A320 STRANGE PTU behaviour

[EIU_EEC]

please see these videos
Barking Airbus A320 on Finals into AMS - YouTube (http://youtu.be/WOpnwa61rMQ)
Easyjet Takeoff with PTU fault - YouTube (http://youtu.be/IFYTGdlJPn8)

There is no engine failure, no ecam,no ptu fault, or gear/slat/flap config change, to make the ptu run.BUT why is it running continuously without any action from crew!
Acoording to ptu logic, the only times it comes in are during 2nd engine start as a self test, if you do a single engine taxi and you start or shut down your eng1 on taxiway, and after landing gear retraction, but definitely not at VR, which would mean your flight had experienced an ENGINE FAIL at VR, nice scenario for a sim sessin though.
This is CERTAINLY abnormal case. remember from cockpit PTU running is barely audible and you can confirm it only by memo 'HYD PTU'. ptu logic prevents it from running in case of hyd reservoir low level, low pressure, ovht, pump low pressure. in a320dlh's video notice how the freq of barking increases on grnd spolier deflection on touchdown!!
in the second video, the pnf could have easily seen ecam memo of hyd ptu, and if in case the ptu was indeed faulty and running for at least 2 mins continuous, no pilot intervention can lead to dual hyd failure in worst case scenario by hyd ovht from yellow to green. just reach up and press that thing OFF, or return to the gate.
Guys, we need to solve this,"out off FCOM stuff". Any A320 AME here in forum?

[Chris Scott]

Hi EIU_EEC,

Not an AME, I’m afraid: just an 11-years-retired A320 pilot. Flying as passenger several times a year, I get to hear the PTU from the cabin almost as much as when I was working. So, just a few thoughts.

(1) Are you sure the PTU could overheat either the Green or Yellow fluid in as little as 2 mins?

(2) On both clips, the PTU is running only at a low height or on the ground.

(3) In the AMS landing, PTU starts at about 300ft, long after the flaps and L/G have been fully extended, and in the apparent absence of roll-spoiler or other flight control movement, except (possibly) the THS. As you say, it continues throughout the landing run. The increased noise during ground-spoiler and engine-reverse extension seems logical.

(4) In the easyJet take-off, it runs throughout taxiing and the take-off run. From start of take-off until rotate, the loads on the hydraulics are minimal. It stops 5 secs after lift-off, at which point the L/G would normally have started to retract. 10 secs after lift-off, the “ping” from the No-Smoking sign relay confirms that L/G retraction is complete.

(5) I wouldn’t expect the PNF to notice the Memo “HYD PTU” in the air at low height, but would certainly agree that it should be noticed eventually while taxiing. However, if there is a software fault, can we assume that the message will appear? I don’t know precisely what generates that message.

(6) Seems silly to ask, but could there possibly have been a mod to the PTU operation criteria? If not, this is a behaviour that needs to be explained.

[737Jock]

Sorry buddy you but you are sadly mistaken, the PTU will not overheat a system when both reservoirs have hydraulic fluid.

The problem happens when one system is empty while the other system is operating the PTU to pressurize the empty system. Since there is nothing tompressurize the PTU will "overspeed" and overheat the full system, leading to a dual hydraulic loss due to overheat.
This problem will be masked by landing and takeoff inhibit. above 1500ft on t/o the ecam will display instructing to turn off the ptu.

This can happen in both directions, green to yellow or yellow to green.

When an engine is shutdown the PTU will operate for far more than 2 minutes. The airbus has no electrical hydraulic pumps (only engine driven, yes there is a yellow electrical but that is not used normally for pressurizing the system in case of engine driven pump failure), so the PTU will pressurize the remaining system.

And it will do so happily for ages and ages without overheating the systems.

There is no danger in these videoclips.

In addition easyjet airbusses have been modified to display an empty reservoir ecam around 400ft on t/o.

[EIU_EEC]

737jock, My question was, Why is the PTU running when it shouldn't in the first place?
Hi, chris, you could reason out any clue Why it is running?

[IFixPlanes]

737jock, My question was, Why is the PTU running when it shouldn't in the first place?
No, the only questionmark in your post was in: ;)
Any A320 AME here in forum?

737Jock gives you some good hints in conjunction with the overheat.
Instead of thanking him for helping you, you bark at him. :ugh:

[Uplinker]

In our company, when we taxy in, we shut down engine 2 after its cool down period. Before we do this we start the APU (to replace the genny) and start the Yellow Electric pump. So no need for PTU to run.

