PTU(Power Transfer Unit) B737NG

[owl-attack]

Hello guys

An elementary question on the working of PTU

I Understand the conditions required for the PTU to operate:

Sys B EDP pressure drops below limit
Flaps less than 15 but not up
Airborne

and I also understand that is used to operate autoslats and LE flaps and slats

But I seem to be getting very confused here. What do they exactly mean when they say PTU is used to operate Auto-slats and LE flaps and slats. Why can't the Sys B EMDP be used to operate these ( only the Sys B EDP has failed , Sys B EMDP is still working fine)


2. When the FCOM says PTU is used to operate LE Flaps and slats " do they mean it is used to retract after take off and extend while landing?


Thanks guys !

[IFixPlanes]

... Why can't the Sys B EMDP be used to operate these ( only the Sys B EDP has failed , Sys B EMDP is still working fine) ...
EDPs can have a output of 36 gpm but value of the EMDP is only 5,7 gpm.
The PTU can add 13,7 gpm.
How quick do you want Slat/Flap movement if you are in a Flight Phase that requires Flaps between 0 and 15? :E

[de facto]

2. When the FCOM says PTU is used to operate LE Flaps and slats " do they mean it is used to retract after take off and extend while landing?

The PTU uses hydraulic A Pressure and system B fluid to operate the LE flaps/slats under the conditions you mentioned when the Hydr sys B PRESSURE falls below 2350 psi.
It can be used to retract or extend them,however the PTU will stop if you use the STanndby Alternate TE flaps in the Arm position and you used the switch to lower the TE flaps electrically.(electrical motor of the Power drive unit).

To answer your question:yes.

[owl-attack]

Thank you very much guys

I have few more questions for you guys

1. As per the FCOM,
if a leak develops in the Sys B line or its related components the quantity decreases until it indicates zero and system B hydraulic pressure is lost. [I get this part very well]

It also says however with the fluid at top of the standpipe, the fluid remaining at top of the standpipe is sufficient for the PTU to operate.

My question here: Firstly I dont understand why the FCOM initially says the quantity decreases to zero (that means there is nothing). Then it says with the fluid quantity at top of the standpipe is sufficient for the PTU to operate(I thought the FCOM said there is no fluid quantity that is why it says Zero, but now how did this fluid come to operate the PTU) Does he mean that there is little bit of fluid left which is considered or sensed as zero but that little bit is enough for the PTU operation?


2. Now if I am at cruise and I have lost EDP in Sys B and I come in for landing. My flaps are still up, now when I want to extend my flaps to flaps 1 the PTU here wont help me in extending the LE devices ( since the condition "flaps less than 15 but not up" is not met.... My flaps are up!!!!)

So does this mean that PTU is only helpful after take off to retract the flaps.
And during approach if I have extended my flaps to 1 and then if the Sys B EDP pressure is below limit this is when PTU can help me extend the rest of the LE devices.

So if in the above case because PTU wont operate, then LE devices will be extended by SYS B EMDP which will be slow

I really cant see PTU as a very helpful component. Please help me get out of this fix:ugh:


Thanks a lot guys:)

[IFixPlanes]

You should first understand the difference between "quantity decreases until it indicates zero" and "quantity decreases to zero".
Then you should start to lock at the pictures as well. (e.g. standpipe)

Sorry, but did you read the headline of this "Question" forum? I can quote it for you:
If you are a professional pilot or your work involves professional aviation please use this forum for questions. Enthusiasts, please use the 'Spectators Balcony' forum.

[owl-attack]

If you are a professional pilot or your work involves professional aviation please use this forum for questions. Enthusiasts, please use the 'Spectators Balcony' forum.


Thank you but I am not an enthusiast. I am a cadet/Trainee pilot with one of the airlines in SE Asia. So I definitely come under professional category.

My questions might be elementary because I am a cadet and learning this aircraft

[de facto]

Does he mean that there is little bit of fluid left which is considered or sensed as zero but that little bit is enough for the PTU operation

I think you may be confusing pressure and quantity.
The PTU is using hydr Sys B fluid (reservoir) and syst A PRESSURE if sys B PRESSURE is too low(under 2350psi).

