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.

This week we take a look at the hydraulic systems we have on board and the best way to remember them.

The basics - The A320 has 3 independent, hydraulic systems. Green, Blue and Yellow. Each system has its own hydraulic fluid reservoir and all three of these reservoirs are automatically pressurised by bleed air from engine 1. If the bleed pressure is too low from engine 1, the system will automatically take air from the cross bleed duct. 

The systems normally operate at around 3000psi

A power transfer unit, commonly referred to as the PTU, enables the yellow system to pressurize the green system and vice versa. This allows the green system to be pressurised by the yellow system when no engines are running via the yellow electric pump. The power transfer unit comes into action automatically when the differential pressure between the green and the yellow systems is greater than 500 psi. The PTU does not transfer actual fluid between the green and yellow system, it can only transfer power.

The green system controls BOTH slats and flaps, the blue controls slats and the yellow flaps.

Reversers – green on the left controls Rev Eng 1 and yellow on the right controls Rev Eng 2.

Flight controls – the Rudder is nice and easy to remember as all 3 systems can power the rudder. The elevator is similar again, the green system on the left controls the left elevator and the yellow system on the right controls the right elevator. The blue system can control both as a backup. The ailerons buck the trend a little, the green and blue systems can power both. Blue is primary on the left aileron and green is primary on the right.

Ground Spoilers/Speed brake – This system uses all 3 hydraulic systems so at least 1 panel would be available in a dual hydraulics failure situation. There are 5 panels on each wing.

Andy's way to remember - panel 1 and 5, the two outer panels are powered by the green system, then the next 2 in, 2 and 4, are powered by the yellow system leaving a solitary panel 3 to be powered by the blue system.

Matt's way to remember (including ailerons) - Going from wing root to wing tip, GYBYG GB

Yaw Damper – follows a similar pattern, Yaw damper 1 on the left green system and yaw damper 2 on the right yellow system.

Landing gear is only powered by the green system. Normal braking is also on the green. Andy's way to remember is G for GOOD brakes. The alternate braking is on the Yellow system as well as the parking brake.

Nose wheel steering is on the green system BUT some older aircraft had it on the yellow system so check your aircraft.

Links & Resources

 Diagrams by Pierre-Michel Gasser - http://pmgasser.ch/

Matt's diagram for remembering the hydraulic systems