[EIU_EEC]

Okay! My question is why should the ptu run in these two videos?

[Uplinker]

Hmmm...

Don't know. It does not seem normal to me.

The PTU works when difference between the hyd pressure is more than 500 psi in either direction, (from memory). It is normally heard on engine shutdown, and as has been said; during a test phase on engine start.

Easy taxy in and out on one engine, so the PTU might be heard then if they don't use the Yellow Electric pump. But on Take off??

Without knowing the answer, I would have to guess that a HYD pump was giving low pressure, or the PTU 500 psi sense was up the bucket.

[737Jock]

Possibly single engine taxiout, followed by a PTU self test after the second-engine start. I believe the PTU self test can last up to 2 minutes.
Shouldn't happen during T/O but no harm either. I would put it down as a glitch (not even a fault) in the ptu selftest system.

[ReallyAnnoyed]

For what it is worth, I have noticed it a few times now while being passenger and I asked the drivers after landing whether they had done OETD which they hadn't. It made me wonder.

[PENKO]

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!

[Right Way Up]

Sounds like an MEL item. I had this a few years back.

"PTU may continuously run when set to AUTO provided that the operation of the PTU in both directions is checked before the first flight of each day."

[Chris Scott]

EIU_EEC,

Sorry, but I don't think I can shed any real light on this one, particularly as I'm retired. In addition to giving you a hard time (while in one case referring to you patronisingly as "buddy"), several posters seem to have missed the original point of your thread completely. (Wasn't it ever thus? ;))

In the case of the landing at AMS (in marginal Cat 1 conditions of cloud ceiling, so it might be an autoland), I can only point out that we cannot rule out the possibility of a Green or Yellow system low pressure causing the PTU to start. I've run the video again.

2:16 before touchdown (video time 0:04), there is that "ping" (chime) associated with the No Smoking signs automatic illumination (yes, I know that the crew overrides the automatics by forcing the signs on throughout the flight, but that doesn't seem to stop the chime associated with L/G position). So the L/G is being extended at that stage, as is normal (~1500ft agl).

Flaps 3 seems to be extended 1:42 before touchdown (video 0:38), and Flaps Full 10 seconds later (for that, the T/E flap motors are clearly audible). T/E flaps use Green and Yellow, IIRC, but these flap selections have not provoked PTU start.

The PTU starts 15 secs before touchdown (video 2:06), so that's about 170'R (170ft agl), and continues at least to the end of the landing run.

Unless there was a low-pressure trigger (genuine or false) for the PTU to start at that point, I can only speculate again that there might be a recent change to the PTU operation logic? For example, what about an autoland, using both APs?

Turning to the easyJet take-off, Uplinker and 737Jock are working on the single-engine taxi theory, which sounds likely. If the PTU self-test lasts up to 2 mins after the second engine start, is there a minimum time between engine start and commanding take-off thrust on the CFM-56?

Right Way Up,

Only just seen your post. Sounds like a fail-safe logic in that, if the differential-pressure sensor fails, the PTU runs continuously?

Not good for the SLFs down the back. I've always wondered (since 1988) why this PTU (the Yanks call them RMPs - reversible motor-pumps - on other aircraft I've flown) is so damned noisy in the A320 cabin...

[EcamSurprise]

Yes, the minimum time is 2 minutes.

[Chris Scott]

Thanks for that, ECAM Surprise. (Thank goodness YouTube didn't exist in my day....)

[OPEN DES]

737 jock, it's not the ECAM inhibit that has been modified to present a HYD SYS LO LVL on the ECAM below 1500ft/2min after lift-off. The PTU inhibition logic mod has been applied to all ezy airbuses. This means that the PTU will not run when either HYD SYS LO LVL to protect against an inadvertent overheat due to PTU overspeed as you stated.
Getting back to the OP. I have found the PTU to run many times as described, albeit not continuously. Surely the 'AUTO conditions to run' were not met at this time. I attribute it to a 'glitch' as you said and nothing to worry about when all other indications/ops normal.

ATB

[737Jock]

OPEN DES check the warnings and cautions section:

The "modified" ones inhibit the RSVR LO LVL for the first 15seconds of flight phase 5, whereas the other ones inhibit up to 1500ft/2min.
I imagine that 15sec will be approximately 400ft, where we can start ecam drills.

Not sure if it is part of the modification, but it seems to be as it is across the entire board of airplanes. But the majority inhibits only for 15 seconds.

Anyway thanks for the information, had tried to find it but couldn't find a ref to the Modification. And I still can't find it, any idea?

