737-NG Hydraulic System Experiences: 737-6/7/8/900

Reservoir quantity changes during flight:

1. Before takeoff extending the Leading Edge Flaps to extend and the Leading Edge Slats to Intermediate position causes 10 % drop in System B hydraulic reservoir level.

2. Retraction of the landing gear causes a noticeable drop in system A hydraulic reservoir level. Retraction of the landing gear causes roughly 20% drop in indicated system A reservoir quantity.

3. Thermal contraction during cruise causes an additional drop in reservoir level. Operators have reported 5 to 10% further drop in system A reservoir level after some time spent in cruise, and calculations indicate that thermal contraction after prolonged cruise at low temperatures could cause as much as 20% drop.

4. Extending the Flight Spoilers causes 6 % drop in System A and System B reservoir quantity.

5. Leading edge flap extension to extend and Leading Edge Slats extension to Full Extend position just prior to landing removes an additional 20% from the reservoir.

6. Extending the Landing Gear causes roughly 20 % rise in system A reservoir level.

7. During Landing applying Speedbrakes (Flight + Ground Spoilers) causes 10 % drop in System A and 6% drop in System B reservoir level.

8. Retracting the Leading Edge devices during taxiing causes 20 % rise in system B reservoir level.

9. Operating the Ailerons, Elevators, Rudder does not cause observable change in system A and B reservoir level.

10. Operating the Trailing Edge Flaps does not cause change in the reservoir level because it is driven by a hydraulic motor, not an actuator.

Experiencing in-flight reservoir level changes as described above should not be concerned, as long as the reservoir levels return to normal after the airplane has been on the ground for long enough for the hydraulic fluid to warm to ambient temperature. When comparing reservoir levels after flight to levels before flight, the airplane should be in the same state as it was before flight. The leading edge, landing gear, spoilers, and thrust reversers should all be in the same position, the pumps should be on/off as they were before, and the brake accumulator should be properly precharged.

Fluid consumption of different Hydraulic components: The EMDP provides a hydraulic flow rate of 6 gpm. The EDP provides a hydraulic flow rate of 37 gpm. If you use only EMDP the EMDP Low Pressure Light will illuminate if you apply too much load to the system. If you excersize with the aileron, rudder, elevator (applying quick movement, great deflection of these flight controls) it can be enough to illuminate of the EMDP Low Pressure Light.

In order to retract the MLG’s within the time limits specified in the Airplane Maintenance Manual (AMM), the hydraulic system has to be capable of delivering 20 gallons per minute (GPM) of fluid flow. In a normal airplane operation, this flow is delivered by the engine driven pump (EDP), which has a maximum flow capacity of 36 GPM. This is supplemented by the System-A EMDP, which provides 6 GPM of flow capacity. If only the EMDP is used to provide hydraulic power to retract the landing gear, it is unlikely that the MLG’s will retract within the specified time due to the limited flow capacity of the EMDP. This is especially true if both gears are being retracted simultaneously. Thus, a hydraulic power cart capable of delivering 20 GPM of flow must be used to test the MLG retraction times.


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