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Hydraulics : some technical questions

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Hi gents, I'm a bit puzzled as to how the hydraulic is working. I got a few questions : * First off, reading the procedures, I have trouble finding the right moment to power up the SYS B ENG2 pump. NP.21.30 section of said procedures is as follows :

Hydraulic panel.................................................................Set F/O If pushback is needed and the nose gear steering lockout pin is not installed: WARNING:Do not pressurize hydraulic system A. Unwanted tow bar movement can occur. System A HYDRAULIC PUMP switches – OFF Verify that the system A pump LOW PRESSURE lights are illuminated. System B electric HYDRAULIC PUMP switch – ON Verify that the system B electric pump LOW PRESSURE light is extinguished.
and then, on next page you can read :
Verify that the brake pressure is 2,800 psi minimum. Verify that the system B pressure is 2,800 psi minimum. If pushback is not needed, or if pushback is needed and the nose gear steering lockout pin is installed: Electric HYDRAULIC PUMP switches – ON Verify that the electric pump LOW PRESSURE lights are extinguished. Verify that the brake pressure is 2,800 psi minimum. Verify that the system A and B pressures are 2,800 psi minimum.
But since I do power up the B system, push back and, then only, power up A system, I refer to the Pushback or Towing procedure on next page which states :
Pushback or Towing Procedure The Engine Start procedure may be done during pushback or towing. Establish communications with ground handling personnel. C CAUTION:Do not hold or turn the nose wheel steering wheel during pushback or towing. This can damage the nose gear or the tow bar. CAUTION:Do not use airplane brakes to stop the airplane during pushback or towing. This can damage the nose gear or the tow bar. Set or release the parking brake as directed by ground handling personnel. C or F/O When pushback or towing is complete: Verify that the tow bar is disconnected C Verify that the nose gear steering lockout pin is removed C System A HYDRAULIC PUMPS switches – ON F/O Verify that the system A pump LOW PRESSURE lights are extinguished Verify that the system A pressure is 2800 psi minimum.
Ok, so when I read : System A HYDRAULIC PUMP switches – OFF I understand that both ENG1 and ELEC2 switches should be turned OFF. Now, when I read : System B electric HYDRAULIC PUMP switch – ON I understand that I should turn on the ELEC1 switch ON. So I pushback with just the ELEC1 switch ON, all others, including ENG2 are off. Then, afer pushback is complete, the procedure states : System A HYDRAULIC PUMPS switches – ON F/O So theres no mention of the ENG2 switch at least as far as I read. I usually end up turning it on at that moment or before taxi along with the ELEC1 switch. But I'd like to understand how this is supposed to be done. * Second 'problem' is that when I do check brake system pressure at that point of the procedure I dont get 2800 PSI on both systems, system A is at like 2780 or around that value. However, once the engines are running I always exceed that minimum by a comfortable margin and the brakes perfom normally. * Finally, I also noticed something while in flight, the fluid quantity in system A tank drops to about about 71-72% . So I refered to the manual, and on page 13.20.8, one can read the following :
Standby Hydraulic System Leak If a leak occurs in the standby system, the standby reservoir quantity decreases to zero. The LOW QUANTITY light illuminates when the standby reservoir is approximately half empty. System B continues to operate normally, however, the system B reservoir fluid level indication decreases and stabilizes at approximately 72% full. Variations in Hydraulic Quantity IndicationsDuring normal operations, variations in hydraulic quantity indications occur when: •the system becomes pressurized after engine start•raising or lowering the landing gear or leading edge devices•cold soaking occurs during long periods of cruise. These variations have little effect on systems operation.If the hydraulic system is not properly pressurized, foaming can occur at higher altitudes. Foaming can be recognized by pressure fluctuations and the blinking of the related LOW PRESSURE lights. The MASTER CAUTION and HYD annunciator lights may also illuminate momentarily.
Ok but, I don't use failures yet and none of those lights/warnings are lit. However on the two flights where it happened I was close or at maximum ceiling operation. Addtionnally, is there a way to know what the quantity in the standby reservoir is? Any help understanding those three points would be much appreciated :) Regards,

Pierre ROBELOT

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General rule of thumb. The Eng Hydraulic switches stay on always. This is the engine hydraulics and shouldn't be switched off. Switch on the Elec Hyd pumps once push is complete and tow equipment is removed. The PSI will vary. 2780 is close enough to 2800.


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I could be wrong but when you activate a Hydraulic pump and send fluid through the system the tank level will drop slightly due to the fluid being sent to the various components.


Ash Keelson

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I hink it was mentioned before, but it's safe to keep the ENG hyd switch 'on' at all times. I think a forum member and tech mentioned it's killer for the pump solenoids to be switched on/off all the time. Here's what I understand a pushback procedure should entail regarding hydraulics.Ready for push, engines off.Hyd b elec on - read ~2800 psi on brake accumulator gauge and sys display.Park brakes on, chicks (haha Chocks I mean) removed then initiate pushback.Parking brakes now off, pushback occurs then all done. Brakes back on.Hyd A Elec pump now on.Do my engines start and everything else and good to go. Remember the ENG hydraulic pumps have been selected on the whole time...but only pressurised when I brought the engines online after the push.