[EIU_EEC]

In addition to giving you a hard time (while in one case referring to you patronisingly as "buddy"), several posters seem to have missed the original point of your thread completely. (Wasn't it ever thus? http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/wink2.gif)
Chris, thank you!
In summary, till now the reason for the "glitch" hasn't been understood. ptu self test for 2 minutes:rolleyes:...and that too in the air in the LVO ops..
This is from the MEL if it helps in getting some clues.

Note: 1. The PTU pressurizes the system to a nominal value of 3 000 PSI PSI (± 200 PSI),
but if no consumers are operated the pressure is not constant. The PTU operates in
a cycle which keeps the pressure in the hydraulic systems between 2 100 PSI and
3 200 PSI.
2. For passengers comfort, the PTU may be set to OFF during the taxi after landing
provided that both engines hydraulic pumps are operative and running. The PTU must
be set back to AUTO before leaving the aircraft.
Can this lead to the PTU servo valves in an endless loop?

The fact is THIS very occurrence is being dismissed as harmless glitch, already in 2 different cases is very disturbing. In aviation NOTHING can be dismissed as unimportant till we get hold of the root cause.History shows this very attitude of us pilots of thinking we know everything and nothing can happen to us has led to the deaths of thousands of innocent lives, who have no idea of whether its a PTU or a dog loose in the Aft cargo barking away!:confused:
So guys instead of criticizing everyone's small errors, please behave constructively and seek help from maintenance ASAP!

[Airmann]

Quoting 737Jock[

The airbus has no electrical hydraulic pumps

I thought the blue system was electrically powered?

[EIU_EEC]

There is no danger in these videoclips
737jock are you doubly sure of this??? unless you pinpoint to me the reason.
ptu self test, SET, etc,are all but guesses.

[PENKO]

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.

[ampclamp]

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 )

[737Jock]

EIU_EEC

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:
http://ad.easa.europa.eu/blob/SIB_201216_R1_Airbus_SA_Hydraulic_System_Improvement.pdf/SIB_2012-16R1_1

The overheat incident:
http://www.aaib.gov.uk/cms_resources.cfm?file=/Airbus_A319-111_Airbus,_G-EZDM_08-09.pdf


Airmann:

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.

[IFixPlanes]

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.

[737Jock]

FCOM dsc-29-10-20 overhead panel

Note:
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?

[Chris Scott]

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.

[IFixPlanes]

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". :E
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.

[Chris Scott]

IFixPlanes,

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.

PENKO,

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.

[Uplinker]

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).


U

[A320skoda]

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.

[737Jock]

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!:D

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.

[Checkboard]

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." :rolleyes:

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

[737Jock]

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.

[TyroPicard]

Is the barking a sign of continuous PTU operation, or an ON-OFF or backwards/forwards operation?

[IFixPlanes]

@ 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)

[TyroPicard]

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?

[TyroPicard]

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".
TP

[Uplinker]

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.


U

[Chris Scott]

Uplinker,

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 (http://www.37000feet.com/report/936143/mechanic-working-an-A320-a-check-in-a-hangar-reports-about-a) 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. ;) )

[IFixPlanes]

@ 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.

[Uplinker]

No problem IFix - we all have our bad days. Apology accepted; I will remove my reference to it :)

Chris; yes, that is exactly what I am refering to; The accumulator smooths pressure fluctuations (and covers short term pressure drops as Ifix says). If an accumulator had lost its nitrogen charge, I thought any pressure fluctuations in the HYD system might trigger the PTU just for a moment, each time, causing the 'barking dog syndrome'.

Not being an aircraft engineer, I don't know what the maintenance procedures are, but no we don't have any cockpit indication of accumulator pressure, so this might go undetected until the next hangar visit?

[Fly3]

Airmann.
You are correct, the Blue hydraulics are powered by an electric pump.

[Checkboard]

I started the second engine the other day with the hydraulic page up on the SD, just out of interest sparked by this discussion - and watched the PTU cycle on as the second engine master was selected on.

I can't say I have ever noticed a PTU message on the memo during engine start before (but I can't say I have looked for it, either.)

[Franklin AME]

Please take your time to read this doc. http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgSAIB.nsf/0/c8cde9e6e5d4931586257aa0004fbdcb/$FILE/NM-13-03.pdf

It is well known that the PTU operation is automatic, it will start working whenever a 500 psi differential pressure exist btwn green and yellow hyd sys, it will be inhibited at first engine start and during cargo door opening among other details (considering parking brake position, and NW steering bypass lever position)

The real problem comes out when there is a leakage on green or yellow hyd system, if you let the ptu working for more than two minutes it will produce an overheat condition in the operative system.

