Message Name:

HYDRAULIC FLUID TRANSFER BETWEEN RESERVOIRS

Identifier:	293OBS001
Effectivity:	AIRCRAFT 20501 - 20999
System:	Hydraulics
System Description:	29-30-00 - Indicating
Schematic Diagram:	29-12-00 - Left and Right Hydraulic Systems
Wiring Diagram:	29-31-01-1 - Hydraulic Pressure Transducer (LHS) 20501 to 20886, 20888 to 20890, 20896 to 20898 29-31-01-2 - Hydraulic Pressure Transducer (LHS) 20887, 20891 to 20895, 20899 and Subs 29-31-03-1 - Hydraulic Pressure Transducer (RHS) 20501 to 20886, 20888 to 20890, 20896 to 20898 29-31-03-2 - Hydraulic Pressure Transducer (RHS) 20887, 20891 to 20895, 20899 and Subs

Hydraulic Fluid can transfer between reservoirs, depending on the operational or maintenance actions performed

• Operation

Advisory Wire/Service Bulletin:

• AW300-29-0056 - Hydraulic Fluid - System Transfer
• AW300-29-0251 - Recent failures of the Engine Driven Pump (EDP)

Full Throttle Blog/Forum Articles/Infoservice/Newsletter: None

Flight Operation Notifications Manual (FONM): 

• [ Challenger 350 ] FON 01- 15 - HYD-001 - Hydraulic Fluid Transfer

NOTES:

• Hydraulic Fluid Transfer from Right system to Aux system

  • Right DCMP selected ON and AUX DCMP Selected OFF.
  • AUX Switching valve is in the normal position with the Right System supplying pressure to the rudder.
  • If the Right system pressure drops below 2500 psig (This can happen if spoilers are cycled), the switching valve will cycle to the back-up position. The 2500 psi fluid between the switching valve and the rudder is dumped into the aux system.
  • When the right system pressure increases above 2500 psig, the AUX switching valve will cycle back to the normal position and the right system will re-pressurize/re-fill the line between the aux switching valve and the rudder.
    This does not happen in flight because
  • The EDP keeps the Right system above 2500 psig for all cases, and
  • The AUX System is pressurized so the lines between the switching valve and the rudder are still pressurized if the Right system pressure drops below 2500 psig.

• Hydraulic Fluid Transfer from LEFT to RIGHT or RIGHT to LEFT

  • This is due to how the Emergency/Parking brake handle is set and released.
  • The OTBD brakes are normally pressurized from right system and INBD from the left system.
  • The Emergency system uses the left system to pressurize all 4 brakes.

General Setup for Maintenance
Standard Aircraft Configuration for Maintenance	AMM 12-00-00-867-803
Connect Electrical Power to the Aircraft	AMM 24-00-00-861-801
Remove Electrical Power from the Aircraft	AMM 24-00-00-861-802
Electrical/Electronic Safety Precautions	AMM 24-00-00-910-801
Electrostatic Discharge Safety Precautions	AMM 24-00-00-910-802
Maintenance Tasks
Pressurize the Left and Right Hydraulic Systems	AMM 29-10-00-862-801
Release Hydraulic Pressure - Left and Right Hydraulic Systems	AMM 29-10-00-862-802
Component Location	AMM 29-31-00-992-801
Removal of the Hydraulic Pressure Transducer	AMM 29-31-01-000-801
Installation of the Hydraulic Pressure Transducer	AMM 29-31-01-400-801

The hydraulic reservoirs fluid quantity changes are largely due to fluid transfer happening between the systems Left Hydraulic, Right Hydraulic, and AUX Hydraulic which can be minimized if proper hydraulic system operation procedures and sequences are followed. 

-Case A) Fluid Transfer from Right to AUX through the switching valve. 

-Case B) Fluid Transfer from Right to AUX due to rudder PCU compensator depletion. 

-Case C) Fluid Transfer from Right to Left through the brake system. 

-Case D) Flap Power Unit (FPU). 

The four possible fluid transfer paths are described below. 

Case A) 
FLUID TRANSFER BETWEEN RIGHT AND AUX THROUGH THE SWITCHING VALVE 

One shared component between the Right hydraulic system and the Aux. System is the Switching Valve. 
NOTE: The Right and Aux Systems are also common to the Intensifier but because the systems are separated by a chamber that is vented, the fluid from the two systems cannot mix. 

The switching valve function is to maintain hydraulic power to the lower rudder PCU during loss of Right System or during a combination of transitory high flow demand and relative pressure drop in the Right hydraulic system. The transfer of fluid from the Right System to the AUX system can be minimized during maintenance if the AUX DCMP is operating when the Right DCMP is selected ON. 

Here is the scenario, when the AUX DCMP is selected OFF and the Right DCMP is operating:

1. Right DCMP selected ON and Aux DCMP Selected OFF.
2. AUX Switching valve is in the normal position with the Right System supplying pressure to the rudder.
3. If the Right system pressure drops below 2500 psig (can happen if spoilers are cycled), the switching valve will cycle to the back-up position. The 2500 psi fluid between the switching valve and the rudder is dumped into the aux system return (reservoir). 
4. When the right system pressure increases above 2500 psig, the Aux switching valve will cycle back to the normal position and the right system will re-pressurize/re-fill the line between the AUX switching valve and the rudder. 
5. Repeat of steps 3 and 4 causes the Right Fluid transfer to the Aux System.
6. Each time the Aux. switching valve cycles per the above scenario, approximately 0.1 to 0.2 in³ of fluid transfers from Right System to Aux System.

