APU to pressurize HYD

[code0]

Greeting all!

Just a quick question, wonder why APU is NOT used to pressurize HYD systems in case of all ENG failure? at least the flight controls can be powered up?

Code0

[hetfield]

On 727 you can't start APU in the air.

[code0]

Type A APUs can not be started in air. Only Type B and C. 737 NGs with Garrett APU can be started at FL100 t for both BLEED and ELEC . . . All new aircraft APUs are type B or C . . .so was wondering why can't they pressurize any ESSENTIAL HYD system in case of all engines out!

Cheers!
Code0

[Beeline]

Hi,

Do you mean fitting a hydraulic pump to the APU or using the bleed air??

Doubt you could power up hydraulics on 50psi. Electrical back ups are the better option, already readily available.

[Admiral346]

Well,
the APU powers a generator.
That generator powers the HYD elec pumps.

APU driven hydraulic backup as it was set up on every type I have flown so far...

Nic

[code0]

@Beeline . . using BLEED ? . . yeah! APU can start an Engine!

@Admiral346 . . what types are those? I am very interested! My study area! probably won't sleep until my mind give me a chance to close my eyes! :)

Thanks

Code0

[Admiral346]

A320, A330, A340, CRJ 200 -700 - 900, EMJ 190 - 195

All the same setup.

Nic

PS: I don't recall exactly if you got back all the hydraulics on each type, but at least some.
And to bridge the gap until APU was started, the RAT (Ram Air Turbine) was deployed and also powered a GEN to supply elec to several systems, including hydraulics.

[Pontius]

B767:

Double engine failure = RAT drops down and powers the flt ctrls portion of the Ctr Hyd Sys, so no need for an APU to power the elec driven Hyd pumps.

Lose both AC buses, then no elec driven Hyd pumps but still got the eng driven hyd pumps. Take away those eng driven Hyd pumps for some reason and you've got the RAT. If you start the APU you'd obviously get AC power, so you'd get your elec driven Hyd pumps back. If the APU generator wasn't working for some reason you'd get air from the APU to the air driven pumps of the Ctr Hyd Sys but it is not clear in the manual if this air is sufficient to power the flt ctrls (by then the RAT should be working but I'm just postulating a scenario where it's not). The APU would, through the ADP (air driven pump), power the Ctr Hyd Sys, which is enough to power the Hyd Driven Gen but this just powers Stby buses and Transfer buses, so doesn't have the Ooomph to power the elec Hyd pumps.

So, in summary, engs fail = RAT. Flash up the APU = elec Hyd pumps ok. No RAT and no APU gen = Ctr Hyd Sys off APU air but probably not man enough to power the Hyd pumps (I'd be interested to know if someone knows differently).

[code0]

As far as I know and checked in 320s . . the APU (HONEYWELL 131-9A) they do not supply any HYD system in any way.

HYD system is powered by Green EDP, Yellow EDP and ELEC pump, Blue ELEC pum and RAT, and NO APU BLEED is supplied to any.

Add to this both Engines failed, do you generally start APU for ELEC power ?

simply my question is based on these two, why APU is not used to supply ELEC power or HYD during engines failures ?

@Admiral346 : as far as I checked no APU BLEED is connected to HYDs in A320.

Also add more hey into the my fire of confusion! in case of an in flight engine shutdown, if the CSD is disconnected, No ELEC could ever get even if the engine is restarted. so we get another abnormal event. So my point is in a acse like that y can't we use the APU to supply ELEC? Am i making sense? :ugh:

@Pontius : yes, all engines failure will bring in the RAT . . that is exactly my point, why can't the APU does what a RAT does? provided fuel, why can't it provide ELEC (or BLEED) to control the airplane more efficiently?

Thanks for replies! i cant dream peacefully! :)

Code0

[Admiral346]

Ok, Code0, rid yourself of the idea that any kind of air would be strong enough to pressurize a hydraulic system. They generally run on 3000 psi, and bleed pressure in aircraft is supplied at around 50psi.

Ao pressurizing hydraulics is always done by pumps. Now where do those pumps get their power? Usually by the engines, the so called EDP or engine driven pump.

