B737 cl PTU

[lunav]

Hello guys,
from AOM:

The purpose of the PTU is to supply the additional volume of hydraulic fluid
needed to operate the autoslats and leading edge flaps and slats at the normal rate
when system B engine–driven hydraulic pump volume is lost. The PTU uses
system A pressure to power a hydraulic motor–driven pump, which pressurizes
system B hydraulic fluid. The PTU operates automatically when all of the
following conditions exist:
• airborne
• system B engine–driven pump hydraulic pressure drops below limits

• flaps are less than 15 but not up.
The questions are:
-OPERATE means retraction AND extension?
-Could you make an example of a scenario where the PTU will start working?(a/c configuration, flap lever position and movement)
Thanks a lot!!!

[Todders]

PTU can be used for retraction and the best situation to think about is after t/o with and engine failure and related loss in the eng driven pump the ptu will allow you to clean up the aircraft to fly away.

[lunav]

thanks todders.very helpful! May I ask you a further question?
-loss of EDP and EMDP of sys B, hyd qty sys B normal - alternate flap extension to 15 -approach - go around - diversion to other field).
If i place the ALTERNATE FLAPS master switch to OFF after TE flap retraction...will the PTU retract The LE devices?

[BOAC]

• flaps are less than 15 but not up

- I suspect this is for a spare sim 5 mins but I would think best to put the ALT switch OFF before TE retraction? CaptainSandL will know In any case, a most unlikely scenario?

[lunav]

got unemployed a couple of month ago, my girlfriend on a long holiday to visit her parents, my best friends busy getting themselves a girlfriend.......so
........plenty of time to have fun with 737 sys!!!!
Anyway,I need to have the ALTERNATE FLAP MASTER SWITCH on ARM to use the Alternate flap pos switch to retract the TE flaps.From AOM:

The guarded ALTERNATE FLAPS master switch closes a flap bypass
valve to prevent hydraulic lock of the flap drive unit and arms the ALTERNATE FLAPS position switch. The ALTERNATE FLAPS position switch controls an electric motor that extends or retracts the TE flaps.

After Alternate flap retraction I will find myself with LE devices full ext...so if you place the ALTERNATE FLAP MASTER SWITCH on OFF will the PTU help you with the LE devices?
Thanks a lot for helping me sleeping well tonight!!!

[CaptainSandL]

Funnily enough I was exercising a 737 PTU this afternoon over E Anglia at 15,000ft!

In normal operation, the autoslat system automatically drives the slats out from extend to full extend about 5-10kts before the stick shake to give you some extra speed protection above the stall. If/when you accelerate away from the stickshake/stall the autoslat system will return the slats back to extend.

The flaps must be at or between 1 & 10 (strictly between not up and less than 15 because they could be in transit) for the autoslat to operate. This is normally powered by the B system.

If the B system pressure is not available (again look in the FCOM for the precise criteria), the PTU uses A system pressure to pressurise the B sys fluid to power the autoslat system. Note that you still require B sys fluid, so you lose autoslat with a loss of B quantity not pressure.

Moving the Alternate Flaps switch to ARM will not affect autoslat or PTU operation. However, remember once you start to move the T/E flaps down with the alternate system the L/E devices will travel inexorably to Full extend, so the autoslat system effectively becomes redundant.

Just for the info of any 737 pilots/engineers, I have found the PTU not to be totally reliable. In about 200 flight tests I have had 5 not operate. No hard faults were ever found, the engineers simply changed the units. I suspect it was just because they are used so infrequently.

S&L

[BOAC]

I'm happy with the autoslat operation, it is the wording of Boeing's operate the autoslats and leading edge flaps and slats at the normal rate (my bold/underline) which has always puzzled me and been something I wanted to try in the sim. Will it retract the LEDs from full to extend if you stop the alt retract at more than 'UP' (and, of course, will it complete the retraction at 'Clean')? The explanation under 'Flight Controls' is a lot more specific, but the quoted 'Hydraulics' extract is, to my mind, confusing - why, for example, is it designed to work at Flap10?

[spannersatKL]

I thought the criteria for this to operate the PTU was loss of Engine 1 (N2 Low) hence no EDP and flaps as per CaptsandL away from up and less than 15....and an autoslat demand due to hi AOA/low airspeed. PTU then runs extending slats fully. What am not sure of is will the PTU retract the slats after this....my understanding is not as they use different hydraulic lines to extend the slats and the PTU wont be running as the demand wont be there in the end as either the T/E flaps will be up or the auto slat demand wont be there? However am new to the 73CG (28 years 74!!) so am open to being told how it really works.... Have to agree the PTU probably never gets used except on maintenance checks or test flights (a good thing).
Fly safe...

