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A320 STRANGE PTU behaviour

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A320 STRANGE PTU behaviour

Old 25th Jan 2013, 05:32
  #21 (permalink)  
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As a regular commuter and Airbus pilot I do occasionally hear the PTU running as a passenger without any obvious cause. In the beginning I used to ask the flight crew what was going on, if they had any hydraulic problems or if they knew the PTU was running all the time, only to be met by blank stares from the pilots. Apparently there is no obvious indication in the cockpit that the PTU is running on these occasions.

So I have no answer to this question, I can just confirm that it DOES happen.
It usually happens after engine start and during taxi, it stops once airborne with the gear up, only to start again on approach when the gear is lowered. Or sometimes only during taxi or only on approach.

An Airbus mystery!
I do not usually repeat myself, but we are going off a tangent. The video shows exactly what I describe above. I have experienced this PTU behavior many times commuting and positioning as crew in the back of the aircraft. Talking to the crew afterwards reveals no defects and the crew are never ever even aware that the PTU was running. It does not even show up on the ECAM memo.
Why this happens remains a mystery to me.
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Old 25th Jan 2013, 07:14
  #22 (permalink)  
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Airmann, 320s do indeed have an elec hyd pump on the blue system. The ones I worked did anyway.
Yellow has an elec pump also.( Cargo door ops iirc )

Last edited by ampclamp; 25th Jan 2013 at 07:18.
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Old 25th Jan 2013, 07:20
  #23 (permalink)  
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Yes I am sure for the reasons I explained! The PTU is designed to run for hours in a row, this is not a problem as long as there is hydraulic fluid in both the yellow and the green system.
If it would always overheat hydraulic systems after 2 minutes it would hardly be a backup for losing one of the engine driven hydraulic pumps now would it?

Overheat can only happen if one of the hydraulic reservoirs is depleted. This means that one system could drive the PTU into overspeed as the PTU is not driving anything.

But this has been modified through a service-bulletin. The PTU will be inhibited for low reservoir levels. So the simple fact is that it won't run if there is a danger of overheating the other system.

Some info for you:

The overheat incident:


We are talking the main hydraulic sources here Green and Yellow im relation to the PTU, the PTU cannot power the blue system. Nor can the blue system power the green or yellow system. Lets Keep it simple ok?

Yes the blue system has an electric pump, but that is hardly relevant, it can also be pressurized by the RAT in case of electric pump failure.

Last edited by 737Jock; 25th Jan 2013 at 08:31.
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Old 25th Jan 2013, 08:10
  #24 (permalink)  
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Maybe some facts about the A320 Fam. and its PTU:
1. There is no selftest of the PTU. The FWC only check that the pressure difference of system G and Y is not more than 650 psi at least 4 seconds after second engine master switch is set to on. (PTU is inhibited with engine master switches in different positions, NLG compressed and either parking brake is on or N/W STRG off)
2. B and Y hydraulic systems have a elec pump. (in normal config system B is only pressurized by elec. pump)
3. PTU connects G and Y hydraulic mechanically.
4. the logic to avoid a overheat situation due to PTU overspeed condition works (simply said) by watching the low pressure status of system G or Y (6 seconds is the trigger). If there is a reservoir low level, the PTU would not be able to build up pressure in that system.

My thoughts to the linked videos:
It does not sound like a hard working PTU so no big problem, but a snag that should be fixed.
Continuous running of the PTU without excessive use of hydraulic (e.g. L/G or flap/slat movement) is not normal. I would first check both EDPs for health.
If one EDP is at the upper end of its range (3200 psi) and the othe one is at its lowest end of range (2800 psi) the PTU can step in.
Other possible triggers for a PTU running during normal operation can be a to high internal leak of a hydraulic system (e.g. partially open check valve) or the breakout pressure of the PTU is to low.
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Old 25th Jan 2013, 08:25
  #25 (permalink)  
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FCOM dsc-29-10-20 overhead panel

The PTU is inhibited during the first engine start and automatically tested during the second engine start.
What is this test exactly? It doesn't run the PTU? What happens if this test fails?

Isn't It just lovely how all information is taken out of the FCOM?

Last edited by 737Jock; 25th Jan 2013 at 12:52.
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Old 25th Jan 2013, 09:40
  #26 (permalink)  
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AC Pumps

Just a quickie on this side-issue of AC pumps. Things may have changed since the end of 2001, when I retired, but in my experience the Blue AC pump was identical and interchangeable with the Yellow AC pump. (Before somebody corrects me on that, I think it may be necessary to adjust the orientation of the exit manifold, or something like that.)

