Hydraulic systems can suffer from a number of abnormal situations. Pump low pressure, reservoir overheat, Reservoir low air pressure and reservoir low level. The electric pumps on the blue and yellow systems can also overheat. All of these will lead to the ECAM requesting that you switch off the pump. If this occurs to the green or yellow systems the PTU, if it is available, will transfer power, not fluid, between the systems recovering the affected system. The blue system can not be powered by the PTU. If the PTU is not available or the procedure ask you to turn it off, the failed system will not be powered. This leads to a single system failure.
In the case of a single system failure the aircraft will remain in normal law so all the associated protections are available. Certain flight controls will be affected based on which system has failed but ultimately Aileron, elevator and Rudder control surfaces will remain powered so controlling the aircraft will be conventional. Flaps and or slats will be slow depending on which of the systems has failed, we covered these in the original hydraulics podcast so it maybe worth having a listen again to refresh your memory. Certain spoilers will be unserviceable.
Things start to become more interesting when 2 hydraulic systems fail. When this occurs the Autopilot will be lost so priority must be given to flying the aircraft and stabilising the flight path, the aircraft will also revert to alt law in 2 of the cases, so this, as usual, means direct law once the gear is extended. The ECAM will display LAND ASAP red, this is a timely reminder that you are now operating on a single hydraulic system, why have we lost 2? what happens if we lose the last system? We will cover all 3 cases of Dual hydraulics failure in some detail shortly but lets just broadly go over what you can expect for each case. If you remember these as a guide.
G+B = Handling Problem
G+Y = Braking Problem
B+ Y = As the green system is available this is the least demanding of the 3 scenarios
Airbus designed the summary pages to give us all of the information we need to help us during the cruise, approach, landing and if necessary the Go around. in the FCOM Pro-ABN-01- Use of Summaries section, more background information is provided. it states that the summaries are QRH procedures created to help the flight crew to perform actions. In ANY case the flight crew should apply the ECAM first, this includes the STATUS page. This is an important point, it is all too easy in a high workload situation to divert our attention to performance calculations and other tasks before completing the ECAM. The ECAM's for dual hydraulics are not actually that long and the status page will give you valuable information as to the state of the aircraft increasing your situational awareness.