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What's the use of AUX pumps for Hydraulics

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Member
Registered: Sep 2014
Posts: 165
Location: Near Paris
Hi everyone,
I have a small question for which I was not able to find an answer (at least not one that I understood !) and that would be the role of the Aux pumps.
I do understand the procedure, but I don't really understand why you need such pumps in the first place ?
Why can't you just use the "regular" demand pump number 4 ? Is it because is would use bleed air required for start ?
And why are there different options regarding pumps (AUX1, and demand pumps 2 and 3 optionally air driven or electrically driven) ? Are there any advantages/drawback to each solution ?

Much apreciated if anyone can shed some light on those mysteries !

Charles
Member
Registered: Oct 2014
Posts: 22
Location: London UK
Hi Charles,

May have something to do with AUX being an electric pump and not engine driven pump. Important on the ground as it is pressure source for the brakes, when eng driven pump is not active (before eng start). But...I'll let the pros weigh in ;-).

EDIT: Obviously not limited to brake accumulator only, but other Hyd 4 'customers'. Not sure though.

Rgds,

Dejan
« Last edit by Deki on Mon, 29 Jun 2015 21:51:40 +0000. »
Member
Registered: Aug 2014
Posts: 375
Location: Camberley, UK
Nos 1 and 4 pumps are air-driven - and as such should not be operating for engine start due to bleed use.

The brake system is normally powered by system 4, with system 1 as an automatic backup, and system 1 also controls nosewheel steering.

Before the engine-driven pumps have pressurised, therefore - it is necessary to have pressure for systems 1 and 4, hence the AUX pumps, which are electric, only operate when engine-driven pumps are inactive, (and only work on the ground btw)

Not all variants have an AUX pump on the no 1 system - in this case it is acceptable to have the number 1 ADP in AUTO.

System 4 AUX pump should always be pressurised first before the other pumps are switched on - as the brake shuttle valves (that provide the automatic changeover if there is a failure) can allow fluid transfer, and excess fluid always drains back into system 4 reservoir - so if you pressurised system 1 first fluid could transfer unhelpfully.

HTH

Peter
« Last edit by Britjet on Tue, 30 Jun 2015 14:59:11 +0000. »
Member
Registered: May 2010
Posts: 843
From a maintenance standpoint, it's also nice to have an electric Aux pump. Engineers don't have to start the APU or the engines to pressurise Hydraulic Systems 1&4. They can just use normal external power.

e.g. If the engineers have to change a light bulb in the nose wheel well, there is a manual release handle for the nose wheel doors (and the doors will drop down with gravity), but hydraulics are required to reclose the doors. Running the APU is not always possible or wanted. It may be broken or the aircraft could be in a hangar (you don't want to set the hangar on fire). APUs are now switched off whenever possible to save fuel and to reduce noise and air pollution. Ground air/pneumatic carts can be just as polluting, expensive to run and are difficult to position when the aircraft has catering vehicles and aerobridges in position.

Of course, there are many other maintenance uses for Hydraulics 1&4, not just for changing light bulbs ;)

Probably the main reason for having air driven pumps on the #2 & #3 Systems is that some aircraft need the extra hydraulic fluid flow of ADPs. ADPs are more powerful than electric pumps. P&W engines, for example, have hydraulically driven reversers, rather than pneumatically driven ones.

Rgds
JHW
Member
Registered: Oct 2014
Posts: 12
Britjet wrote: System 1 AUX pump should always be pressurised first before the other pumps are switched on - ................

Additional for airplanes with only an AUX pump #4:
"When the parking brake is set, the first hydraulic system that is pressurized may supply a small amount of fluid to the brake lines. When brakes are released, that small amount of fluid normally returns to system 4. Pressurizing system 4 before pressurizing the other three systems precludes the transfer of the hydraulic fluid from system 1 or 2 into system 4".

Therefore, for 744's with only an AUX pump #4, system 4 AUX pump should always be pressurized first before the other pumps are switched on.

HTH
Walter
Member
Registered: Aug 2014
Posts: 375
Location: Camberley, UK
Sorry, yes, a typo, I meant system 4, of course. Thanks!
Peter
Member
Registered: Sep 2014
Posts: 165
Location: Near Paris
Hardy,
You should call Boeing and ask them to authorize this forum as an official Boeing help desk 24/7 !
Thanks a lot guys that's exactly the information I was looking for !
And that might sound stupid but when trying to think about the reasons for this system layout I totally forgot the maintenance point of view, which is clearly less known.
John, your insights into this unknown world (to me I mean !) are always fascinating!

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