3.2 aicraft systems and flight hydraulic system

    The hydraulic system on an aircraft (that has one, some aircraft such as the OH-6 do not have a hydraulic system) provides flight control, breaking, landing gear deployment/retraction, breaking, and starting on some aircraft such as the AH-64 with its hydraulically operated starters for both APU and main engines. The hydraulic system of an aircraft is one of the most important systems on any aircraft equipped with it. Without this system and a failure in any back up systems can spell disaster for anyone on board. According to chapter 7 of the Pilots Handbook of Aeronautical Knowledge, A basic hydraulic system consists of a reservoir, pump (either hand, electric, or engine-driven), a filter to keep the fluid clean, a selector valve to control the direction of flow, a relief valve to relieve excess pressure, and an actuator (PHAK, 2020).




    The list of potential failures in a hydraulic system can go on and on but in my experience with hydraulic systems, the most common I've seen were filter button pops. this is easily mitigated with a simple filter and indicator change and any additional services required IAW the aircraft's maintenance manual. A potential failure could be and servo cylinders which actuate flight controls. These cylinders work like any other hydraulic cylinder would, hydraulic fluid is either pushed or out of the cylinder or push a piston in or out, this actuation can either extend a flap, deploy landing gear, move ailerons up or down, pitch a main rotor blade up or down, the list goes on. These failures can range anywhere from a leak to a crack in the part to a wire that's not telling the cylinder to operate normally. This failure in a servo cylinder can knock out one of your flight controls, for example, in a helicopter if you lose the cylinder that controls your collective, you wont be able to pitch all three, four, five, six, however many main rotor blades you have, up or down to either gain or lose altitude under control. In the event of a failure such as this mid flight the pilots can auto-rotate to the ground. Such the case as in a lose in more then one fight control in a helicopter. A failure in the directional cylinder can cause a total loss of directional control, which will cause the helicopter to counter rotate to the ground (counter rotation is when the aircraft spins opposite to the direction of flight of the main rotor blade). To  Some helicopters have a vertical stabilizer which allows the pilot to pitch down and gain enough speed to where the stabilizer will keep the aircraft relativity straight so the pilots land it much like an airplane, once on the ground (if on a wheeled helicopter) the pilots can taxi and use their break system to turn right or left.

        Any failure in hydraulic systems can be mitigated by either pilot training or ground maintenance. such examples can include:

  • Hydraulic filter replacement
  • Regular checks on the hydraulic systems
  • Inspections for leaks, corrosion, and damage
  • Auto-rotation 
  • Use of back-up systems in the event of hydraulic failures
  • Using a secondary hydraulic system if your aircraft is equipped with two systems in the event of hydraulic failure

        Reference 

 https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/09_phak_ch7.pdf

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