Hydraulic System

Gulfstream GVII

Eddie sez:

The G500 hydraulic system is another step in the evolutionary progress from the original G159 (landing gear, brakes, main entrance door), to the GII/III (1500/3000 psi flight controls added), to the GIV (3000 psi flight controls), to the GV (simplification of landing gear and flap hydraulic lines), to the G650 (fly by wire). ATA 29 covers most of what you need but does not touch on back up hydraulics for the flight controls; for that you need to visit: G500 Flight Controls.

For a quick refresher or a good intro, be sure to see Ivan Luciani's notes: G500 Hydraulic System Notes.

Sometimes when you are troubleshooting it helps to have a look at things when they are normal. I took a video of the hydraulic system synoptic from engine start, taxi, takeoff, a short pattern, landing, taxi in, and shut down. Everything was normal except we had the APU flameout on us so there are a few things we had to do prior to shutdown, but the rest is okay. There isn't any sound and it isn't a captivating video, but it may give you the information you need: Hydraulic system synoptic page engine start to shutdown.

There are also a few flash cards here: G500 Flashcards and Quizlet.

Everything here is from the references shown below, with a few comments in an alternate color.

images

Photo: G500 Hydraulic System, General Description, PAS, p. 8-1
Click photo for a larger image

How it works . . .

The Components in Greater Detail . . .

Limitations and Abnormal Procedures . . .

Last revision:

2020-08-02


How it Works . . .


Where hydraulic systems are advantageous

You may have wondered why some systems on the aircraft are hydraulic and others are mechanical, electrical, or pneumatic. Hydraulic systems advantages:

  • High power to weight ratio; small components can handle large loads
  • Transportability of power; you can provide a large amount of force a large distance away from the power source without the space and weight requirements of equivalent pulleys, cables, push/pull rods, and other mechanical devices.
  • Precise motion control; since the fluid is relatively incompressible, the amount of movement can be exact.
  • Instant stopping and reversibility.
  • Constant torque with variable speed.
  • Automatic braking; stopping the flow from the source will instantly stop at the objective.

So how does it do all that? Blaise Pascal came up with the concept in 1647. The law states: "Pressure exerted on a fluid in an enclosed container is transmitted equally and undiminished to all parts of the container and acts at right angle to the enclosing walls."

To visualize, consider a sealed container filled to the brim with a fluid and compressed by downward force on its cork:

images

Photo: Pascal's law, Eddie's drawing
Click photo for a larger image

Pressing down on the cork of a bottle filled with a relatively incompressible fluid will in theory expose every side and corner of that bottle with an equal amount of pressure. You might argue about the friction exerted on the cork and the effect of gravity, but for our purposes the forces everywhere are equal. You can exert a great amount of force by taking advantage of the area of force application:

images

Photo: Pascal's law application, Eddie's drawing
Click photo for a larger image

In the example, you increase the force by ten times by increase the area of the piston by ten times. It has become common on many aircraft to use a 3,000 pounds per square in (PSI) system to transfer force from a pump to an actuator. That's a lot of pressure to be contained. The other constraint on the system is heat generated by the pumps and actuators that must be cooled and of course the fluid must be stored . . .

How the fluid is stored

images

Photo: Example hydraulic system reservoir with bootstrap, Eddie drawing
Click photo for a larger image

There are two hydraulic system Reservoir Assemblies, one each for the left and right hydraulic systems, located in the tail compartment. Each reservoir contains a piston that pushes the fluid to maintain pressure (60 PSI) to prevent air from entering the fluid and to keep the pressure moving towards the pump, preventing cavitation.

The left system is considered full at 2.5 gallons and supplies fluid to the Left Engine Driven Pump (EDP), the Aux Pump, and the Power Transfer Unit (PTU) Pump. The right system is considered full at 1.3 gallons and provides fluid for the Right EDP.

Each system has an accumulator to absorb shocks in the fluid caused by various components suddenly starting and stopping. The accumulator uses nitrogen to do this; a pressure gauge in the tail compartment should read 1200 PSI at 70°F with a 25 PSI difference for each 10°F change in temperature.

