[29液压] 接通外电源或者APU电源时，备用液压泵会工作4秒？

The phenomenon is related to the auto-standby system, which can be seen in System Schematic Manual (SSM) 27-23-11 and 29-22-11 Flagnote 1. Auto-standby turns on the Standby hydraulic pump and opens the Standby rudder shutoff valve under the following condition:

Flaps "NOT UP" switch actuated

 --AND--

MLG wheel speed > 60KT ---OR--- Air-ground in AIR mode

 --AND--

Either /A/ or /B/ flight control hydraulic pressure < 1200 PSI --OR-- Yaw damper coupler turns on System A Flight Control Low Pressure Light L3 (airplanes with rudder pressure reducers)

On airplanes with the Rudder System Enhancement Program (RSEP) incorporated, the rudder Force Fight Monitor (FFM) can also turn on the Standby pump if a rudder force fight (greater than 3600 psi difference in delta-P between A and B sides of Rudder PCU) is detected.

The Standby pump may run momentarily when hydraulic system pressures (S142 & S141) are less than 1200 psi and the flaps are extended (flaps switch S1051 is "NOT UP"). The momentary running occurs when external ground power is applied.

If the Reverse Thrust Lever is not STOWED or if the Speed Brake Lever is not DOWN, then upon application of electrical power to the airplane, relay R591 (AIR/GND) moves to the GROUND position. As part of a self-test, the auto speed brake module (ASM)(M980) then closes an internal wheel speed relay for four seconds, connecting pins 47 and 49 in connector D2586A. This provides a "WHEEL SPEED GREATER THAN 60 KT" signal to the auto-standby system, and the Standby pump comes on. After 4 seconds, the ASM wheel speed relay opens, the "WHEEL SPEED GREATER THAN 60 KT" input goes away, and the Standby pump turns off. The circuits may be seen in SSM 27-23-11 and 27-62-11.