737NG PTU

[PosClimb]

Something I don't get about the 737NG PTU..

1. The PTU is only used when Engine #2 EDP is lost to provide the extra volume of hyd fluid required to retract the Leading Edge devices.

2. The PTU itself is powered by Hyd System A pressure.

3. The Hyd B reservoir has a hole in the very bottom of the reservoir that is used to supply the PTU.


Question is then: Why does the Hyd B reservoir supply anything to the PTU? I thought only Hyd A pressure is used to power the PTU.

Does the PTU provide hydraulic pressure directly to the LE devices via its own separate line???

That's the only reason I can think of why the Hyd B reservoir would supply fluid to the PTU.

[Cirrus22]

The PTU consists of a motor and a pump. System A fluid drives the motor which drives the pump. The pump pressurizes System B fluid to power the Leading Edge Devices. There is no transfer of fluid through the PTU, it is simply a 'power transfer' from A fluid to B fluid.

[PosClimb]

I understand that perfects, but my original question is why does the Hyd B Reservoir have a hole in the bottom of it (not the stand pipe) that is, from what I've read, somehow associated with the PTU.

Since Hyd A powers the PTU, I don't see why the Hyd B Reservoir would provide any sort of fluid to the PTU.

[Cirrus22]

System A powers the PTU, but the PTU uses (pressurizes) System B fluid to power the LEDs.

The standpipe is there so that in the event of a leak in the hydraulic pumps (or downstream thereof) there will be enough fluid left in the reservoir to be used by the PTU.

[framer]

This is one thing that I too have struggled to get my head around. I think I understand it. One way I look at it is that if there were no fluid in system B then the PTU wouldn't work at all because it is still B fluid powering the LED, it just gets it's pressure from sys A fluid thats all.
I stand to be corrected.

[Prober]

I am not too familiar with the 73, but with the 75, which cannot be so very much different. The PTU is there so that, if the L HYD (main system on the 75) is degraded, the R HYD can drive what is left of the L HYD system. If the L HYD leak is below the stand pipe, make your own arrangements. If the leak is above the stand pipe, its remains are driven, via the PTU, by the R HYD system providing power to a limited number of items (flaps, gear, nose wheel steering and hyd gen). Hope this helps!
Prober

[Pub User]

framer has it. However, to say the PTU 'gets pressure' from system A fluid is not quite right.

The PTU is a pump, which pressurizes part of the B system, this pump is driven by a hydraulic motor, which is operated by system A. There is no fluid transfer, the link is purely mechanical.

This part of the B system takes fluid from below the standpipe, in case there is a loss of fluid from the main part of the system. However, as framer correctly states, if there was no fluid (at all) in system B, the PTU would not work.

[ImbracableCrunk]

Yeah, don't get this and the landing gear transfer valve confused. On the gear system, you're actually swapping fluids.

Precious hydraulic fluids.

[framer]

Really? So it's actually B fluid raising the gear? Not just used to drive a motor like A fluid is in the PTU?

[punk666]

Hydraulic system B pressure is available for raising the landing gear through the
landing gear transfer unit. Hydraulic system B supplies the volume of hydraulic
fluid required to raise the landing gear at the normal rate when all of the following
conditions exist:
• airborne
• No. 1 engine RPM drops below a limit value
• LANDING GEAR lever is positioned UP
• either main landing gear is not up and locked.

[M.82]

There is a difference between the Power transfer unit and he Landing gear transfer unit, they are different systems.

FCOM Vol2 13.20.3

"With fluid level at the top of standpipe, fluid remanining in the system B reservoir is sufficient for power transfer unit operation"

FROM HERE I GUESS THAT PTU USES FLUID REMAINING FROM THE SYSTEM B RESERVOIR.

"The purpose of the PTU is supply the additional volume of hydraulic fluid needed to operate the auto slats and leading edge flaps and slats at the normal rate... The PTU uses system A pressure to power a hydraulic motor-driven pump which pressurizes system B hydraulic fluid"

FROM HERE I GUESS THAT SYSTEM A FLUID MOVED THE PTU MOTOR DRIVEN PUMP TO COMPRESS HYDRAULIC SYSTEM B FLUID REMAINIG.

So, there is no change of fluid between system A and system B.
:ok:

[framer]

OK.... Think Ive got it.
PTU is definately no fluid transfer. Just A fluid operating a motor that is attached to a pump that has B fluid in it.
The LTU is a different animal, in normal operation it sends A fluid to the landing gear selector valve. If • airborne
• No. 1 engine RPM drops below a limit value
• LANDING GEAR lever is positioned UP
• either main landing gear is not up and locked.
then a shuttle valve slides within the LTU and it sends B fluid to the landing gear selector valve.
Anyone disagree?

[microbus37]

There is a small detail about PTU which is why I had a discussion with my instructor this morning. The conditions for the PTU to activate automatically are;

-Airborne
-System B engine driven pump drops below limits
- Flaps not up (flaps not up but not more than 15 for some versions)

Which makes me think that this is a protection for engine failure #2 after V1 so that system allows to retract LE devices. But I think it doesn't work the same way when you arm Alternate flaps master switch since it closes trailing edge bypass valve. You may extend the TE devices but can't retract them. So we can say PTU is deactivated during stand by system operation. Please correct me if Im wrong.

Similar thing for the Landing gear transfer unit. You can retract gear by using System B in case of an engine failure #1 but you can not retract it when you lose System A. In that case you need to shut down engine #1 in order to retract gear :)

[de facto]

Microbus,
Correct,the PTU shuts off when the alt flaps switch is moved to arm position and TE flaps switch selected to the down position.
Correct,you cant retract the gear if you lose system A,the Landing gear transfer unit works when Eng 1 fails,therefore its main pump as well.The remaining electrical pump would not be sufficient to retract the gear in the time limit imposed by certification(8 secs i think)so extra sys B pressure is given to retract it.

[microbus37]

de facto

Thanks for the reply. So can we say as long as we have sufficient fluid in system reservoir B, we can retract LE devices even if we lose whole system B? What I'm trying to say is following a total loss of system B right after V1, LE devices will retract with the help of PTU but TE devices will stay where they are when we put the flap lever up. Is that correct?

[repulo]

No, there is a system blocking valve that is closing the retract line with sys B pressure below 2000PSI. The PTU will not overpressure that (Source AMM manual). Think of the PTU as a back up for auto slats. During an engine # 2 failure auto slats may not react quick enough to counter a high AOA scenario, so the PTU jumps in to help extend the slats to the full extend position. This may be the case during an immediate turn after an engine failure. The QRH does not tell you to retract the leading edge devices with the help of the PTU.