Some of the hydraulic fluid from the inlet of each Engine Driven Pump (EDP) is allowed to bypass into the pump housing to keep the fluid moving during times of low demand; this helps to cool the fluid and lubricate the EDP. There is also a heat exchanger inside each fuel hopper to cool the hydraulic fluid. It takes about 2 hours after takeoff to cool the hydraulic fluid within minimums; if a failure were to occur within 2 hours of takeoff, the aircraft must be landed within 4 hours of the failure. The concern is that hot fluid which isn't moving could melt a seal or other component.

How hydraulic fluid is pressurized

images

Photo: G500 Hydraulic System, System Synoptic, PAS, p. 8-23
Click photo for a larger image

An Engine Driven Pump (EDP) is mounted on each engine accessory gearbox. The pumps produce a constant 3,000 PSI pressure at variable volume, anytime the engine is turning. The Electronic Engine Controller (EEC) will offload the pump if engine HP RPM drops below idle while the aircraft is inflight; while the pump still turns, the torque needed to do so is minimized.

A Power Transfer Unit (PTU) in the tail compartment contains a motor and a pump. The motor is powered by the right hydraulic system when turned on with an OHPTS switch, or when armed and the left system pressure < 2400 PSI, each system has at least 0.36 gallons of fluid, the right system pressure is > 2850 PSI, and the right system temperature is < 107°C,

An Auxiliary (Aux) Pump in the tail compartment is electrically powered by the L ESS DC bus and uses left system fluid for utility systems during ground / maintenance activities (gear, flaps, inboard brakes), nosewheel steering, and the main entrance door. The aux pump comes on automatically on the ground when armed if the toe brakes are applied when the inboard accumulator is < 1500 PSI. It comes on automatically inflight when left system pressure 1500 PSI provided there is at least 0.36 gallons, the fluid is < 107°C, and the flaps or gear positions do not match handle position when over 100 KCAS. Selecting the Aux Pump manually removes the minimum constraints, but inflight it will time out after 2 minutes. The timeout can be reset by cycling the switch OFF then ON.

Left System

images

Photo: G500 Hydraulic System, Left System, PAS, p. 8-11
Click photo for a larger image

[PAS, p. 8-2]

  • Independent.
  • The left system is independent and isolated from the right system.

  • Powered by.
  • It is powered by the left Engine Driven Pump which is mounted on the engine gearbox. This pump outputs a constant pressure (3,025 ± 50 psi), variable volume. The volume is low at idle, higher at cruise. The pump offloads when in flight if an engine drops below idle (< 55% N2), (automatically controlled by Electronic Engine Controller). This reduces pump outlet pressure so as to reduces drag on the engine during an airstart. The system has a pressure relief valve to relieves pressure during surges at 3500-3750 psi.

  • Reservoir.
  • The left hydraulic Reservoir is larger than the right and is considered full at 2.5 gal. It supplies hydraulic fluid for: Left Engine-Driven Pump, Aux Pump, PTU Pump.

    The AFM, §1-29-10 says the left system quantity is 2.4 gallons. In my experience the 2.5 is probably right.

  • Accumulator.
  • The left hydraulic Accumulator is installed in the pump pressure line to absorbs shocks to the left system. The minimum nitrogen pressure to check on preflight is 1200 psi at 70°F / 21°C ± 25 psi per 10°F / 5°C difference. The gauge located in tail compartment (System 1).

  • Filter Manifold.
  • The filter Manifold in the tail compartment controls the direction of fluid travel. It contains 6 filters to provide filtered fluid and there are no pressure differential indicators to check.

  • Functions.
  • The left system powers the primary flight controls (elevators, ailerons, rudder) the midboard spoilers, as well as the left thrust reverser. The left system also performs a number of utility functions: Flaps, Gear, Brakes (Inboard), Nosewheel steering, Main Entrance Door.

  • Failure of Left System Pump Only
    • Results in loss of: Left Thrust Reverser, Midboard Spoiler Pair
    • Pilot Considerations
      • Airspeed 285 KCAS/.90M max
      • If Failure occurs ≤ 2 hours after takeoff: Land at nearest suitable field within 4 hours of failure, Flight control failure a concern due to viscosity issues related to possible high temp of EBHA trapped fluid
      • If failure occurs > 2 hours after takeoff: Land at nearest suitable field with no time limit, Hyd fluid temp stabilized → Flight control failure not a concern

Right System

[PAS, p. 8-4]

  • Independent.
  • The right system is independent and isolated from the left system.

