Flap and Slat Wing Tip Brake Location

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Why Wing Tip Brake of Flap and Slat Installed at outboard of wing?

The function of wing tip brake to hold and stop rotation of flap/slat transmission shaft when slat/flap control computer sense asymmetry, uncommanded or runways condition of flap/slat.

Wing tip brake(WTB) work similar to pressure  off brake(POB).Pressure off brake installed in power control unit (PCU) and wing tip brake separately placed at wing tip.
The contraction of WTB and POB similar to brake of an aircraft. As the brake has stator, rotor and piston housing similarly we have stator, rotor and piston housing  in WTB and POB .

Pressure Off Brake 


                                                             Fig 1 Pressure  Off Brake

POB consist of friction disc, spring and piston assembly for actuation of spring.
When hydraulic pressure available spring compressed. Spring pressure removed from friction disc and transmission shaft turns freely.
In absence of hydraulic pressure spring pressure acting on friction disc which hold transmission shaft.

Wing Tip Brake


                                                             Fig 2 Wing Tip Brake

                                                   
                                           Fig 3 Wing Tip Brake Installed on Aircraft


WTB is electrical control and hydraulic actuated component. WTB mainly consist of friction disc, piston and solenoid valve.
When Solenoid valve  de-energized hydraulic fluid pressure not acting on piston. Due to absence of piston force on friction disc transmission shaft turns freely.
When Solenoid valve Energized hydraulic fluid pressure act on piston. Piston apply force on friction disc which hold transmission shaft.
Solenoid valve get energization signal from slat/flap control computer.

The function of POB and WTB is same. Both apply brake and hold transmission shaft. We need WTB only when runaway, uncommanded and asymmetry condition of flap/slat arises but POB is sufficient to hold transmission shaft. So, why we need two brake assembly to hold transmission shaft rotation? 

Location: POB installed inboard of wing and WTB installed at outboard of wing.


Aircraft Without WTB

Consider the situation,Flap is extended and transmission shaft broken into two parts due excessive aerodynamic force. Inboard part of transmission shaft hold by POB and outboard part of transmission shaft free to move because we don't have any braking device (WTB) which hold transmission shaft. Thus, there will be movement of flap outboard part which is critical for flight operation and safety of an aircraft. 

Aircraft With WTB 

Aircraft with WTB, If transmission shaft broken then POB will hold inboard part and WTB will hold outboard part. Free Movement of  flap outboard part stopped by placement of braking device WTB.

Thus, aircraft manufacture installed WTP at outboard end for protection of transmission shaft if shaft broken and in normal operation provide addition support to POB when asymmetry, uncommanded and runaways condition occurred. 

Outboard Location of WTB ultimate enhancing the reliability of an aircraft.


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