I think that story as told is bogus. Why would turning the APU off pressurize system A ? That is probably an example of the ramp fiction Kyle refers to.

 

If anything you would leave the airplane without electrical power as you have not switched to engine driven generators yet.

 

BTW, the electrical hydraulic pump is not as strong as the engine driven.

 

 

No assumptions here kyle! Thats what the pilot said to me. Just for precaution.

 

You did not assume, the pilot you talked to did. These cases are well documented. Never heard of one where the tow truck broke. All injuries were caused by the bar. Kyle is right.

That story could be bogus. Admittedly I have no clue. I should probably stop perpetuating the ramp fiction, as my answer to the original questions were answered a while back but..

 

The story would work if - the engines were running the hydraulic pump, and not pressurizing System A because they were clicked off in the cockpit, AND the servos that keep the system from not pressurizing only work under power... wouldn't clicking off the APU depower the servos and allow System A to pressurize? I guess that only works if the hydraulic pump is mechanically driven by the engine... I guess if the hydraulic pump were electrical it would shut off too.

 

Again, no clue on my part.

I don´t think so. The only thing that pressurizes system A hydraulics is bleed air from engine number 1. As soon as it starts turning, bleed air starts to pressurize the system A. The system A hydraulic pump switch merely isolates the pressurized fluid, but the system is still pressurized.

 

So, why relate the APU being switched off to system A pressurization ? It pressurizes when you start number 1.

 

Edited to add: The bleed air for the system A should not be confused with the engine bleeds for the pneumatic system.

Sorry i mean't tow bar

I don´t think so. The only thing that pressurizes system A hydraulics is bleed air from engine number 1. As soon as it starts turning, bleed air starts to pressurize the system A. The system A hydraulic pump switch merely isolates the pressurized fluid, but the system is still pressurized.

While the system is "pressurized" good luck moving something without the pumps on providing fluid flow. 

 

 

Edited to add: The bleed air for the system A should not be confused with the engine bleeds for the pneumatic system.

Last I checked, the hydraulic reservoir pressurization came from the on-side bleed manifold, which is downstream of the EBV. 

No assumptions here kyle! Thats what the pilot said to me. Just for precaution.

 

Precaution? Sure. Moving a tug and killing people? No. The shear pin would snap before the tug moved too much. For what it's worth, my answers here are from working on the ramp with IDE, UAX, and several FBOs from BCB, to PHX and DVT. Most ramp accidents/deaths were from accidents in driving (most tugs we used didn't have seat belts, and people drove like idiots). Plus, it's not like the crew is going to be randomly up there swinging the tiller around quickly from lock to lock. If anything, they might nudge it and move the towbar through a partial turn. It's not swinging like an angry old dude chasing you off of his lawn swinging a bat around.

 

Heck, tugs and towbars are pretty much a thing of the past now anyway with the cradle-based systems most airports use now, so the point (and practice) is slowly becoming moot.

While the system is "pressurized" good luck moving something without the pumps on providing fluid flow.

 

Lets not confuse the terms here. The swiches don´t turn the pumps on, the engine rotating gets the pumps going, evenif the switches are off. The switch opens the gate for the pressurized fluid to operate the systems.

 

However, of course you need the switches to on to have available pressure.

 

 

 

 

Last I checked, the hydraulic reservoir pressurization came from the on-side bleed manifold, which is downstream of the EBV

 

Yes, but I mentioned that to illustrate that reservoirs pressurized by bleed air is independent of the engine bleed switches position in the pneumatic system.

So the story went:

 

The A pumps had been selected off prior to engine start. Engines were started during pushback. In accordance with procedures (in this case), the generators had not been selected after both engines were running. Pilot for whatever reason turns on the nose light, temporarily blinding the tug operator. Pilot acknowledges mistake and goes to turn off the light but instead turns off the APU.

Since the blocking valves on the Hyd engine driven pumps are powered to close, the A system suddenly became pressurized. Tug driver was injured (not killed) when the tow bar broke.

 

Fact or fiction? I don't know. But I learned a lot today, thanks all.

Found this - I have read reports that ground crews have used the wrong diameter steering pin or placed it in a rigging hole instead of the lock out hole, thus failing to properly lock out the steering

I will not follow Southwest SOP anymore of A pumps off for pushback and will instead follow AA. I think Alaska might be the same, that is a thing of the past with the classics. I will leave pumps on from now on.

Each to there own i guess! But the RW Uk operator SOPs i follow defo have A pumps off.

Each to there own i guess! But the RW Uk operator SOPs i follow defo have A pumps off.

 

If it's who I'm thinking, it's most likely for the same reasons I mentioned with SWA more than anything else. Simply a carryover from the mixed fleet with 200s and 300s. Since 2005 it's been in there because "that's how we've always done it."

Hi kyle. They only have 800s and a few 700s. Got the recent update of SOPs and no fleet less than 700s ?

I don´t think so. The only thing that pressurizes system A hydraulics is bleed air from engine number 1. As soon as it starts turning, bleed air starts to pressurize the system A. The system A hydraulic pump switch merely isolates the pressurized fluid, but the system is still pressurized.

 

The primary (3000 psi) hydraulic pressure comes from the engine-driven hydraulic pump, which is mechanically-driven by the engine accessory gear box. A secondary source of hydraulic pressure is the electric pump.

 

Bleed air does not provide motive force to pressurize the hydraulic fluid to do any useful work. The purpose of the bleed air is to pressurize the hydraulic RESERVOIR to provide a positive "push" on the fluid flowing to the inlet side of the engine-driven pump, to help prevent the pump from cavitating. (Cavitation is the fluid equivalent of a compressor stall. It can damage the moving parts of a pump very quickly.)

 

I don't know the exact amount of bleed air pressure applied to the reservoir on a 737, but on other aircraft I have worked on, it is typically between 15 and 40 psi.

 

You may be thinking of larger Boeing aircraft like the 747, which does indeed have bleed-air driven auxiliary hydraulic pumps.

Lets not confuse the terms here. The swiches don´t turn the pumps on, the engine rotating gets the pumps going, evenif the switches are off. The switch opens the gate for the pressurized fluid to operate the systems.

 

However, of course you need the switches to on to have available pressure.

 

 

 

 

 

Yes, but I mentioned that to illustrate that reservoirs pressurized by bleed air is independent of the engine bleed switches position in the pneumatic system.

You are the one confusing things by talking about reservoir pressure. This has nothing to do with pressurising the hydraulic system, which is done by the engine turning the pump and electrical power enabling the pump to move into the running position.

 

Whether the hydraulic reservoir is pressurised by bleed air or not has no bearing on the hydraulic pressure in the landing gear system. Bleed air pressure might be 40 psi, hydraulic system pressure is 3,000 psi.

 

Edit, I see Jim beat me to it.