Hydraulic Pump 4 to Aux why?

[PatrickWarner 0]

Can anyone give the true background reason why hyraulic demand pump 4 needs to be switched to Aux, rather than auto, for pushback?

 

I was puzzled by this, because from what I read in the manual, once the APU is running, the demand pumps can be fed from the APU, so why not just switch pump 4 to "auto" like all the others?

 

I have read a few contradictory answers to this but nothing that fully explains it.

 

It's clear that the Aux position provides power to the brakes, and that the brakes are needed for stopping during pushback (is it normal to have to use the aircraft brakes during pushback or is this just backup for emergencies if the tow truck cannot stop the aircraft?).

 

However the other things I've read on this seem contradictory.

 

I've read that this is because the nose gear steering needs to be disabled for pushback, but this doesn't make sense because the nose gear steering is powered by pump 1 according to the manual, and this is in fact disabled by a physical device by the ground crew.

 

I've read that this is because we want to keep the air supply for engine start, but again this seems odd because at least one (and sometimes three) of the other demand pumps are all air powered.

 

So, if the pump 4 is switched to "on" rather than "aux", does this mean there will literally be no brakes, and if so why? (and why wouldn't the alternate brakes kick in if the other pumps are pressurized?).

 

My best guess is that the real reason for this is to make double sure that the brakes system is getting full pressure, and there is a risk that the APU can't provide enough full pressure to make sure the brakes are getting sufficient when the engines are not running.  Can anyone confirm this?

 

Finally, are the hyraulics actually needed before engine start, or do you start them before the engines purely in order to validate that the demand pumps are working?

 

cheers

[Werner747 107]

Patric,

 

You are quite right it seems difficult to get a definitive answer LOL!   what I have found that is interesting and gives you a bit of an insight to the function of the #4 system, have a look at this:

 

http://www.pprune.org/tech-log/353754-747-400-aux-hydraulics-brakes-question.html

 

I hope this helps a little bit!

 

Kind regards

[PatrickWarner 0]

Hi Werner thanks for your reply.  This is somewhat helpful, but I had already read that thread, and whilst it does provide some useful information, it still doesn't seem to given a definitive answer.

[scandinavian13 5,192]

 

 

is it normal to have to use the aircraft brakes during pushback or is this just backup for emergencies if the tow truck cannot stop the aircraft?).

 

Emergency stopping only.

 

 

 

and this is in fact disabled by a physical device by the ground crew.

 

Correct (often called a bypass pin - they refer to it in the pushback dialogue).

[Marc320 2]

To have brake power at p/b.

[PatrickWarner 0]

FYI to all - I was able to dig out my old copy of the 747 manual from Hardy Heinlin's PS1.3 software (old DOS simulator for 747-400).

 

In this it says that the aux setting causes the demand pump to operate continuously and not only when there is a demand for pressure.  The implication of this I guess is that if it is in auto, the pump will only start when it senses that there is a demand for pressure.  If the pilot hits the brakes and the pump is not running, there will be no pressure, and by the  time the demand pump kicks in it will be too late. 

[Lukas_Mathijsen 41]

FYI to all - I was able to dig out my old copy of the 747 manual from Hardy Heinlin's PS1.3 software (old DOS simulator for 747-400).

 

In this it says that the aux setting causes the demand pump to operate continuously and not only when there is a demand for pressure.  The implication of this I guess is that if it is in auto, the pump will only start when it senses that there is a demand for pressure.  If the pilot hits the brakes and the pump is not running, there will be no pressure, and by the  time the demand pump kicks in it will be too late. 

From memory, doesn't the switch have AUX-OFF-AUTO-ON positions? What's the purpose to setting it to AUX instead of ON?

[tcas_climb 45]

 

 

From memory, doesn't the switch have AUX-OFF-AUTO-ON positions? What's the purpose to setting it to AUX instead of ON?

 

Been years since i toyed with the pmdg 747-400, but i believe the demand pumps run off air, while the AUX mode runs an electrical pump.

[PatrickWarner 0]

 

 

From memory, doesn't the switch have AUX-OFF-AUTO-ON positions? What's the purpose to setting it to AUX instead of ON?

 

Good point.  The on setting also says "pump runs continuously", whereas the aux setting says that the auxiliary pump of system 4 runs continuously.  There should be air available from the bleed air from the APU by the time you are pushing back, so it's still not 100% clear.  I can only guess that the aux pump somehow prioritizes the brakes to be sure that they will work during pushback.