As the Blue system on the A320 (unlike other Airbuses) has no ED pump, robbing the Yellow system of its AC pump can be done to "get you home".

Don't know current practices, but - despite something posted earlier - we used to use the Yellow AC pump quite a lot in normal ops. Single-engine taxiing on the No 1 engine retains the Green ED pump for brakes and steering, but if it fails you needed the Yellow system to pressurise the Green via the PTU (which, for any readers not au-fait, is simply a reversible motor-pump). The Yellow AC pump is also the only way to pressurise the Green system on the stand for servicing purposes (without turning No. 1 engine), including opening and closing the L/G doors.

Last edited by Chris Scott; 25th Jan 2013 at 09:56. Reason: 3rd paragraph added. Title added.
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Old 25th Jan 2013, 09:48
  #27 (permalink)  
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I do not think that "all" details have to move into the FCOM.
IMHO Pilots have to know the system not in that detailed form. You have to know that the funktion of the PTU will be tested during second engine start, but is it really relevant for you, how it is done in detail? I think not. If you do not get a PTU Fault during second engine start the PTU works fine.

OK, how does the "test" work? There is no PCU (PTU Control Computer), so someone else have to do the "test".
So how can i test the PTU in the normal operation? Airbus take a situation where the PTU have to work (albeit for a short time).
Best point is the start of the second engine.
In DSC-29-10-20 you find the inhibition logic for the PTU. The inhibition is off with both engine master lever in on.
Hydraulic situation before the second start:
- one engine running and EDP pressurize related system (B or Y system)
- other hydraulic system is depressurized
- PTU pb in auto
- PTU did not run due to inhibition

With second master lever to on, the inhibition is not longer valid and due to pressure difference between B and Y the PTU starts running.
The EDP of the starting engine needs some speed of the engine to provide sufficient pressure. Up to these speed the PTU hold up the pressure.
If the PTU is not OK, the pressure would not rise immediately after master sw in on.
Now we need a computer which have access to all inhibition parameter. The FWC have!
Simply said: As soon as both master levers are in on, the FWC gives a PTU FAULT if there is now a difference in the G and Y hydraulic system of more than 650 psi during at least 4 seconds. (the starting engine EDP needs more time to pressure up its related system during engine start.)
As soon as the engine speed is high enough the EDP pressurize the system and the PTU stop working.

Do you really want this all in the FCOM? I think that "The PTU is automatically tested during the second engine start" is enough info.

Last edited by IFixPlanes; 25th Jan 2013 at 14:06. Reason: 29 not 39
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Old 25th Jan 2013, 10:22
  #28 (permalink)  
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Thank you for aapparently confirming what I have always suspected: there is no such thing as a programmed self-test sequence of the PTU during the second engine start. The PTU runs at that stage simply because - now that both Engine Master Switches are open - it is no longer inhibited from doing its normal job.

As soon as the second-engine ED pump gets a bit of RPM, it will pressurise its system enough to stop the PTU. This happens well before the second engine reaches idle RPM, because there is no load on the system.

So, to return to the two flight-videos that caused EIU_EEC to start this thread:

(1) why did the PTU start at 170R on the approach to AMS?

(2) why did the easyJet PTU run continuously long after the second engine was started, and even at take-off RPM?

In case (1), I have speculated that there might be a change in PTU operation-logic to increase flight-control redundancy during an autoland. But that's only off the top of my head, and seems unlikely. The more likely alternative is that there was either a real or false warning of low pressure in either Green or Yellow.

The other possibility may be that, when the system detects a fault in the differential pressure switch, the PTU is forced to run continuously as a fail-safe measure.


For what it's worth, I can identify with your frustration when travelling as SLF with some technical (and type-specific) understanding of unexpected noises in the cabin! We would like to know if it is ever possible for the PTU to run with no Memo message to the crew, and no auto-presentation of the ECAM HYD page.

Last edited by Chris Scott; 25th Jan 2013 at 11:19. Reason: Last paragraph added.
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Old 25th Jan 2013, 11:17
  #29 (permalink)  
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IFiX, That is really interesting stuff; (I was an electronics engineer (not aircraft) before becoming a pilot). A little gem for the examiner on my next line check !