  • Powered by.
  • Powered by the right Engine Driven Pump which is mounted on the engine gearbox. This pump outputs a constant pressure (3,025 ± 50 psi), variable volume. The volume is low at idle, higher at cruise. The pump offloads when in flight if an engine drops below idle (< 55% N2), (automatically controlled by Electronic Engine Controller). This reduces pump outlet pressure so as to reduces drag on the engine during an airstart. The system has a pressure relief valve to relieves pressure during surges at 3500-3750 psi.

  • Reservoir.
  • The right hydraulic Reservoir is smaller than the left and is considered full at 1.3 gal. It supplies hydraulic fluid for: Right Engine-Driven Pump.

    The AFM, §1-29-10 says the right system quantity is 2.3 gallons. In my experience, it seems to stay around 1.7 or so and the "full" mark might be around 1.5 or so.

  • Accumulator.
  • The right hydraulic Accumulator is installed in the pump pressure line to absorbs shocks to the left system. The minimum nitrogen pressure to check on preflight is 1200 psi at 70°F / 21°C ± 25 psi per 10°F / 5°C difference. The gauge located in tail compartment (System 2).

  • Functions.
  • The right system powers the primary flight controls (elevators, ailerons, rudder) the inboard and outboard spoilers, as well as the right thrust reverser. The right system also powers the outboard brakes and drive the PTU motor.

  • Failure of the right system results in the loss of the right thrust reverser. There are several pilot considerations:
    • Airspeed 285 KCAS/.90M max
    • If failure occurs ≤ 2 hours after takeoff: Land at nearest suitable field within 4 hours of failure (Flight control failure a concern due to viscosity issues related to possible high temp of EBHA trapped fluid)
    • If failure occurs > 2 hours after takeoff: Land at nearest suitable field with no time limit, if Hyd fluid temp stabilized → flight control failure not a concern

The Components in Greater Detail . . .


Accumulators

images

Photo: G500 Hydraulic System, , PAS, p. 8-4
Click photo for a larger image

[FSI G500 MTM, p. 38]

  • There are two 50 cubic-inch hydraulic accumulators, one for each engine driven system, located on either side of the tail compartment. They are installed in the hydraulic pressure lines downstream of the filter manifold. Their purpose is to act as hydraulic shock absorbers and dampen hydraulic pressure surges within the system caused by the rapid loading and unloading of the various system components. They are also used to supply initial quick flow demands until the EDP can react to meet a rapid increase in system demand. The accumulators are serviced with a precharge of 1200 psig of nitrogen (at 70°F) via a combination service valve and pressure gage located on the accumulator servicing panel in the tail compartment. The nitrogen precharge ensures proper operation of the accumulator.

Aux Pump

If activated in flight, the Aux Pump automatically shuts down after 2 minutes.

images

Photo: G500 Hydraulic System, Aux Pump, PAS, p. 8-6
Click photo for a larger image

[PAS, p. 8-6]

  • Logistics.
  • The aux pump is located in tail compartment below left hydraulic reservoir, it electrically powered by the Left Ess DC, and uses left system fluid.

  • Primary Function.
  • The aux pump provides hydraulic pressure for utility systems during ground / maintenance activities (gear, flaps, inboard brakes), nosewheel steering, and the main entrance door. The aux pump comes on automatically if the toe brakes are applied when the inboard accumulator is less than 1500 psi.

    Everything you need to land.

  • Secondary Function.
  • The aux pump provides an automatic backup to assist the PTU if necessary.

  • Auto Operation In Flight
  • Normally inactive, automatically provides power for flaps and gear after a dual engine failure, dual engine driven pump failure, or left engine pump and PTU failure.

    To activate: aux pump needs to be armed and not overloaded or overheated, left system pressure needs to be less than 1500 psi, the left system needs to be available ( > .36 gal and < 107°C), and the flap or gear positions do not match handle position when over 100 KCAS.

