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Trimeresurus
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How much hydraulic pressure does the APU provide vs the engines?

Fri Dec 11, 2020 3:20 pm

For example in the 737NG, there are two hydraulic pump systems, engine pumps and electric pumps(can be either operated via the APU transfer buses or external power) but the FCOM says they've 6 times less output pressure than the engine driven pumps. So any command on a hydraulic actuated surface will be 6 times slower.

Question comes in pushback operations. During pushback, the pilot is instructed to set the parking brake after it's completed(at that point one engine is running but the other one still starting up most of the time), in that case, are the parking brakes pressurized via the EMDPs? Or is the engine pump switched on after the first engine comes alive, to give the full available pressure? If EMDP is used, do you have to press in the brake pedals 6 times longer than you otherwise would before pulling the barking brake handle?
 
Dalmd88
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Re: How much hydraulic pressure does the APU provide vs the engines?

Fri Dec 11, 2020 4:01 pm

It doesn't really effect the brake system much. The brakes have a large hydraulic accumulator the provides a backup of full pressure even with no hydraulic systems pressurized. It's been I while since I've been around the 737, but I think it gives you 2-3 full applications.

I think the pressure from the electric is the same but they provide much less volume of fluid so the actions are slower. When we did the flight control checkouts in the hangar I don't recall the pressure being lower. Defiantly not 6x lower.
 
Woodreau
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Re: How much hydraulic pressure does the APU provide vs the engines?

Fri Dec 11, 2020 4:36 pm

APU doesn't provide any hydraulic pressure - it's just a pneumatic air source and an electric power source.

even though the engine driven pumps supply a higher flow than the electric pumps, the system pressure is still 3000psi whether it comes from the engine driven pumps or the electric motor driven pumps.
Bonus animus sit, ab experientia. Quod salvatum fuerit de malis usu venit judicium.
 
DualQual
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Re: How much hydraulic pressure does the APU provide vs the engines?

Fri Dec 11, 2020 4:38 pm

The APU doesn’t provide any hydraulic pressure so the answer is zero. The APU can provide an electrical power source for the electric hydraulic pumps. The electric pumps provide the same pressure as the engine driven pumps but a lower volume. As you’ve already learned, a high demand system will operate slower on just the electric pumps (the flaps as an example). In terms of setting the parking brake on just the electric pumps or one engine pump, no difference in performance of that system.
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Okcflyer
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Re: How much hydraulic pressure does the APU provide vs the engines?

Fri Dec 11, 2020 7:27 pm

Brakes are almost entirely pressure driven and require very little fluid flow (volume).

As stated, aircraft run constant pressure hydraulic systems. Flow rate (ie volume) through the pump / system depends on the “demand”.

Brake demand is next to nothing. Further, the demand is consumed from the accumulators storing the energy.

Does the 737 have a PTU between hyd systems to assist the electric pumps for high demand applications once in the air?
 
unimproved
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Re: How much hydraulic pressure does the APU provide vs the engines?

Fri Dec 11, 2020 11:47 pm

Okcflyer wrote:
Brakes are almost entirely pressure driven and require very little fluid flow (volume).

As stated, aircraft run constant pressure hydraulic systems. Flow rate (ie volume) through the pump / system depends on the “demand”.

Brake demand is next to nothing. Further, the demand is consumed from the accumulators storing the energy.

Does the 737 have a PTU between hyd systems to assist the electric pumps for high demand applications once in the air?

737 has quite a weird hydraulic system. The EDP is the main source for both system A and B, with an EMDP on each of them as well. Then a standby system driven by its own EMDP.

IIRC the EDPs handle everything while engines are running. No electric pumps needed.
 
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452QX
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Re: How much hydraulic pressure does the APU provide vs the engines?

Wed Dec 30, 2020 7:22 am

Dash 8 APUs will give you nothing at all, you either need a DC power GPU or engines running. Brake pressure can come from a manual hand pump in the #2 engine
 
T prop
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Re: How much hydraulic pressure does the APU provide vs the engines?