This does not normally happen in flight because the EDP keeps the Right system above 2500 psig for all cases, and opposite transfer from AUX to RH will happen if the Right system pressure drops below 2500 psig
NOTE: The difference in hydraulic fluid volume between the minimum and the maximum AUX reservoir servicing limits per the AMM is approximately 28 in³ (66% @ 70°F @3000 PSI). This means that, for this case alone, it may take about 140 AUX switching valve cycles (at power-off) to transfer sufficient fluid from the Right System to the AUX System to cause the Auxiliary Hydraulic System fluid level indication to turn white "AUX reservoir High/overfill level". 

Case B)
FLUID TRANSFER BETWEEN RIGHT AND AUX THROUGH THE PCU COMPENSATORS

The purpose of the rudder PCU compensator is to provide hydraulic fluid internally to the PCU for flutter and ground gust protection when the hydraulic system supply is unavailable. The lower rudder PCU is supplied by the Right hydraulic system, therefore, there is only one PCU) compensator contributing to fluid transfer. 

• With the aircraft parked and all systems off, the compensators will slowly deplete.
• The compensator normal pressure is about 100 PSI.
• Each Compensator contains approximately 5.34 in³.
• When the A/C is parked, with all hydraulic systems off, the AUX hydraulic system Switching valve is in the alternate position connecting the lower rudder PCU to the Aux. hydraulic system.

The following table presents means by which fluid can be transferred from the lower rudder PCU compensator and the AUX system.

CREW/MAINTENANCE ACTION	AIRCRAFT EXPECTED STATUS	FLUID PATH/MOVEMENT
1. The aircraft is parked and all systems to OFF.	The aircraft hydraulic systems are off, the Aux Switching valve is in alternate position.	The LOWER rudder PCU is connected to the AUX system.
2. Aircraft power-up, Select L and R DCMP ON	L & R hydraulic Systems @3000 PSI The Aux Switching valve will instantly shift to the normal position. This is the normal operating mode.	The Right system supplies pressure and fluid to the Lower rudder PCU and, replenishes as required the lower PCU compensator, (about 5.34 in³).
3. Aircraft shut-down, Select L and R DCMP OFF	Right hydraulic Systems @ 0 PSI As the Right system pressure dissipates below 2500 PSI, the Aux Switching valve will shift to the alternate position.	Now again, The Lower rudder PCU is connected to the AUX system.
4. Aircraft Parked	The Aux Switching Valve is in the alternate position.	As the lower rudder PCU compensator depleted fluid will return to the AUX system, the potential fluid volume transferred depends on how long the aircraft is parked. It can be as much as 5.34 in³ when the compensator is totally depleted.

NOTE: The Rudder Lower PCU compensator is filled by the Right system however, when the aircraft is parked, the Lower PCU compensator discharges the fluid into the Aux. system when the Right system is

It is possible to transfer sufficient fluid from the Right System to the Aux. to cause the hydraulic fluid level indication to turn "white" for the Aux. System after being parked for approximately one month (due to the rudder compensator depletion only). 

Releasing the Park/Emer Brake handle without pressing the toe brakes causes the following to happen:

CREW/MAINTENANCE ACTION	AIRCRAFT EXPECTED STATUS	FLUID PATH/MOVEMENT
5. Release Park/Emer Brake without pressing the toe brakes	"The Park/Emer brake valve releases the four-wheel brakes pressure. Since the shuttle valve has not moved from the previous step, it does no allow flow returning through the toe brake direction, bit it allows the flow/pressure to dissipate through the Park/Emer valve return port only. The four-wheel brake pistons are now all connected to the left system return line.	Releasing the pressure in the Park/Emer circuit allows all wheel brake pistons to retract slightly (to avoid brake drag) the entire equivalent amount of fluid generated from each piston retraction stroke now goes to the left system return line only. The inboard brakes have no impact because they were previously filled by the left system. However, the outboard brakes retraction delta volume that was filled the right system now also goes to the left system, altering the fluid quantities in the reservoirs by that quantity. This represents about 1.9 in³ for each cycle (steps 1 to 5), for the 2 outboard wheels.

NOTE: Based on tests performed by Parker, it takes approximately 16 times through steps 1 to 5, to decrease the right system reservoir quantity by 5.6%.

The delta volume of the Right reservoir serviced to the AMM requirements, (62% @ 70°F @3000 PSI), and "white" EICAS Quantity appearing at 40%, is about 115.5 in³.

This means that for this case alone; it will take about 61 Park/Emer brake setting (with the incorrect procedure) to transfer sufficient fluid from the Right System to the Left System and cause the hydraulic fluid level indication to turn “white” for the Right System.

• If you set the brake handle with the pedals pushed you must release the handle with the pedals pushed to prevent fluid transfer. If you set the handle without pushing the pedals you must release the handle without pushing the pedals to prevent fluid transfer. It takes 8 to 12 cycles of the parking brake with the toe brakes selected and releasing the parking brake without toe brakes selected to move 2% of fluid.

Case D)
FLAP POWER UNIT (FPU)

• Enter the MDC and pull up the SYSTEM PARAMETERS and go to page 2. Select on only the left hydraulic system and observe the Flap Xducer Press. It is normal to see 100 to 200 psi on this gauge in static conditions. If left hydraulic pressure is showing on this Flap Xducer Pressure, consider an internally leaking FPU as the culprit

Approximately 1.9 in³ is transferred between systems each time the procedure is completed. It takes approximately 14 times through the procedure to increase the Right System reservoir quantity by 5%.