  • When Manually Selected ON
  • The minimum and maximum constraints when armed don’t apply. Activates Aux Pump on the ground without time limit, in-flight with a 2 min limit but it can reset by turning OFF then ON

  • Much slower operation of gear and flaps on Aux Pump alone due to a low flow rate
  • PTU → Approx. 23 gal / min, AUX → Approx. 2.5 gal / min

images

Photo: G500 Hydraulic System, OHPTS, PAS, p. 8-16
Click photo for a larger image

[PAS, p. 8-16]

  • Armed / Not Armed Switch
    • Downstate
    • Armed: Default position at power up, Aux pump auto operation is armed and triggered by DCN when left pressure < 1500 psi, left fluid available, left fluid not hot, and flap or gear handle doesn't equal actual positions.

      Armed: Aux pump has failed.

    • Upstate
    • Not Armed: Aux pump auto operation inhibited.

  • Off / On Switch
    • Downstate
    • On: Aux pump forced to operate.

    • Upstate
    • Off: Default position at power up, Aux pump off.

      On: Aux pump on (if activated by Armed Mode).

      Off: Aux pump off and failed.

      On: Aux pump forced to operate.

Engine Driven Pumps

images

Photo: G500 Hydraulic System, Pump, PAS, p. 8-2
Click photo for a larger image

[FSI G500 MTM, pp. 29 - 30]

  • An engine-driven hydraulic pump (EDP) is mounted on the aft face of each engine accessory gearbox. The variable volume and displacement, pressure compensated pumps can vary their outlet flow, based on system demand, to maintain a constant system pressure of 3000 psi. The pumps rotate any time the HP compressor of the engine is turning.

  • With the engine operating, fluid is directed from the reservoir through the motor operated shutoff valve to the EDP suction port. A portion of the incoming fluid is used internally for pump cooling and lubrication. This hot bypass fluid exits the pump via the Case Drain port and is routed to heat exchangers in the fuel tanks. Pump outlet pressure is maintained at 3000 psi throughout the aircraft pressure lines and is available for use by hydraulic user components.

  • The G500/G600 operates a hydraulic offload (depress) function, which when energized, minimizes the pressure and flow from the EDP and effectively unloads the pump from the gearbox. This function is designed to assist with engine windmill starting performance following an in-flight engine shut down. The pump remains connected to the gearbox and continues to rotate, however hydraulic pressure and flow from the pump is minimal which reduces the torque needed to turn the pump. Hydraulic offload is controlled by the associated Electronic Engine Controller (EEC) via a depressurization solenoid valve mounted to each pump. Power to the solenoids is provided from the on-side Essential DC Bus via MPTs 6 and 12.

  • The EDP will be offloaded (Depressurization Solenoid energized) if engine HP RPM drops below idle with the aircraft in-flight. The affected pump remains off-loaded until successful completion of an in-flight start or the aircraft lands.

We've noticed anecdotally that you need 8% N2 on the ground to get hydraulic pressure, 58% N2 when in flight.

Filter Manifolds

Sometimes you have to wonder, "what am I supposed to do with this knowledge?" These manifolds were a preflight item on previous Gulfstreams because there were a total of 10 Differential Pressure Indicators to check. Those DPIs are gone and these manifolds are more a maintenance concern than one for us pilots to check.

[PAS, p. 8-9]

  • Left, Right, Aux
  • Located in tail compartment
  • Controls direction of fluid travel
  • Contains filters to provide components with filtered fluid
  • Some by-passable, some not; disposable micron filters; replaced at scheduled intervals; no pressure differential indicators to check; Left System Manifold → 6 filters, Right System Manifold → 3 filters, Aux System Manifold → 1 filter

images

Photo: G500 Hydraulic System, Right System Manifold, PAS, p. 8-9
Click photo for a larger image

images

Photo: G500 Hydraulic System, Left System Manifold, PAS, p. 8-9
Click photo for a larger image

images

Photo: G500 Hydraulic System, Aux System Manifold, PAS, p. 8-9
Click photo for a larger image

Heat Exchangers

images

Photo: G500 Hydraulic System, Heat Exchangers, PAS, p. 8-10
Click photo for a larger image

[PAS, p. 8-10]

Radiator-type heat exchangers submerged in corresponding fuel hopper used to cool hydraulic fluid while warming hopper fuel. It is always circulating with no pilot controls.

