Why do you pressurize hydraulics before pushback (744)
Hi, I apologize in advance if this seems obvious, but I've asked a few co-workers and can't seem to get a straight answer.
On the 747 the procedure is to pressurize the hydraulic system prior to pushback. If I understand things correctly, the ground crew uses the steering lockout pin to stop the flow of hydraulic fluid to the nosewheel steering system and enable the pushback tug to steer the aircraft.
So, my question is, why not wait until after pushback to pressurize hydraulics, as an additional margin of safety for the ground crew? Are the pumps turned on simply to ensure adequate parking brake accumulator pressure? Is it done so that the body gear steering can actuate on sharp turns? Is it to ensure pedal brake authority in case of inadvertent towbar breakage/disconnect?
I'm on the -400 if it matters; on other airplanes I've flown you simply didn't pressurize hydraulics until after pushback, granted they didn't have a lockout pin. I'm just trying to reason out the differences since I'm new to Boeings. Thanks for the help.