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.

On the 737 we leave the EDPs on even when shut down as recommended by Boeing to increase system component life. That being the case, pressurising before pushing back would probably prevent nasty surprises as the engines are started.

Hi RandomPerson8008,

It's been a long while - but from memory SYS 4 did Normal brakes, Sys 1 did Alt brakes & steering. Although the hydraulics to the nose wheel steering is isolated by the steering pin - the body steering is still available and it helps with the tight turns during push back. We pressurised sys 4 first to prevent hyd fluid migrating from sys 1 to 4 through a shuttle valve in the brakes system.

We do that on our 737NG shorthaul operation, in our case it's SYS A pumps off before pushback. The reason given is an additional margin of safety and any additional component wear is an acceptable cost of doing business.

RRR is correct. Without hydraulic power to sys #1 & #4 you have no brakes and no NLG & BLG steering, risking tyre damage. It also allows the control check to be carried out at a relatively quiet time during departure.

The procedure is #4, wait for LPWL out, #1, wait for LPWL out then #2 and #3. Waiting after #4 and #1 prevents fluid transfer by ensuring the brake shuttle valves are in the normal positions.

Unless the ground crew is abnormally tall, there's little threat from live flight controls.

Thanks for the replies.

Safety-wise, I was referring to inadvertent actuation of the nosewheel steering in case there was a problem with the lockout pin. I suppose this possibility is remote enough that it is not a major concern. My operator doesn't do flight control checks during pushback and usually not engine starts either so those reasons didn't come to mind initially. They are of course, valid reasons, in addition to the need for brakes and body gear steering.

A related question: Why do all the 744's (at least the ones I've flown) have a #4 aux pump while only some have the #1 aux pump? Obviously the aux pumps are there to supplement the relatively limited bleed capabilities of the APU's bleed air and prevent it from having to supply the Air Driven #1 and #4 pumps during engine start. Is the #1 aux pump just a nice luxury that some operators elected to pay for in their orders from Boeing or is there a reason why some planes need it (pax, BCF) and others don't (freighter)?

Thanks again.

The system 1 Aux pump is/was a customer option. A nice to have!
Allows for, (amongst other things) system pressurisation when in a hangar, no need for a mass air supply for the ADP.
Also some operators elected for electric demand pumps in the Sys.2 + 3 instead of the (more usual) ADP's.

Have known the steering bypass pin to fall out during pushback! Tow bar shear pins normally go.

Flying control check on the 744 is always done after engine start and with flaps normally extended.

There's no problem with the bleed capabilities of the apu to start 2 engines at the same time and run air driven pumps.

1 and 4 AUX AC pumps are standard on the -8.

All the above plus your downlock actuators are pressurised on all gear.

Not all airlines do that. Some at the gate when pressurised.

We used to do flying control check after engine start, before taxy, flaps selected after brake release and as taxying starts.

You leave them on because they are "powered off". If by component life you mean the battery, then yes.

Unless somebody grabs the tiller and intentionally moves it there is no risk of injury to the ground crew. The only thing which will happen with NWS pressurized for push is the A/C will go backwards in a straight line, the first sign of "trouble" will be when the ground crew attempts to turn the aircraft.

EDPs are the engine driven pumps. Component life is related to the solenoids. I don't understand it beyond that.

I know EDPs are engine driven.

They do not consume power in the on position as they are "powered off". ie. if you lose all electrics (battery included) the EDPs will still pressurize the hyrdaulic system. They consume electricity off a battery bus (which particular bus varies based on the model) when selected to OFF.

Yup, seems to mention that in the FCOM. But it specifically mentions solenoid life, not battery :confused:

The B744 uses the same size and shape of lockout pin as the B767 and B757. But the position of the locking pip is different. On the B757/767 it is at the end, and on the B744 it is in the middle.
So the ground crew takes a B767 pin, and pushes it in, and it stays in. During the pushback, the steering moves and it falls out. Good murphy proof design from Boeing. :ugh:
The steering pin fits upwards into the mech, and so falls out when it fails.

Re EDP's yes it save the on/off solenoids wearing unnecessarily and getting very hot as they are powered off / fail safe on with power failure. Same for engine bleeds on some aircraft.
My employers pressurise pre push back and do control checks on bay.

The #4 aux pump provides brake pressure without needing a running engine or APU. Handy for all sorts of reasons. The optional #1 aux pump allows for body gear steering in the same configuration. In plain English, an airplane with both #1 and #4 aux pumps installed can be towed with just a GPU plugged in.

It does not have to do with bleed demands on the APU (MEX excepted!). As noted, not all airplanes have the #1 aux installed and the MEL allows the #4 to be inop; the end result then being that the push back and engine starts are done with the #1 and #4 demand pumps working via the APU bleed.

Fair enough, I didn't consider towing too much since I'm rarely, if ever, there for that, but it makes sense. The aux pumps are on the ground handling bus so I guess there's not much else they could be used for anyway. Thanks again for the responses.