[CaptainSandL]

BOAC,

I don’t fully follow your question so if I missed your point please try me again…

In my earlier post I spoke about slats, that was sloppy, I should have used the phrase LE Devices.

The autoslat system will automatically move the LE Devices from Ext to Full Ext whenever the speed is v low and the LEDs are only at Extend. When the speed is restored it will automatically move them back to Ext again. It works at F10 because the LED’s are only at Extend in that config. Why? I guess to protect during flap retraction in the F15 T/O or G/A case or during approach when F10 was used (remember F10, 160?).

The “at normal rate” only applies to PTU operation (of the autoslats). If there were no PTU the autoslat would probably not work without B sys press, or only at a reduced rate with only the Elec hyd pump because it is not as powerful as the Eng hyd pump.

“Will it retract the LEDs from full to extend if you stop the alt retract at more than 'UP' (and, of course, will it complete the retraction at 'Clean')?” – Yes but only if the autoslat system had moved then to full ext because you had got to close to the stall speed. Did you mean to say “alt retract” because retracting flaps with the alt system will not move the LEDs.

Think of the autoslat system like the flap load relief. Flap load relief automatically and temporarily retracts the flaps to prevent you damaging them during an overspeed. The autoslats automatically and temporarily moves the LEDs from Ext to Full Ext to help prevent you from stalling. Both systems automatically return the aircraft to the selected config after the condition is over.



SpannersatKL,

I think you are confusing the activation criteria of the PTU with the LGTU. I promise you the PTU (usually) works quite well with both engines running.


Hope this helps.

S&L

[CaptainSandL]

BOAC,

Re your question “Will it retract the LEDs from full to extend if you stop the alt retract at more than 'UP' (and, of course, will it complete the retraction at 'Clean')?” I think the penny has dropped with me now. Are you asking if you can use the Autoslat system to retract the LEDs after alternate flap extension/retraction? If so, I don’t know the answer, but I believe theoretically yes. However you would have to be mad or v desperate to deliberately bring the aircraft back to the stick shaker just to retract the LEDs, esp when you already have a flap problem.

If I find the answer I may post it here but it is definitely in the “dangerous information” category.

S&L

[lunav]

Sorry but it's more difficult than I tought.
For todders:
It came to me that You can't clean the a/c using the PTU for the fact that Its operation is between flaps less than 15 and not up!!means 1 to 10!And at Flaps 1 we have a lot of things out the Leading edge!
For CaptainsandL:
Autoslats operation is between flaps 1 and 5 .From AOM:

At flap positions 1, 2 and 5 an autoslat function is available that moves the LE slats to FULL EXTEND if the airplane approaches a stall condition.
Becouse:

When the FLAP lever is moved from the UP position to the 1, 2, or 5 position, the TE flaps extend to the commanded position and the LE:
• flaps extend to the extended position, and
• slats extend to the extended (intermediate) position.
When the FLAP lever is moved beyond the 5 position the TE flaps extend to the commanded position and the LE:
• flaps remain at the extended position, and
• slats extend to the full extended position.

The LE devices sequence is reversed upon retraction.
So for sure the PTU will drive LE slats to full extended pos when moving flaps beyond 5 when conditions are met, but when will it move the LE flaps as they are already in the extended position from flap 1?
Isn't it written on AOM?:

The purpose of the PTU is to supply the additional volume of hydraulic fluid needed to operate the autoslats and leading edge flaps and slats at the normal rate when system B engine–driven hydraulic pump volume is lost.

• airborne
• system B engine–driven pump hydraulic pressure drops below limits

• flaps are less than 15 but not up.

Getting cofused here!Thanks for any help!

[captjns]

While not explained in great detail in Volume II, with the flaps and slats at takeoff position when system “B” is lost, and provided the loss of System “B” is above the stand pipe level, can retract the flaps electrically, and in theory, the LEDs via the PTU. Looking at the schematic in the manual, the PTU valve opens with a loss of System “B” pressure.

If there is anyone going to visit the simulator, give it a try.

Fail Hydraulic System "B".

Place the Alternate Master Switch of “ARM”.

Trailing Edge Flap Bypass Valve closes;
LEFAS Valve associated with the Standby Hydraulic System is armed; and
the normal LED Control Valve closes.

The LEDs will remain in their current position as the Alternate Flap Switch has not been positioned to the DOWN position causing the LEFAS Valve to open.

