The current hydraulic braking system is particularly reliable: its failure probability is only one per 100 million landings! However, if a failure did occur, although highly improbable, a totally independent hydraulic alternate circuit is included to take over immediately and automatically. At several different points sensors are included to detect anomalies, for instance in the circuit oil pressure, and report them immediately to the BSCU.
In this "backup" configuration, the braking function of the BSCU is itself short-circuited. When the pilot presses the brake pedal, directly and mechanically, he operates the master cylinders, which translate quantitative information into the oil pressure to be fed towards the brakes. This is referred to as a "hydro-mechanical" control. In this operating mode, the BSCU and the servo-valves only maintain the antiskid function.