[FSI G500 MTM, p. 33]

  • The engine driven hydraulic pumps rotate anytime the engines are running. When there is no demand on the hydraulic system, system pressure stabilizes at 3000 psi but there is very little flow of fluid. A quick overheat would occur if there were no transfer of fluid through the pump when there is no demand on the system. To cool and lubricate the EDP, some hydraulic fluid from the suction (inlet) port is allowed to bypass into the pump housing. This fluid removes heat and may contain contamination from pump wear and is removed from the pump via the case drain (bypass) port.

  • The purpose of the heat exchanger is to cool the case drain fluid heated by the pumps by transferring the heat to the fuel in the hopper. The heat exchanger is a radiator type heat exchanger installed close to the floor of the hopper section of the wing fuel tanks. During cold weather start-up, cold case drain fluid coming from the pumps will have a higher than normal viscosity and will bypass the heat exchanger through a bypass valve located in each filter manifold. When the case drain fluid warms and the pressure drops, the bypass valve will close directing the warm fluid to the heat exchanger.

Power Transfer Unit (PTU)

images

Photo: G500 Hydraulic System, PAS, p. 8-7
Click photo for a larger image

[PAS, p. 8-7]

  • Located on left side of tail compartment above Fluid Quantity Indicator
  • Primary backup for left utility systems when Left Engine-Driven Pump Inop
  • Gear, flaps, inboard brakes, nosewheel steering, main entrance door

  • Consists of a motor and pump assembly
  • Uses Right System pressure and Left System fluid. Motor is driven by right System Pressurized fluid, motor drives pump, pump pressurizes left system fluid to operate utilities.

  • When armed on OHPTS → DCN will trigger operation immediately when left system pressure < 2400 psi, fluid available in left and right system > 0.36 gal, right system pressure > 2850 psi, and right system fluid not hot < 107°C.
  • There is no time limit for operation. DCN will trigger deactivation 7 seconds after left system pressure recovers ≥ 2850 psi or immediately if right press drops < 2400 psi.

  • When manually selected ON
  • Minimum and maximum constraints when armed don’t apply, activates without time limit.

  • PTU shutoff valve
  • Located in PTU Motor Pressure Line, controlled by Armed and On switches on OHPTS → Hyd/CPCS, prevents right system from pressurizing PTU Motor when PTU motor not commanded to run.

images

Photo: G500 Hydraulic System, OHPTS, PAS, p. 8-12
Click photo for a larger image

[PAS, p. 8-12]

  • Armed / Not Armed Switch
    • Downstate
    • Armed: Default position at power up, PTU Auto Operation is Armed and Triggered by DCN when left pressure < 2400 psi, left and right fluid available, left and right fluid not hot. DCN will trigger deactivation 7 seconds after left pressure recovers > 2850 psi or immediately if right pressure drops < 2400 psi.

      Armed: PTU has failed.

    • Upstate
    • Not Armed: PTU operation inhibited.

  • Off / On Switch
    • Downstate
    • On: PTU forced to operate.

    • Upstate
    • Off: Default position at power up, PTU shutoff valve closed

      On: PTU SOV Open (if activated by Armed Mode)

      Off: PTU SOV Closed and PTU Failed

      On: PTU SOV Open and PTU Failed( if activated by Armed Mode)

Reservoir Assemblies

images

Photo: G500 hydraulic system reservoirs, Eddie's aircraft
Click photo for a larger image

[FSI G500 MTM, p. 15]

  • The hydraulic reservoirs are located in the tail compartment, at approximately the 10:00 (left) and 2:00 (right) o’clock positions. The purpose of the hydraulic reservoir assembly is to store hydraulic fluid under pressure and to remove entrained air from the fluid. Each bootstrap type reservoir is hydraulically pressurized from its respective system pressure. This is accomplished by a pressure operated piston within the reservoir.

  • When the small end of a piston within the bootstrap cylinder is actuated by pressurized fluid from the EDP, the fluid supply within the reservoir is pressurized by the large end of the piston. When bootstrap pressure is 3000 psi, the reservoir fluid is pressurized to approximately 60 psi. This provides the pumps with pressurized fluid for operation.

  • Each reservoir assembly incorporates a pressure relief valve, a manual bleed valve, fluid quantity transmitter and a visual quantity indicator. A standpipe is attached to the top of each reservoir to collect any air that may be introduced into the fluid during system operation.