Place the Flap Lever Up, and place the Alternate Flaps switch to the UP position which will retract the flaps electrically.

After the flaps are up, turn the Alternate Flap Master Switch off.

All valves should assume their normal positions, and via the PTU, the LEDs should retract as operation of the LEDs is a function of Flap Lever position and flap position. Looking at the schematic, it does not appear the normal LED Control cares where hydraulic pressure comes from either normal System "B", or via the PTU.

[lunav]

first i need to find myself a new contract, but after that my goal will be to try this on the first sim.
Captjns,You really got what I meant!
If someone tryed this scenario on a sim please report. Thanks a lot!

[plain-plane]

captjns,
true, yep you can retract the LEDs with the Alternate Master Switch in “ARM”.
at least on the NG. Though obviously not an approved procedure... and would you really want to...

[captjns]

Plain-plane

The original post was a queery as to whether or not the LEDs could be retracted with loss of Hydraulic System"B"... not if I would want to.

The simulator can give one a better understanding as to how systems and controls really work without hurting yourself... wouldn't you agree?

[plain-plane]

captjns, true, true.

I am more thinking of the fact that boeing have chosen not to supply a NNC for this......they must have their reasons...


Then again in situations where there could appear a need for such a NNC, you might just be better off with not messing too much about with the flaps...

[RYR-738-JOCKEY]

From what I understand, we have two systems aiding us in cleaning up in a OEI scenario. There's the landing gear transfer valve and the PTU. Regarding the PTU, again from my understanding, it ensures that the clean up can be done without delay. Also, it provides you with autoslats. However, when it comes to the alternate system, when you put the alternate flaps switch to the down position your LEDs will move to full extend.
From what I gather from the initial post, the question is; can the LEDs be retracted from this position? I think they can. But entirely in a test situation, because you wouldn't have used the alt system unless SYS B was gone. So, my answer will be you can retract them provided you have SYS B pressure and fluid, however not by PTU (A pressure, B fluid).
Hope that helps.

[CaptainSandL]

Lunav,

Re “For CaptainsandL: Autoslats operation is between flaps 1 and 5 .From AOM:”

In most 737s yes, but not all. I know the thread title is for Classics but there will be some NG readers here. On the 1/200’s (not Adv) only the outboard slats were on the autoslat system, and it only kicked in at flap 25. On the new SFPs it is all the slats but it works from flap 1 to 25 inclusive.

Don’t get too hung up on the numbers, it is the principle which matters. I.e. autoslat moves slats from ext to full ext & back again. The PTU assists if you have no B pressure.

Your original questions were:
“-OPERATE means retraction AND extension?” – Answer yes.

“-Could you make an example of a scenario where the PTU will start working? (a/c configuration, flap lever position and movement)” – Answer only as given in the criteria you gave in your original post. But if you are looking for ways to retract the LEDs after an altn extn, there are ways but they are complicated and/or risky. Best stick to the QRH unless the chips are really down.

Regards

S&L

[BOAC]

Originally Posted by CapatainS&L

However you would have to be mad or v desperate

- well, we are getting there, but slowly! I agree with the sentiment of that (as per my post #4), but the problem we/I have is that I can only work on very limited information. Right from Day1 of my 737 life, I was told once the LEDS are out on the alt system that is it. Later, a bright, enthusiastic BA training Cpt went to great lengths on a white board to prove this was not so. Looking at the Tech Manual gives very little clue. There, there is NO suggestion that autoslat needs to be 'triggered' to power the LED line (hence no need to 'stall'?) - the 'feed' from the PTU goes amorphously into the 'autoslat and lE slats and flaps' box suggesting that with simple loss of no 2 EDP (etc) the line may well be 'presurised'. IMO it is not "dangerous information” at all but useful background. Unfortunately 'Todders' did cause some confusion by introducing an engine failure into the thread

So - not that a sim trial would be conclusive, of course, but I would be interested to know the answer. Any idea why Boeing made it work at Flap10?

[CaptainSandL]

Any idea why Boeing made it work at Flap10?

Great question, sorry I didn’t pick up on it earlier. I have just had a look in the 737CL AMM and the electrical schematic for PTU (29-22-00 Page 8 for those with access). It shows that one of the switches to activate the PTU is the LAND WRN SW which closes when the t/e flaps are less than 15. NB it is the same in the 737NG AMM (29-25-00 Page 17). So it appears to me that Boeing have used the same switch that is used for the landing config warning horn. Since the criteria are more restrictive, ie <15 and not just up to and including 5 this is safe. I would guess this was for reducing cost & complexity, if not redundancy!