and landing are not used during cruise flight because they create induced drag. Although flaps and slats can be controlled independently from each other in old type aircrafts, this is not possible in new generation aircraft. The flap and slat are controlled by a single lever in the center pedestal area and open and close at the same time. (The slats close later than the flap to help prevent stall.)

Although many types of aircraft have different components with different names, the operating principle is generally very similar. In this article, we will talk about some Flap/Slat components and working principles used in Airbus Aircrafts.


Flaps are the secondary control surfaces that are deployed and retracted by the rotation force provided by the Rotary Actuators on the trailing edge of the wing through the PCU (Power Control Unit) shaft.


Slats are secondary control surfaces on the leading edge of the wing. The force received from the shaft of the Slat PCU (Power Control Unit) is transmitted to the slat by the actuators for each slat and the slat deployment-retraction operation is provided.

Slat and Flap Control Lever:

Slat and Flap control is provided by this lever in the cockpit. There are positions 0,1,2,3, FULL. 0-Cruise, 1-Hold [If the speed decreases below 210 KIAS (Knot-Indicated Airspeed) the flaps retract automatically.], 2 and 3-Take off, FULL-Landing.

WTB (Wing Tip Brakes):

WTB that located on the transmission shaft near wingtip is used to lock the flap or slat in cases such as asymmetry, overspeed, unwanted movement. However, it can be reset on the ground.

Interconnecting Strut:

It is located between the inner and outer flap. If it senses a possible out-of-limit differential movement difference between two flaps, it sends information to SFCC (Slat And Flap Control Computer). In addition, it absorbs energy in case of disconnection of one of the elements that provide the flap movement.

FPPU (Feed-back Position Pick-off Unit) – IPPU (Instrumentation Position Pick-off Unit)APPU (Assymmetry Position Pick-off Unit):

FPPU, APPU and IPPU are units that can be used interchangeably. FWC (Flight Warning Computer) receives information from IPPU for warnings and position information on ECAM. SFCC (Slat / Flap Contol Computer) monitors Slat and Flap by FPPU on PCU.

Flap Track 4 Sensor Strut:

It allows limited movement between the flap and the intermediate station and provides position data to SFCC.

Torque Limiter

There are 4 ea Torque Limiters for flaps and slats on the right and left wings. It protects the structural elements in the system against a possible overload as a result of transmission failure.

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