  • The air collects around the top of the reservoir and migrates upward into the standpipe. The reservoirs are mounted in the aircraft with a 5° aft tilt to assist with air collection in the standpipe. This allows for the air to be captured and bled, precluding high concentrations of air from entering the EDPs.

images

Photo: G500 Hydraulic System, Right system reservoirs cutaway, PAS, p. 8-8
Click photo for a larger image

[PAS, p. 8-8]

  • Bootstrap Chamber (Yellow)
  • High pressure chamber (3000 psi), compresses one of walls of system chamber to maintain 60 psi inside the system chamber. (“Bootstrap” means system is self-pressurizing).

  • System Chamber (White)
  • Considered full: Left → 2.5 gal, Right → 1.3 gal

    Considered low (no dispatch): Left < 1.75 gal, Right < 0.81 gal. Note: Dispatch decisions should be based on presence of L-R Hyd Quantity Low CAS message.

    Fluid quantity is derived from the movement of a Rotary Variable Differential Transformer (RVDT) connected to a large internal piston within the reservoir. The reservoir must be pressurized to indicate accurately. Can be read electronically on synoptic pages or at a sight gauge on the high pressure end of the reservoir.

    An RVDT is defined as a Rotary Variable Differential Transducer through most of this manual.

  • Hydraulic Firewall Shutoff Valves
  • Located between each reservoir and Engine-Driven Pump. Stops flow of fluid in event of engine fire. Protects remaining fluid in reservoir if hyd line ruptures around engine. Electrically controlled by Left and Right Fire Handle (L / R Essential DC). When pulled → closes valve.

images

Photo: G500 Hydraulic System, TSC, PAS, p. 8-20
Click photo for a larger image

[PAS, p. 8-20] Quantities are available on TSC #1, #2, #3, and #4 ground service pages or TSC #5 fluid quantity page. Note: Quantities on TSCs may differ from synoptic page by 0.2 gallons.

images

Photo: G500 Hydraulic System, 1/6th Synoptic, PAS, p. 8-25
Click photo for a larger image

[PAS, p. 8-25] Hydraulic Remote Replenishing System

  • Located on right side of tail compartment at top of ladder, used to add hyd fluid to reservoir when needed.
  • Exterior Preflight Inspection (Expanded): replenishing Selector Valve → OFF to avoid transmission of fluid in flight, Hydraulic Replenisher → Verify fluid level cap secure, Hydraulic Accumulators → 1200 psi.
  • Separate replenishment pump (oil has its own pump too).
  • Replenishing considered a maintenance function; aircrew can perform if properly trained; see OM Handling and Servicing Procedures → Pilot Hydraulic Fluid Servicing Procedure.

[PAS, p. 8-25] Fluid Quantity Indicator (FQI)

  • Located on left side of tail compartment at top of ladder on the oil replenishment panel.
  • Dual purpose as hydraulic and oil quantity indicator; Hydraulic quantities shown on left for Left and Right Systems; SELECT switch (bottom left) used to change left or right digital indication; displays digital quantity; also displays “LO”, “OK”, or “HI”; Activates replenishing pump; HYD or OIL switch (bottom center) used to select which quantity indications to view; OFF / ON or TEST switch (bottom right) turns FQI display only ON and OFF; FQI still functional; initiates test of the FQI Display.

Limitations and Abnormal Procedures


Limitations

[G500 AFM, §01-29-10] Hydraulic Servicing

  • Maximum reservoir quantities (pressurized) as indicated on the Hydraulics synoptic page: Left Hydraulic System → 4.6 gallons, Right Hydraulic System → 2.8 gallons.
  • Left and Right Hydraulic System Accumulator precharge: 1,200 psi at 70°F / 21°C, ± 25 psi for each 10°F / 5°C difference in temperature from 70°F / 21°C.

[G500 AFM, §01-29-20] Flight Time Limitation after Hydraulic Failure

  • Land within 4 hours of a hydraulic system failure if the failure occurred within 2 hours of takeoff.
  • Note: If a hydraulic system failure occurs > 2 hrs after takeoff, the hydraulic fluid temperature has stabilized and there are no flight time restrictions.


See Also:

Gulfstream GVII-G500 Airplane Flight Manual, Revision 3, July 16, 2019

Gulfstream GVII-G500 Production Aircraft Systems, Revision 3, July 15, 2019