Now that engine fails: An electrically driven pump now is activated and pressurizes the hydraulic system. Easy.

Now both engines fail: Too bad, no elec power to run the elec pumps. Here the ADG or RAT comes into play - an air driven propeller or turbine is released into the slipstream of our now powerless aircraft. That thing usually supplies a generator that has enough power to energize some hydraulics and the most esential instruments.

An uncomfortable situation that any pilot would want to solve for the better. So descending through the appropriate altitude for APU start, you get it running, and voila: An even more powerful generator kicks in, you come back to full instruments and more of the hydraulics are now pressurized.
And you get bleedair - nice, it has gotten quite cold on our way down by now so the packs start running again, you all get to take off the O2 masks.
Even better: Now you can use that bleed air to start the engines back up. Or at least try.

You get it now? There is no link between bleed pressure and hydraulic pressure. It done by changing power into different forms and finally supplying elec pumps to build up pressure.

Nic

[Meikleour]

ADMIRAL346: B747 Classic has one AIR DRIVEN HYD PUMP to supplement/replace each of the four EDP powered systems. So your statement is a bit " sweeping"!

[NSEU]

Admiral346, there are also bleed air-driven hydraulic pumps on 747-400's (up to four) and one on a 767.

However, altitude restrictions may apply to APU bleed supply. Also, the 747-400 APU cannot be started in flight (but will continue to run in the air if started on the ground).

Rgds
NSEU

[glum]

Admiral346, yet again you come over as a know-it-all, when your level of knowledge doesn't support your claims...

Some APU's can be started throughout the full flight envelope - Assuming the fuel supply is not exhausted, and is the reason the engines stopped working in the first place.

Many engines will provide enough hydraulic power through windmilling alone.

Saying 50psi is not enough to power hydraulics is plain daft.:ugh:

Do you understand the difference between pressure and flow rate? Can you appreciate how a pump being driven by a low pressure but high flow rate can produce 3000psi?

[Admiral346]

I stated the types I have flown.
If some other aircraft runs differently, then so be it.
Nothing to pound me for.

Nic

[code0]

Thanks Admiral!

thanks others for all replies, but lets not take it personal as everybody has their own experience with things!

Admiral may be right or wrong but glad that he shared his views! :ok: so that we can always refer the "things"

lets make it a collective things rather pointing fingers! :cool:

Code0

[Beeline]

@glum er 40-50psi with sufficient flow rate = every engine start.

I was thinking Airbus with no way of driving Hyds with APU apart from motoring the engines for five minutes before you blow the starter off the pad!! 50psi Accumulator pressure would be only px source.


Excuse my discrimination against Boeing. As previously said you could run Hyds from the ADP but what has caused your Total Engine Failure. Fuel??

[Yeelep]

Everybody needs to identify the aircraft type they are talking about. There's quite a few different design choices the manufactures have made over the years. I'll go with my favorite, the 737NG.

737 NGs with Garrett APU can be started at FL100 t for both BLEED and ELECI may be wrong, but I believe all NG's have Allied-Signal (Honeywell) 131-9(B) APU's. Can be started up to FL410, with Pneu and elect. available up to FL150. On the 737NG you can run hydraulics using the EMDP's powered by the APU generator. Full APU power (90KVA) is available up to FL320 and 66KVA up to FL410.

As for powering a hydraulic EDP on bleed air. I do every time I motor or start an engine, 3000psi is no problem, although any substantial flowrate demand would drop the pressure right quick.
During a inflight shutdown, if the fire handles are pulled your hyd. supply to the EDP's is cutoff. If not, you may get some benefit from the engines windmilling. I suppose you could motor a engine on the starter to get some hydraulic press./flow. The starter duty cycle is 15 minutes on the NG.
Of course, when all else fails you have your final backup, manual reversion, yeehah.

[code0]

Yes Yeelep you are right! 737NGs are fixed with Allied Signal 131-9 . . . , yet to put ma hands on 737! my fav. that Boeing face is legendary!
wonder what will be on Max? thanks to southwest they kept the 737 face! as Boeing wanted to do a clean sheet approach!