Aircraft fails Rudder PCU internal leakage check 737-6/7/8/900

Applicable Reference:
AMM 27-21-00-820-809, 27-21-00-700-822, WDM 27-25-11, SSM 27-25-11, 27-51-11
Aircraft fails Rudder PCU internal leakage check
Possible Fix:
Adjust Rudder PCU control Rods, Replace FSEU, Rudder PCU
When you perform the Rudder control rod adjustment you adjust each hydraulic system rod input to a preset
dimension of the Rudder. Following this adjustment you go to D10064 pins 18 and 20 and verify the voltage is in the
4.5 to 5.5 VDC range. This is the null adjustment of the Rudder PCU.
Next in the Rudder PCU internal leakage check you will command the Rudder Pressure Limiters (RPL) to the “LOW”
pressure state from the FSEU. During this test you are instructed to push the Rudder pedal until it touches the
forward quadrant stop and hold. When you do this you measure the voltage from the sensor on the Rudder PCU and
verify it is within the required range. You do this with each hydraulic system.
If the values are off for both systems you can try a FSEU because it is what sends the power to the PCU to engage
the RPL solenoid valve. Then a Rudder PCU rod adjustment, or a PCU replacement.
If a individual system fault is found. There are A and B system relays in the P6 panel that could be at fault. The
output from the FSEU is connected on the E1-1 rack so it is not likely that the output from the FSEU causes a single
system fault.
The authority limiter reduces the maximum rudder movement. At specified air speeds, the FSEU opens the main
rudder PCU authority limiter solenoid valve. This limits the hydraulic force produced by the PCU, which limits the
rudder authority.
During the Rudder PCU internal leakage check you verify the initial position of the PCU by it’s feedback voltage.
Then you go to the FSEU and command the Rudder pressure limiter solenoid valves to the low pressure state. Then
when you push the rudder pedal to the forward quadrant you look for a corresponding voltage feedback for the
position that is expected for that displacement. If the Rudder PCU is leaking internally the voltage will be off. Also if the circuit that operates the “RPL” solenoid valves has a defect the voltage will also be off.

Thanks to Tom S. for his input !

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