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.

My statements that you quoted sir, with all due respect, are they false ?

Do the pumps not turn with only the engine turning independent of the switch position ?

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

 

Back in the day, the operator I'm thinking of had 200s and 300s up through 2005. Since then, as I mentioned in my earlier post, they just carried the procedure because it's what they'd always done, so it continues on the 700s and 800s (as they operated those concurrent with the 200 and 300 back in the early 2000s).

My statements that you quoted sir, with all due respect, are they false ?

Do the pumps not turn with only the engine turning independent of the switch position ?

The pumps turn but there is no pressure output with the switch OFF. The confusion I referred to was your statement that hydraulics were pressurised by the bleed air system. On the 737 the hydraulics are not pressurised by bleed air. The reservoir is, but not having that air pressure will not prevent the engine pump (or the electric pump) from pressurising the lines.

I see where the confusion is--I stand corrected. So this is what I understand:

 

Bleed air pressurizes fluid in the reservoirs.

 

Engine turning gets the pump turning.

 

The switches dont turn the pumps on, but open the gate and allow pressure to pressurize the system components.

 

Correct ?

Yeah, that's correct from the 737 manuals I've seen. 

 

The EDPs are driven by the accessory gearbox, and provide high-pressure and high-flow to drive big items. It can not be turned off, but can be isolated.

The EMDPs are electrically powered and provide high-pressure but significantly less flow.

 

The reservoirs are pressurized to prevent cavitation from the on-side bleed air manifold. 

One thing remains to be explained to me please, because I don´t know the innards of the pumps themselves. Boeing now recommends leaving the engine driven pumps on at all times "to prolong solenoid life".

 

And if an airline is fleeted only with NG´s, and their SOPs are in line with this, they will leave these switches alone in the on position all the time.

 

There is a downside to having these switches off while the engine and consequently the pumps are turning and I would like to know exactly where the damage is done and why ?

The default (unpowered) state of the valves at the outlets of the engine-driven hydraulic pumps is open. When you select an engine pump "off" with the toggle switch on the overhead panel, you are applying electrical power to the solenoid on the valve to move it to the "closed" position. Leaving that switch in the "off" position when the engine is not running will result in power being applied to the close solenoid continuously, which can cause the solenoid to become quite hot - thus, Boeing's recommendation.

 

Ordinarily, the only times one would close that valve would be in the case of an engine fire, or a failure of the engine-driven pump, or when there is major leak in the hydraulic system downstream of the pump - otherwise the valve is always left open.

Thanks for the explanation Jim. More doubts remain, because I am still wondering then why these switches exist at all. You mentioned those cases where they would be turned to off. But, in all those cases isnt there already a mechanism to shut off pressure to the systems ? for example, I uderstand that when engine fire when you pull the fire handle this cuts the fluid flow so that it is impossible for the engine driven pump to receive fluid.

Then, in the case of a leak, I don´t recall exactly I would have to go back to the books but are there not other detection mechanisms that accomplish shutting off pressure and activating the standby pressure system ?

I don´t think that the switches are there for show maybe an added feature and just because they existed and were necessary and the overhead is still the old overhead with few modifications. It is just that apparently you seldom have to fiddle with the switches.

 

 

If it was dark enough, I just threw my sunglasses on and the pilots usually got the point.

 

It wasn't me!

Ok cheers kyle! Do you know roughly when the tow truck connects to the acft for pushback?

Ok cheers kyle! Do you know roughly when the tow truck connects to the acft for pushback?

 

Depends - I'd say a good average is 5-10 minutes prior, but for more detail:

 

If it's the first departure, I'll hook the tug up to the first plane as soon as I get a chance to get that action out of the way. This could be as early as right after it arrives. From there, the tug just moves down the line of departures as it becomes available. If the gate planner has any brains, they'll stack the area with departures that are spaced out reasonably. At IDE, as an example, our ramp teams each managed a group of about 6 gates, and the planes were all about 8-10 mins apart.

 

I'd say it was "normal" to have everything hooked up and ready to go at latest 5-10 min prior to departure on a normal day where we weren't running behind.

 

I'm not sure how other airlines run their ramps, but I do know that for the CRJs at IDE, we'd pull the torque links apart (disables nose wheel steering physically) prior to connecting the towbar, mostly eliminating the towbar hazard (if one was being used). With the SAABs (UAX), they all had their own lockout pins that stayed with the plane (we'd pass it up to them after the push through a special door on the pilot side of the deck). Those were installed during the pilot post flight walk around.

 

Disclaimer, of course, being that I haven't been out on the ramp to run it since April of 2010, so my memory is kinda foggy on timing. Like I said, I usually put the tug on the first departure as soon as I could to get that task out of the way (though, if it was a cradle-type, and not towbar, it wouldn't be up in the cradle until the main cabin door closed - it was just strapped up with the cradle lip where the front chock would be).

 

 

because I am still wondering then why these switches exist at all.

 

Isolate the pumps from the system when required.  The "when required" seems to be your question, which is getting off topic.

Jim's insight that the isolation solenoid's powered off state is open makes that gate story I mentioned earlier at least feasible.

 

As lame as I am I think this is fascinating stuff. When downloading this plane one of my long term goals was to understand what all the switches do, why you press them, and when.

 

Great stuff.... great plane!

I will find out if it is possible to de energize the solenoid by replicating the story, I am going to have the NGX running on APU gens and I will deliberately switch the APU to off, the pressure on system A should start to rise to 3,000 psi. If not, there is a stby system continuing to power it.