Boeing 737 Hydraulic Fluid Transfer

Hydraulic systems on the Boeing 737 power essential components such as flight controls, landing gear, thrust reversers, and brakes. Generally, if a hydraulic system is low on fluid, it should be topped up with fresh hydraulic fluid to maintain performance and prevent contamination. Transferring hydraulic fluid between System A and System B is generally not recommended. However, in rare situations, such as the need to equalize fluid levels between the System A and System B reservoirs, specific transfer procedures can be performed. If required, the process must be carried out with great care to avoid system contamination or damage.

Before proceeding with any hydraulic transfer, always ensure that the source of the fluid loss is identified and resolved. Additionally, confirm that personnel are clear of areas where hydraulic components may move unexpectedly when the hydraulic system pressurizes.

Below is a detailed guide for transferring hydraulic fluid between System A and System B.

Transferring Fluid from System A to System B

1. Place wheel chocks to ensure the aircraft remains stationary during the process.
2. Turn off both System A and System B EMDPs to isolate the systems.
3. Turn on the System A EMDP to pressurize System A.
4. Set the parking brake to align hydraulic pressure paths.
5. Turn on the System B EMDP to enable fluid transfer to System B.
6. Release the parking brake to allow fluid movement.
7. Turn off both System A and System B EMDPs to complete the cycle.
8. Repeat the procedure as needed, depending on the amount of fluid required.

Each cycle transfers approximately 15–20 cubic inches of hydraulic fluid from System A to System B. For larger fluid transfers, use standard reservoir servicing procedures.

Operate EMDPs intermittently, with a maximum of five cycles in a five-minute period. Allow at least 30 seconds of cool-down time between pump operations. After five cycles, either run the pumps continuously for five minutes while monitoring for overheat indications, or turn both pumps off and allow them to cool for at least 30 minutes before resuming operations.

Transferring Fluid from System B to System A

1. Ensure the area around Thrust Reverser No. 1 is clear to avoid accidental movement.
2. Turn off both EMDPs for System A and System B.
3. Set the FLT CONTROL switch to SBY RUD to activate the standby hydraulic system.
4. Select Thrust Reverser No. 1 to the OUT position to use standby hydraulics for fluid transfer.
5. Switch the FLT CONTROL switch to ON to deactivate standby mode.
6. Turn on the System A EMDP to begin transferring fluid into System A.
7. Stow Thrust Reverser No. 1 using System A hydraulics.
8. Repeat the process as needed, depending on the desired transfer amount.

Each cycle transfers approximately 4% of System B reservoir capacity into System A. Typically, 1–3 cycles are sufficient for small corrections.

While transferring hydraulic fluid between Systems A and B on a Boeing 737 is technically feasible, it should only be performed when absolutely necessary, such as for minor adjustments or to equalize fluid levels. For significant fluid replenishment or maintenance, always follow standard servicing procedures outlined in the AMM.