Does memory serve me right that 'PTU' is annunciated in green on ECAM memo when PTU is running? And also that PTU running* during normal 2 engine ops is not normal?

So should we be putting this in the Tech log because a HYD pump may be giving low pressure or low flow? (but not low enough to trigger ECAM caution).


Last edited by Uplinker; 25th Jan 2013 at 13:20. Reason: *320skoda: I should have said '...running for more than a few seconds...'
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Old 25th Jan 2013, 12:13
  #30 (permalink)  
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Ptu will run when one system is in high load, Gear up flaps and slats running this high load will cause ptu to start as system pressure will drop. Also they could have turned off one of the EDP's for some reason. This will cause the PTU to do its job.
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Old 25th Jan 2013, 12:39
  #31 (permalink)  
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Actually Ifixplanes, yes I would like that in the FCOM. Understanding something is much easier than trying to drill little bits of loose information in ones brain.

This dumbing down of the FCOM really does nothing for understanding the aircraft systems.

Thanks for the info!

The reason I associated it with the test, is that I have heard the same thing develop after a single engine taxiout where I was paxing. The PTU started to run slightly before the second engine start, then stopped for a bit during the start, and then started and did not stop before being airborne.
So knowing the procedures, I know that the PTU will have engaged after switching off the yellow elec pump before engine 2 start.
And I associated the second run of the PTU with the test during the 2nd engine start.

For me there is no logical explanation, unless there was some kind of unstable pressure delivered from the yellow edp.

Last edited by 737Jock; 25th Jan 2013 at 12:47.
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Old 25th Jan 2013, 12:46
  #32 (permalink)  
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Do you really want this all in the FCOM?
Yep - I would like it as well. This very discussion shows that the PTU running goes unreported in the Tech Log, because pilots misunderstand comments like "The PTU ... [is] automatically tested during the second engine start."

... and the PTU FCOM reference is DSC-29-10-20, BTW - and I thank you for the info as well - interesting!

Last edited by Checkboard; 25th Jan 2013 at 12:47.
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Old 25th Jan 2013, 12:47
  #33 (permalink)  
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Damn vistair.... it seriously indicates it as 29-20 controls and indicators overhead panel.

As the Blue system on the A320 (unlike other Airbuses) has no ED pump, robbing the Yellow system of its AC pump can be done to "get you home".
Sure, but the system was designed for the PTU to power the other system in case of an engine shutdown or EDP failure. There is no issue with running the yellow elec pump to makeup for lost EDP output either (PRO-ABN-29 HYD Y ENG 2 PUMP LO PR (PTU inoperative)).

As such a continuous running PTU is not a danger, as long as there is fluid in the G and Y reservoir.

Last edited by 737Jock; 25th Jan 2013 at 13:01.
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Old 25th Jan 2013, 13:52
  #34 (permalink)  
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Is the barking a sign of continuous PTU operation, or an ON-OFF or backwards/forwards operation?
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Old 25th Jan 2013, 14:56
  #35 (permalink)  
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@ Uplinker
A running PTU gives "HYD PTU" Memo in green.
Right, 2800 to 3200 psi does not trigger ECAM caution but it is also in the normal operating range for EDPs (FCOM LIM-29).
And yes, it is not normal that the PTU is continuous running during "normal 2 engine ops".
When you make a techlog entry, it would help if you pressure values of both systems with PTU in off.
In addition with PTU in off make some sidestick inputs (diagonal to activate elevator, aileron and spoiler), watch the pressure drop and report deepest values.
Shorten the T/S time...

@ TyroPicard
to keep it simple: every "bark" is a pressure transfer. High demand forces high "barking" frequency.

@ all pilots, who wants more information than just the FCOM
try to get a copy of the AMM (PDF) from a greasemonkey during a visit of your maintenance. (btw: to big to E.Mail it)

Last edited by IFixPlanes; 26th Jan 2013 at 17:24. Reason: Bad day behaviour removed.
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Old 25th Jan 2013, 17:50
  #36 (permalink)  
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Thanks IFixP..
So the barking PTU is "pulsing" not running continuously. Not running for long enough to cause an ECAM memo.

Electrohydraulic valve pulsing open/shut? Or partially?
Genuine very slightly low pressure one side which is cured by a pressure pulse then falls again?
Some mechanical effect inside the PTU?
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Old 25th Jan 2013, 18:00
  #37 (permalink)  
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Stolen from another thread, post by I-2021.......

