Question about the test of hydraulics pump during start up

[lucky-hendrix]

Hi all,

 

I have a question about the purpose of these tests. I like to understand the aircraft's systems in depth. And neither the manual or training mission explained this.

 

I understand that the hydraulic transfer pump purpose is to pressurise the flt hydraulic from the combined hydraulics (and vice versa) when a motor is stopped.

 

Why then does the startup make you pressurise the combined hydraulics first by cranking the left engine ? Because with both hydraulics circuit at 3000psi. You won't see it if the hydraulic transfer pump is not working ?

 

Other point : the emergency flight hydraulics pump.

I believe this pump is for providing flight controls when both engines(or both hydraulic pump are down ). What are the high &low mode ? When would you use them?

 

Also when you start to crank the right engine for the first time, for a few seconds you can here the emergency hydraulic pump turn on. Is this normal ? Why wasn't it running before starting the engine crank.

 

 

Thank you

 

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[QuiGon]

I understand that the hydraulic transfer pump purpose is to pressurise the flt hydraulic from the combined hydraulics (and vice versa) when a motor is stopped.

 

Why then does the startup make you pressurise the combined hydraulics first by cranking the left engine ? Because with both hydraulics circuit at 3000psi. You won't see it if the hydraulic transfer pump is not working ?

Great question! I was also wondering about this (besides other things) when going through the checklist in the Heatblur manual. It doesn't make sense to me as well :dunno:

[iRocco]

I leave them alone and nothing ever failed or happened, it's either not modeled or bugged or it's OK not to do anything with those switches.

 

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[lucky-hendrix]

Of course

 

But I'm asking to understand how the systems works. We are not just clicking switches for fun. There is a reason this test is in the natops ...

 

Feel free to not do it. But I want to understand why this test is asked

 

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[sLYFa]

You won't see it if the hydraulic transfer pump is not working ?

 

Hydraulic pressure will drop without the respective engine spinning. So if your combined pressure remains at 3000psi with left engine RPM at zero you will know the transfer pump is working.

 

What are the high &low mode ? When would you use them?

 

These are different power modes for the backup pump. High mode will eventually overheat the pump and cause it to fail, therefore it should only be used for a brief amount of time. HIGH mode increases the rate at which you stabilizers will move compared to LOW mode, which is very important for inflight refueling and landing. You would use HIGH mode for these situations and LOW for any other part of the flight (i.e. cruise, descend).

[lucky-hendrix]

thanks for the reply !

 

Does it drop in the Sim ? it seemed to stay at 3000psi. maybe I didn't wait long enough

[Victory205]

The momentary spin up on the engine is to "prime" the hydraulic system and put head pressure on an accumulator to prep the system for the subsequent checks.

 

The emergency flight hydraulics were designed to get you feet wet so you could eject. There were a couple of disasters with crews trying to make it all the way to a landing on that system alone. A close friend had testing a fix for that as his first assignment as a test pilot at PAX. They put a system in fkr shunting normal hydraulics in order to test the backup. AFAIK, it was never put on the aircraft for whatever reason. Maybe on the F14D? I never heard.

 

The combined system has the major components on it, gear, flaps, etc, so the left engine was thought of as the "main" engine, while the right was the "auxiliary" engine, so to speak.

 

It explains why the left engine required more indicated airspeed for a successful air start.

[lucky-hendrix]

thanks for the input, very interesting.

 

 

I just tried in the Sim.

Turn ON Right engire.

Hydraulics transfer pump to SHUTOFF

Crank left engine untill Combined flight hydraulics is 3000psi

Stop left engine crank.

Put hydraulic transfer pump in "Normal"

 

Combined flight pressure is still slowing dropping (I waited to 1000psi). Is this a bug ?

[Jowen G. Bruère-Dawson]

Hi guys, I up this because, few years later, I still had the same question as lucky-hendrix and I found the answer, but it also bring another question about the hydraulic transfer system :

If you turn off electrical ground power before cranking left engine, the HYD transfer pump will pressurise the COMB hyd. system from the FLT system.

But if you don't turn off electrical gorund power, the HYD Transf. Pump wont work. It means this HYD transf. pump is not working under GPU ? 

Thanks

 

EDIT : It's in the NATOPS : When ground power is on, no working hydraulic transfer pump, except if the ground crew make it running. I still wonder why this choice.

 

Edited February 7, 2022 by Jowen G. Bruère-Dawson

[RustBelt]

If you're on Ground power, you're mostly doing things that you would rather have the systems stay isolated for. Like running one engine, while changing the hydraulic fluid on the other side etc...Since you're supposed to pull Ground power once your Right engine stabilizes (because Ground power is right up next to the left intake and so you wouldn't want to unplug it after starting L), it's no big deal in normal operations since you check the Hydraulic Transfer pump between starting R and Starting L.

[Jowen G. Bruère-Dawson]

On 2/9/2022 at 1:13 AM, RustBelt said:

If you're on Ground power, you're mostly doing things that you would rather have the systems stay isolated for. Like running one engine, while changing the hydraulic fluid on the other side etc...Since you're supposed to pull Ground power once your Right engine stabilizes (because Ground power is right up next to the left intake and so you wouldn't want to unplug it after starting L), it's no big deal in normal operations since you check the Hydraulic Transfer pump between starting R and Starting L.

Ok, thanks for the answer