Operation of the WTB The WTB's lock the transmission system if a failure occurs. If one or the other computer finds a failure: - the computer arms its own WTB circuits ( flap or slat channel ) - the other computer receives a WTB-arm discrete signal. If the second computer also finds the failure: - the computer arms its own WTB circuits - the first computer receives a WTB-arm discrete signal. When both SFCC's receive a WTB-arm signal they energize the related WTB solenoid valves. The WTB's lock the transmission system. If one SFCC does not operate, the other SFCC receives a WTB-arm signal automatically. If the other SFCC finds a failure, one solenoid valve on each WTB gets electri-cal power The WTB can only be reset on the ground through the CFDS.
The WTB will be applied in case of: - asymmetry - runaway - uncommanded movement - overspeed. When both SFCC's find any one of these failures: - the valve block solenoids on the PCU are de-energized - the POB's in the PCU are applied and stop their related hydraulic motors - each computer arms its own WTB circuits and sends a WTB-arm signal to the other computer - the WTB's lock the transmission system - the SFCC's give a class 1 level 2 caution and the ECAM display unit shows a failure message.