Wed Dec 30, 2020 7:41 am

[url][/url]
452QX wrote:
Dash 8 APUs will give you nothing at all, you either need a DC power GPU or engines running. Brake pressure can come from a manual hand pump in the #2 engine


DC GPU won't run the electric hydraulic pumps on the Dash, they're AC powered.
 
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zeke
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Re: How much hydraulic pressure does the APU provide vs the engines?

Wed Dec 30, 2020 8:14 am

T prop wrote:
DC GPU won't run the electric hydraulic pumps on the Dash, they're AC powered.


Surely there is a static inverter off the DC bus, how else is the AC bus powered when in an emergency situation where the aircraft is just powered off batteries ?
“Don't be a show-off. Never be too proud to turn back. There are old pilots and bold pilots, but no old, bold pilots.” E. Hamilton Lee, 1949
 
T prop
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Re: How much hydraulic pressure does the APU provide vs the engines?

Wed Dec 30, 2020 9:12 am

zeke wrote:
T prop wrote:
DC GPU won't run the electric hydraulic pumps on the Dash, they're AC powered.


Surely there is a static inverter off the DC bus, how else is the AC bus powered when in an emergency situation where the aircraft is just powered off batteries ?


Yes, there are 3 inverters on board that provide single phase fixed frequency AC power for avionics. Heavy demand AC items like the standby hydraulic pumps, fuel pumps anti ice heating etc. use 3 phase variable frequency AC power that is only available with either engine running with it's propeller unfeathered...or if you have an AC GPU connected.
 
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zeke
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Re: How much hydraulic pressure does the APU provide vs the engines?

Wed Dec 30, 2020 9:32 am

T prop wrote:
Yes, there are 3 inverters on board that provide single phase fixed frequency AC power for avionics. Heavy demand AC items like the standby hydraulic pumps, fuel pumps anti ice heating etc. use 3 phase variable frequency AC power that is only available with either engine running with it's propeller unfeathered...or if you have an AC GPU connected.


I guess on the dash brakes are not really required, beta or whatever it’s called would be fine.

Just had a look at the system diagram, the electric pumps are only really standbys with lower output pressure/flow by the look of it, EDPs are then main source.
“Don't be a show-off. Never be too proud to turn back. There are old pilots and bold pilots, but no old, bold pilots.” E. Hamilton Lee, 1949
 
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452QX
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Re: How much hydraulic pressure does the APU provide vs the engines?

Wed Dec 30, 2020 7:48 pm

T prop wrote:
[url][/url]
452QX wrote:
Dash 8 APUs will give you nothing at all, you either need a DC power GPU or engines running. Brake pressure can come from a manual hand pump in the #2 engine


DC GPU won't run the electric hydraulic pumps on the Dash, they're AC powered.


Mistyped them, I meant the AC port in #2, not the standard 28VDC up front
 
m1m2
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Re: How much hydraulic pressure does the APU provide vs the engines?

Wed Dec 30, 2020 10:09 pm

The #2 engine is generally started first on the Dash 8 during push back, or during maintenance runs. The #2 hydraulic system powers the park brake on the Dash 8, (all of them). Even with the prop feathered, the engine driven pump (EDP) produces 3000 psi, so no problems with the brakes. Once the #2 is unfeathered, then you have ac power available which enables both SPU's (electric hydraulic pumps) to be switched on, also providing hydraulic pressure (although the SPU produces 2750 psi rather than 3000) to the #1 system. The prop on the Dash 8 100 through 300 has to be at 785 rpm to provide enough rotation for the ac generators to come on line. On the Q4 that's 660 rpm.

This is for the 100 through 300 series, the Q4 only has one SPU, it's on the #1 side and powers the #2 side via a PTU (Power Transfer Unit). As posters above have said the APU on the Dash 8 does not produce any Hydraulic pressure and I've always wondered why they didn't put an ac generator on the thing, would be awesome to be able to run the hydraulics when carrying out maintenance without dragging out an ac cart.

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