B737-200 System A Pumps off for engine starts

[Centaurus]

With the B737-200 for push back without lock-out pin installed the A Pump switches are turned off to prevent hydraulic pressure from pressurising the noswheel steering system. After the towbar is removed the A Pump switches are then turned on prior to stsrting the first engine. A defect in an A Pump blocking valve (which has been known to occur) could cause uncommanded pressurisation of the mose wheel steering system even with both System A pumps switched off.

The thought occurs it might be prudent to leave both System A Pump switches off until AFTER engine starts for all starts, whether push back is envisaged or not. The theory is that a faulty Blocking valve will be picked up this way. Certainly a reasonable point of view.

There may be good reasons why (apart from push-back requirements of course) Boeing state the System A pump switches are normally ON prior to engine start. This is beside minimising maintenance on the solenoids. Instructive comments most welcome.

[Tree]

I know of several companies who operated large 73-200 fleets and the procedure was as you suggest:

A pumps off/interconnect closed-start engines-remove towbar-turn on A pumps during the after start checklist. Their maintenance dept. indicated that the solenoids should not be left in the off position(high stress/heat) for long periods hence they were not turned off until cleared for pushback and start. They were not turned off for open ramp starts. Valve failures were very rare; pressure lights/guages were monitored as a backup.

[Rotates Lowly]

My company B732 SOP requires that we turn the system A pumps off until after both engines have been started and the towbar has been disconnected. In addition to this we have a nosewheel steering lockout pin inserted. This allows start during the push.

Happy landings