On the A320, the "barking" sound you hear is typical of a situation where the PTU runs to pressurize a hydraulic reservoir with a low hydraulic output. What happens is that it takes only a few seconds to power the reservoir up to 3000 psi and then the PTU stops. In another few second the pressure drops... and at 2500 psi the PTU kicks in again to bring the pressure back to 3000 psi and so on... here you get this barking sound. If you want to hear another sound from the PTU, try pressurizing a reservoir, activating the PTU and using some flight controls, such as flap/slats, you will hear a costant sound from the PTU and not the barking sound you are used to.
Disregard "reservoir", insert "system".
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Old 26th Jan 2013, 07:59
  #38 (permalink)  
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Ifix; Can you tell me how long the PTU must run before the ECAM memo displays HYD PTU? - is it instantaneous, or after so many seconds? I fly the thing every week, but cannot remember what the PTU indications are on 2nd engine start!
(My thinking is that if the PTU is constantly cycling when it shouldn't be, we in the cockpit may never see it on the memo and would therefore not know if it was happening, unless we happened to see it during our system pages scan.)

Another thought is; could this be a sign that the hydraulic accumulator(s) need recharging?

I will endeavour to record details in the Tech Log as you suggest, should a fault ever arise.


Last edited by Uplinker; 27th Jan 2013 at 10:11.
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Old 26th Jan 2013, 14:16
  #39 (permalink)  
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System Accumulator - low "air" precharge pressure


You raise an interesting point, It looks as if the self-styled "greasemonkey" is now away fixing planes, so, FWIW, here's my take on it.

As you well know, an accumulator precharge pressure can only be checked when its system is depressurised.

AFAIK, there is no ECAM warning for low air pressure in an accumulator. (We used to have LAF accumulators in the flight-control system, and they did have an ECAM warning of low precharge pressure, which frequently used to appear on shutdown after a long flight [cold soak], until the ambient air warmed them up.)

For beginners, a system accumulator stores fluid to helps cater for sudden changes in demand. To reduce surges in system pressure, a cushion of "air" is provided. This is separated from the fluid by a diaphragm or piston. The accumulator itself can be likened to the cylinder of a car engine. (In practice, it looks like an oxygen bottle, with a pressure gauge at the end where the "air" (nitrogen) is.) The fluid pressure forces the diaphragm to squash the air until the air pressure balances the fluid pressure. When the system itself is depressurised, the pressure of the air forces the diaphragm to the end of the accumulator, forcing fluid back towards the hydraulic reservoir. The air precharge falls to its nominal pressure, which can now be checked and adjusted by your "greasemonkey". Typically, it may be between 1000psi and 2000psi, but I forget the figure for the A320. (Remember, system pressure is 3000psi.)

What happens if the precharge is lost completely, due to a faulty accumulator? Next time the system is pressurised, the diaphragm will shunt rapidly the full length of the accumulator, hitting the end-stop. Thereafter, the pressure in the system will be less stable than usual.

This is, I think, what Uplinker has in mind. Could this rapidly-fluctuating pressure lead the PTU to "bark" intermittently, perhaps without triggering either a "SYS LO PRESS" warning, or even the PTU message on the ECAM Memo?

By the way, a quick look on the 'net shows there may have been a batch of faulty accumulators in circulation a year or two ago.

IIRC, one or both of the Green and Yellow system accumulators, like the Green reservoir, may be up on the keel beam, and therefore only accessible by dropping one or both main L/G doors. (That's easily done, but standing on the main wheels is not high enough unless you are very tall, so you need steps. Perhaps it is a job for IFixPlanes, after all. )
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Old 26th Jan 2013, 17:26
  #40 (permalink)  
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@ Uplinker
Sorry for that offense. Had a bad day. I have removed my misbehavior now.

"HYD PTU" Memo:
I did not find any delay in the ECAM system logic Data so the memo should come as soon as the PTU starts. But it did not find any description how long it stay.
If there is only a low demand on pressure due to lack of F/CTL movement the MEMO would cycle is there is no delay of fade out. I assume there is a light delay for the fade out.

I do not think that the system accumulator can force this behavior. The system accumulator covers the time of hydraulic pressure demand to the time the EDP response. The accumulator also smoothen the output from the EDP.
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