[kevinh 409]

Good point. The on setting also says "pump runs continuously", whereas the aux setting says that the auxiliary pump of system 4 runs continuously. There should be air available from the bleed air from the APU by the time you are pushing back, so it's still not 100% clear. I can only guess that the aux pump somehow prioritizes the brakes to be sure that they will work during pushback.

See tcas_climb's answer to this point above. Systems 1 and 4 demand pumps are pneumatic so need duct pressure to run, which affects engine starting during pushback. The AUX pump is electric so can be run without affecting duct pressure.

 

 

Sent from my iPhone using Tapatalk

[G.Franco 0]

Hi Patrick,

Before the push back&towing the Hyd pumps that have to be switched are two.

The ADP1 in AUTO position and the AC pump to ON position.

The ADP1 to pressurize the related Hyd system n°1 and to provide hyd pressure to the Body Gear steering.

The AC pump to provide the Hyd pressure to the brakes.

I remember you that the push back&towing can be performed with engines shut so to provide the necessary Hyd pressures the APU Air&electric power is needed.

Cheers Gianfranco

[PatrickWarner 0]

 

 

See tcas_climb's answer to this point above. Systems 1 and 4 demand pumps are pneumatic so need duct pressure to run, which affects engine starting during pushback. The AUX pump is electric so can be run without affecting duct pressure.

 

That's true, but pump 1 is pneumatic and is left on, and as I understand it, pumps 2 and 3 are also pneumatic sometimes depending on customer options.  However, I guess an aux electric pump will continue to provide hydralic brake power even if bleed air is in short supply due to engine start.

[Marc320 2]

Hello guys. found on AFR FCOM.

 

Engines start with pneumatic starter. All APU bleed is for engines during start. Engine n*4 have an EDP on accessory gear box as primary, and a pneumatic pump ADP on demand (is EDP fail) and ACMP (electrical Pump) on AUX. Normal braking system is on hydraulic n*4. To have brake as p/b you set the switch to Aux to have a braking source power. 

 

Hydraulic Systems

Each system is powered by an engine driven pump and a demand pump installed

in parallel.

 

Engine Driven Pumps

Each system has an engine driven pump (EDP). The EDP is pressurized when the

engine is running and the engine pump switch is ON.

 

Demand Pumps

The demand pumps supply normal system demands if an engine or EDP fails.

Systems 1 and 4 have air driven demand pumps. The bleed air manifold provides

pneumatic power for the air driven pumps. Systems 2 and 3 have electric motor

driven demand pumps. In the AUTO position, the demand pumps operate during

periods of high system demands, such as flaps in transit or gear retraction.

 

Auxiliary Pump

System 4 has an electric auxiliary pump for ground handling operations.

 

Normal Brake Hydraulic System

The normal brake hydraulic system is powered by hydraulic system 4. The brake

pedals provide independent control of the left and right brakes.

 

Alternate Brake Hydraulic System

If hydraulic system 4 pressure is low, hydraulic system 1 supplies pressure to the

alternate brake hydraulic system. If hydraulic pressure in systems 4 and 1 are low,

system 2 powers the alternate brake system.

Loss of hydraulic systems 4, 1, and 2 causes the brake source light to illuminate

 

See you.

[G.Franco 0]

All four system Pumps are  engine driven or pneumatic driven.You have to switch the pump1 to auto because until the engine 1 is shut the body gear steering is activate by the air provided from APU.It's possible only move the plane around the airport using the APU air&electric power and having the body gear steering and the brakes activated.

Gianfranco

[G.Franco 0]

demand pumps???Maybe you call demand pumps the ADPs.These air driven pumps automatically start when the HYD pressure drops below 2600psi?Electric motor driven pumps for systems2&3??? I flew 21 years as F/E of Alitalia B-747 and I can assure you that on board of B-747 there are 4 EDP(engine driven pumps) and 4 ADP(air driven pumps).There's also an auxiliary electric pump operative only on ground for ground necessities as provide HYD pressure to set parking brake or push back&towing the plane around the airport.

When you'll be able to build your B-747 you can make the electric motor driven pumps for the systems2&3.???

It's a pity but until now these pumps even do not exist on board of the B-747.

Regards  Gianfranco