Patrick Houghton

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First problem: Engine hydraulic pumps for system B always stay on, i think. Since the engines aren't running, they aren't doing anything, as illustrated by the "Low pressure" light that's illuminated before engine start. Second problem: Engine driven pumps produce a lot more pressure than electric driven pumps, and that is where the extra pressure is coming from. The electric pumps are design to produce the bare minimum. In case of an engine failure, you want to put as little strain on the other engine's generator, so it makes sense to not have the pumps any bigger than they need to be. The same applies for dual engine failure, where you're either running on APU or battery. Third problem: Hydraulic quantity is measured in the reservoirs, i believe. The 20-28% quantity of fluid that is missing is found in various other systems (like gear, flight controls, etc.). A drop in quantity of that magnitude is perfectly reasonable.


Cristi Neagu

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Ah thanks for the quick and numerous answers ! Good to know there's nothing wrong. I got a tad bit alarmed by the drop of quantity on system A. At that value the square surrounding the numbers is filled white instead of black. Regards,


Pierre ROBELOT

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Another quick hint, dont forget to put the gear lever in off after takeoff. Will help keep those electric pumps from overheating.


Patrick Houghton

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That I always do :) I'm your average procedural pilot ^^ Regards,


Pierre ROBELOT

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Hmm. A bit confusing. I too have been curious about the decreasing fluid level in system A. A word of caution, however. In RL, a ground service start is done with Engine 1 which will pressurize hydraulic system A if that is left selected ON. If a pushback is then required without having an installed steering lockout pin the correct configuration is Hydraulic Sys A = OFF, Both Electrics = ON, and leave Hydraulic Sys B = ON. If an APU start, this should not be necessary since Engine 2 is started first hence if pushback is done before Engine 1 start, Sys A is still unpressurized even if set to ON. Oftentimes if APU start, both engines are started after pushback so again having both Hydraulic systems left set to ON would not matter. This is actually pretty clearly explained in FCOM page NP.21.30-31


Craig Williams

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There is a little bit of confusion about ENG and ELEC pumps.The main Hydraulic systems on the 737 are 2, System A and B.Each system, for redundancy, have 2 different main pressure sources, one engine, and one elec, the electric power for the elec pump comes from the cross side engine generator(A= ENG1, ELEC2), (B=ENG2, ELEC1).Each system (A or B ) feeds its users, For example Flaps and slats are powered by System B, Landing gears extension/retraction are powered NORMALLY by system A, Brakes are normally powered by system B,primary flight controls from both systems, steering with A system, thrust reversers by the onside hydraulic system 1=A,2=B. There are some logic that let the other system to power the system in alternate mode, sometime limited (Sys B only for gear retraction for example) and I've not mentioned a Stby system or a PTU.Elec pumps are electrically powered, they are used each time the engine pumps are not avaiable and helps the engine pumps during hard loads. Selecting the relative switch ON will power a contactor that feeds the pump with 3 phases 115VACEngine pumps are different, they are not electric pump, they are geared to the engine, if engine N2 turns, pressure is avaiable, if not, pressure remains low disregarding cockpit selection.So, if engine is stopped, leaving the switch in ON position will result in 0 pressure.The engine pump has a solenoid, it puts the pump to "idle" or we can say tat it unload the pump, or turn it off. So as you can read it uses an opposite logic, Pump ON soleenoid de-energized, pump off, solenoid energized. This is for safety reason, if you lose power on the aircraft the pump will continue to work if the engine is operational.So, basically, you must understand that if you want to unpower, unload an hydraulic user (the steering in this case) you need that the entire system A must be turned off.To unload the system you must know the engine status for the rest of the pushback manouver.The best way to pushback is always with steering bypass pin installed that let you use system A pressure.


Regards

Andrea Daviero

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And during pushback I do remember the ground crew saying "pushback pin installed". So... no reason to turn off any pumps.On a side note, even if you didn't have the bypass pin, I don't think it would make a difference whether or not hydraulics are off. I mean... PMDG have simulated a lot of failures, but I don't remember "pushback car tow bar" being one of them. That, and you cannot steer during pushback because FSX won't have it.


Cristi Neagu

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I've seen it done two different ways:

  1. Elec Hyd Pumps on before pushback on at shutdown
  2. Elec Hyd Pumps on during preflight off during secure checklist at the end of the day

Eng driven pumps are always on except by QRH. A sys qty does go down in flight.


Matt Cee

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As stated in previous posts, the excessive use of the OFF function of the engine pumps (that powers the depressurizing solenoid) will worn this solenoid. This can be dangerous because in case of fire the system won't shut off hydraulics and skydrol (the hyd fluid) can burn extending the fire.On Classics I've found few times worn depressurizing solenoid connectors, in more than one occasion one of the wires was broken.During maintenance I've changed my procedures and when I do dry motoring, I test the depressurizing functionality by switching the pump switch OFF. If it works pressure will decrease, if not, I have a starting point for troubleshooting. NG has a different engine, maybe the problem is now solved.


Regards

Andrea Daviero

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General rule of thumb. The Eng Hydraulic switches stay on always. This is the engine hydraulics and shouldn't be switched off. Switch on the Elec Hyd pumps once push is complete and tow equipment is removed. The PSI will vary. 2780 is close enough to 2800.
That is not quite true ! You leave them on at shutdown to prolong the life of the solenoids.( see FCOM VOL 2 page 1165)There are occasions when they need to be turned off.

Frederic Steiner.

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Very interesting read Andrea. Thanks for the answers everyone, I know what to do now. Regards,


Pierre ROBELOT

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