If sys B quantity is lost,The PTU will not work and you will follow loss of sys B (quantity) NNc which tells you arm the stby hydr.(alt flaps to arm).
The stby hydr has its own electric motor and hydr pump for the LE flaps/slats,this will allow their extention.
It is safe to take off with LE down but less to land with the LE up:E
Pls note: As far as I think the Standby Hyd pump cant retract the LE devices only if it has been extended by the ALTN FLAP switch.

Correct and your QRH says so.
3. Now if I am at cruise and I have lost EDP in Sys B and I come in for landing. My flaps are still up, now when I want to extend my flaps to flaps 1 the PTU here wont help me in extending the LE devices ( since the condition "flaps less than 15 but not up" is not met.... My flaps are up!!!!)
Correct,you will use the standby hydraulic fluid and pressure to extend your LE flaps and the stanby electrical motor to get your TE flaps down.

Try to think the PTU as an automatic help to get your LE flaps up and down if sys B PRESSURE is a bit low.

[owl-attack]

I think you may be confusing pressure and quantity.

Thanks De facto. I get it now


Correct,you will use the standby hydraulic fluid and pressure to extend your LE flaps and the stanby electrical motor to get your TE flaps down.


I don't get this part. Why will stand by pump be activate ( it definetly wont be activated automatically for sure since the conditions are not met for automatic activation) but why would you even activate stand by pump manually. In the example scenario I gave, I had just lost EDP and not both EDP&EMDP. Thus I still have Sys B EMDP. Now since flaps are up, PTU wont help me extend LE devices, the LE devices will be extended using Sys B EMDP right

Am i correct on this

Please excuse my ignorance. I really have a deep desire to learn and questions things and learn more

[de facto]

I had just lost EDP and not both EDP&EMDP

Sorry ,i read too quickly,thought you mentioned loss of all B pumps.

If your sys B Engine driven pump shows low pressure then the electrical pump will do the work to extend the TE flaps and the PTU will use A pressure to extend the LE flaps/slats once your TE flaps are selected and move past the UP position.

Have a look at your QRH for failures and scenarios, and if you dont understand the sequence in it then study the systems in more details,otherwise going deep in theory before the basic you may go crazy:E

[IFixPlanes]

... I am a cadet/Trainee pilot ...
OK, so some extra info about the indicating system.
0% in the B-System means that the fluid level is at the top of the standpipe. EDP and EMDP get their fluid through this pipe and with indication 0% they are out of business.
But the PTU pump gets the fluid from the bottom of the reservoir. The hight of the standpipe is equivalent to 1,3 gallons.

As i already mentioned above:
One of the differences between EDP and EMDP is the capacity of fluid flow.
Slats need massive fluid flow.
Nice reading in conjunction to that is FCOM 9.20. "Autoslats"

... study the systems in more details,otherwise going deep in theory before the basic you may go crazy ...A very wise advise... :D

[owl-attack]

Thanks Ifixplanes and De Facto

I get it now

[yotty]

I always thought the primary reason for the PTU was to transfer pressure to aid gear retraction when the No1 engine was below 56% N2. The last thing you want is not to be able to raise the gear when you've got an engine failure during the climb out!:eek:

[IFixPlanes]

Yotty, do not mix PTU with the "landing gear transfer valve". (See AMM Part 1 Pageset 32-31-00-010 for details of the valve)
This valve switch to system B under given conditions and the retraction will be done with system B fluid.

[yotty]

Excellent IFP. I don't have NG cover or access to the AMM. :O

[IFixPlanes]

Similar system in the B737CL.
FCOM also tell some details. Search for "Landing Gear Transfer Unit".

[Vessbot]

0% in the B-System means that the fluid level is at the top of the standpipe

Do you have a reference for this? It seems very odd. My manual (company-tailored) says 20% for the system A standpipe, and does not say anything for system B standpipe % indication.

[HOVIS]

It's a long time since I worked the 737, but wasn't this all to do with ensuring there was sufficient pressure and fluid flow in the event of a go around, gear up and auto slat operation.
Apologies if I'm way off the mark here.

[IFixPlanes]

Do you have a reference for this? It seems very odd. My manual (company-tailored) says 20% for the system A standpipe, and does not say anything for system B standpipe % indication.
Yes, AMM Part I - System Description Section ATA 29-10-00.

https://cimg9.ibsrv.net/gimg/pprune.org-vbulletin/944x680/29_10_00_bfd7e707e547d2dc9ae2704f7f730101045b7b6d.jpg