What is Wing Tip Brake ( WTB )on a320 aircraft
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.
Function of Wing tip brake
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 ( Pressure off brake) POB and WTB is same. Both apply brake and hold transmission shaft.
Purpose of WTB
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 of wing tip brake :
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.
Why Wing Tip Brake (WTB) C/Bs Can't Reset In-flight
The Wing Tip Brake (WTB) C/Bs have red caps on them to prevent them from being reset. The C/B TRIPPED warning on the ECAM indicates the location of the affected C/B. The following panels are monitored.
Activation of Wingtip brakes (WTBs)
Wingtip brakes (WTBs), activated in case of assymetry, mechanism overspeed, symmetrical runaway, or uncommanded movement of the surfaces.
They cannot be released in flight. They use blue and green hydraulic power for the slats and for the right wing flaps, and blue and yellow hydraulic power for the left wing flaps.