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Monday, March 26, 2018

Aircraft Hydraulic Systems - Part 01

Basically an aircraft is controlled in longitudinal, horizontal and vertical axis with the help of flight controls. Flight controls are categorized in to two main groups as Primary Flight Controls and Secondary Flight Controls. Primary Flight Controls including Ailerons, Elevators and Rudder, controls the aircraft in its 3 main axis while Secondary Flight Controls assist the Primary Flight Controls. Spoilers, Slats, Flaps and Trimmable Horizontal Stabilizer (THS) are reckoned as Secondary Flight Controls. As stated earlier they assist Primary Flight Controls and change the Lift and Drag parameters according to various flight phases. These flight control surfaces are actuated by actuators which are powered by the aircraft hydraulic systems.




Modern day aircraft incorporates three hydraulic systems, Green, Blue and Yellow. These three hydraulic systems are separate systems which supply its power for various aircraft units. Normally a hydraulic system receives its power from an Engine Driven Pump (EDP) or an Electric Pump. EDPs are driven by the Engine Accessory Gear Box (AGB) and Electric Pumps are powered by the aircraft electrical system. As an example, in A320 aircraft, Green and Yellow hydraulic systems are driven by separate EDPs. Yellow system can be pressurized from an Electric Pump when the aircraft engines are not running. But the Green system can only be directly pressurized through its EDP (PTU has been considered as in-direct pressurization). Blue system has no connection to any EDP as other two hydraulic systems and it’s powered by an Electric Pump in normal operations. In an emergency situation Blue system can be brought up to act by a Constant Speed Motor Generator (CSMG) which receives power from Ram Air Turbine (RAT). Any of these hydraulic systems are not connected in a manner where hydraulic fluid can be interchanged. 


A unit called Power Transfer Unit (PTU) is used to pressurize a low pressure system with the aid of another healthy system. PTU connects Green and Yellow systems. When the pressure of one system drops, making the discrepancy of pressures between two systems to 500 psi, PTU comes in to act and pressurizes the low pressure system by running a hydraulic motor which is powered by the healthy system. PTU makes the annoying sound which is well known as barking dog sound. A330 aircraft does not incorporates a PTU as it has EDPs for all three hydraulic systems. PTU is inhibited in few aircraft configurations and these configurations and other facts related to aircraft hydraulic systems are discussed in the next article. 



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