Electric Hydraulic Pumps

[Guest Hugo Frey]

HiI have a question, how to use the electric hydraulic pumps in the 737NG.Are they on for takeoff and landing or only in case of an engine driven pump failure?Regards:

[marcom 139]

They are always left on during normal operations. They serve as backups for the engine driven pumps (just as you said). Because of this reason, the electric pumps are "cross wired" meaning the left elec hyd pump is supplied by the right generator and vice versa.Regards,Mark

[Qavion 19]

"Because of this reason, the electric pumps are "cross wired" meaning the left elec hyd pump is supplied by the right generator and vice versa."Interesting.... although, technically speaking, the pumps are powered by busses. If you lose an engine, both electrical pumps should remain powered (thanks to the automatic Bus Transfer system). ;-)Cheers.Ian.P.S. Has anyone noticed that the flaps now retract in stages when you cycle all the hyd pumps off then on again repeatedy? (An improvement on the previous version, but....) ;-)

[marcom 139]

Hi Ian :-)Unfortunately, I don't have that much NG experience, but on the classics (again there are airline options for the electrics!) if you lose an engine (thus a generator) you will NOT have all electric equipments powered (like on the 767/757/747 etc)!!! Some high load equipment will be shed in EVERY case if the aircraft is airborne. This includes the electric hydraulic pumps (aswell as pitot heat, fuel pumps etc)! This is not because of the Bus Transfer System (if you isolate the bus with the inoperative generator you will lose even more equipment!) but because the generator cannot supply enough electricity to all the equipment. The APU can be started and can supply the AC bus on the side of the inoperative generator and will be able to power the hyd pumps.Here's an example: (again, 737-300/400/500)Cruising at FL200, engine 2 has a catastrophic failure (the engine stops suddenly) and thus the generator is inoperative. What will happen is that you will hear a loud click (like the generator pop sound) as the Bus Transfer System automatically kicks in and supplies the right AC Bus but sheds a few systems. As the right generator is missing, and the right AC Bus powers the left electric hydrualic pump, it will stop operating. The right elec. hyd. pump will however operate as it is supplied by the left AC Bus (powered by the left generator). Now, if you isolate the right AC Bus completely by putting the Bus Transfer Switch to off, the First Officer will lose all his CRT's (showing that not all equipment has been shed which was previously powered by the right AC Bus). As I said, even more systems will be shed such as fuel pumps and pitot heat. Exactly which depend on airline options.I am not sure if this is the same on the NG. Can someone check this?According to PMDG's simulation either generator is capable of powering every single electric component. I doubt this is realistic as something is usually shed in any case.Regards,Mark

[Qavion 19]

"I am not sure if this is the same on the NG. Can someone check this?"Looking at the NG wiring schematics for the elec hyd pumps, there doesn't appear to be specific load shedding circuits, Mark. Most of the loadshedding seems to be concerned with the galley busses with only extreme cases of overcurrent resulting in complete aircraft busses being shed: Because the pumps are on the (Main) Xfer Busses, you would probably have to lose a complete Xfer Bus to stop the associated pump running. This would seem to be similar to switching the Bus Transfer switch to OFF.Looking forward to a definitive answer on this also :-)"According to PMDG's simulation either generator is capable of powering every single electric component."Although loadshedding may not be modelled, I have seen numerous systems in PMDG lose power if you both lose a generator AND switch the Bus Transfer switch to OFF. The modelling may be better than you think ;-)Unfortunately, I'm not familiar with the Classic at all, so I can't compare the two generations. Are the generators outputting the same power? The NG has 90Kva gennies.Cheers.Ian.

[marcom 139]

Ian,Sorry, but I don't know what the output of the classic's gen is, but I'll try to find out!Another electric question:Is it realistic (on the NG) that the battery will power the left out/in board DU, the CDU and the upper DU (as PMDG does)Again, from the classic, on the ground the battery won't supply any CRT and will only power the Captain's EADI, Captain's EHSI and the left FMC & CDU when airborne.Regards,Mark

[Guest lbb]

Hi guys,I remember having a similar discussion with Ian just after the PMDG737NG was released, because I too thought that the PMDG electical system was quite flawed, based on the manuals I had on the Classic. Ian was kind enough to help me through this phase :-)One tip for Mark: http://www.smartcockpit.com/b737/B737%20ELEC.PDFwill give you an interesting insight into the differences between the Classic and the NG.As far as battery power goes, in the air the NG manual that I have shows that the displays you mention are still functioning on battery power alone. Not only that, but with the STBY POWER switch in AUTO the battery will also power these systems on the ground. (It seems to me that the BAT position of the STBY POWER switch is not very relevant on the NG. Which would explain why it isn't modelled in the PMDG!?)Quite a few differences between the Classic and NG!Leo Bakker

[marcom 139]

Hi Leo!Thanks a lot for that PDF!Indeed, I assumed that the NG behaves similar to the classic. I guess the 20 years of difference do show somewhere :-)Regards,Mark

[Guest Hugo Frey]

Hi Mark, Ian and LeoThanks for all the interesting answers to my post and also the pdf.Regards from LSZH

[Qavion 19]

Thanks Leo!The only other comments I can add is that:1. There is no mention of an air/ground input to the Stby Power System either in the Boeing Maintenance Manual Description & Operation or in the Wiring Schematics, so the 3 displays, FMC and L CDU should also be available on the ground.2. The BAT position on the Standby Power Switch allows you to have the Main Battery switch off and still run Standby Power systems, so it may be of some use (still trying to figure out what that use is tho'). :-)Cheers.Ian.