Asking 737 experts
 

Hello, I am studying on the system of Boeing 737-300 non-EFIS. On an issue of the hydraulic system, there's a designation of turning System B "Electric" pump before turning System A "Electric" pump. What is the reason for this? please kindly advice. I would really appreciate.

Without getting my books out it probably has something to do with checking a buss. If you check A first you will not be able to check some buss has power. It may be something like that.

Since B system is tied to the standby system, turning on the B sys pump first insures that pressure observed on the B sys gauge is not supplied through the A sys via a faulty Power Transfer Unit (PTU) control valve.

thank you for your kind advice.
as you mentioned that it ensures the conflict reading of sys A pressure drive on the gauge, I am so all with you.
However, what I see is that the sys B ENGINE pump (with a sensor) is the only system which could only drive hyd shutoff valve to PTU OPEN, not the ELEC pump. Accordingly, that is the reason I am so doubt about Turning sys B "ELEC" pump ON before the sys A "ELEC" pump on. with this issue, it might not have something to do with the PTU.

Please kindly advise