Please check the link above...:ok:

[Franklin AME]

:confused:
What????

Yellow and blue sys have EMDP!:ugh:

[pax2908]

Pax here - was intrigued this morning by the PTU running continuously during taxi (OK) but also without interruption until ~30 seconds after takeoff. Easyjet, 1st flight of the day (I suppose!).

[gegemi]

was it an old a319?

[neilki]

737jock are you doubly sure of this??? unless you pinpoint to me the reason.
ptu self test, SET, etc,are all but guesses.

Airbus Safety First Issue #4 Page 24.
My operators' QRH had an OEB for a while directing the PM to IGNORE ECAM and follow the QRH procedure.
As far as these videos are concerned; there was a detected 500 PSI difference between the 2 systems, and the PTU operated per design. Either an EDP/accumulator wasn't functioning properly, or the pressure sensor was incorrect...
To the poster who said there aren't any electric pumps on the A320... Except on the Blue System and the Yellow System...

[pax2908]

HB-JZZ, A320

[Uplinker]

Pax here - was intrigued this morning by the PTU running continuously during taxi (OK) but also without interruption until ~30 seconds after takeoff. Easyjet, 1st flight of the day (I suppose!).

Are you certain it was the PTU and not an electric motor running? They have different sounds, but I cannot easily explain in text how they differ, except to say that an electric motor driven hydraulic pump has a clean, ‘single’ high sound, whereas the PTU has a deeper ‘multi component’ sound.

Anyway, my guess would be the electric hydraulic pump in the yellow system running. This is done when single engine taxiing, but should be selected off when the second engine has started, before take-off.

If it was the yellow pump, perhaps the crew got distracted and missed turning it off before take-off, then spotted it after getting airborne?

[pax2908]

@Uplinker yes, I am reasonably sure. Where I wrote "continuously" I did mean "continuously pulsing" ("of course"). I did record the sound, rather low quality, starting when we were lining up for t/off ... attached here as .amr.zip

[Uplinker]

That file won’t download for me. Sorry, I don’t know the answer. The only reason I can think of if the PTU was running after the second engine start is a faulty PTU or low pressure in the green or yellow hydraulic systems. But any of those are a no dispatch item.

[pax2908]

But technically, assuming that a non-normal condition "a no-dispatch item" was only detected after (single-engine) taxiing has started, then it could pe possible to continue? For example if a problem was only detected during or after 2nd engine start?

[Uplinker]

By the letter of the law, perhaps. However, I for one would not take-off with a known hydraulic problem that was a ‘No dispatch’ item in the MEL.

It must have been something else in this case. I wonder if a faulty hydraulic accumulator could have been the problem? This has been discussed on Pprune elsewhere, and might occur too briefly to bring up a fault indication on the flight deck.

[dream747]

May I revisit this thread with another query about the PTU.

https://youtu.be/sYihjNpxPjQ

If I am not mistaken, the distinct sound of the PTU can be heard when the landing gear is retracting and extending. There are also a lot of videos on YouTube taken in the cabin and the PTU can be heard when the gear is in the process of being retracted or extended.

Is the PTU working during landing gear operation? I don’t remember ever seeing “HYD PTU” on the ECAM Memo for this, but that’s not where I am usually looking during those times anyway.

[Cough]

Can't see any ground hydraulics plugged in, so guessing they're just using the yellow pump for this test swing. Landing gear is on the green system (if my memory serves me right - It's been 6 years!), so without the left engine running, the PTU has to be used to retract the gear.

[dream747]

Thanks Cough.

What about under normal LG retraction and extension? For example in this video at 1:53 you can hear the PTU (unless that’s something else) during gear retraction. The same can be heard for gear extension quite often and I’ve heard it before as a passenger as well though you typically got to be seated at rows close to the landing gear.

https://youtu.be/nQqQdmVeKBs

[Check Airman]

I've heard the PTU during gear movement while in the back. Usually, it's one of the older planes.

[Cough]

I can only guess dream. Typically during landing gear extension the landing gear is often dumped at the same time as some flap/slat which puts a load on the green system. The PTU jumps in if the pressure differential exceeds 500psi between the two systems, so I could only summarise that if the yellow is running at a slightly higher pressure, green drops a bit when the services are called for then you get a few moments of PTU. End of the world? I dunno, not an engineer, but I guess not. Like you I've never seen HYD PTU either, more things to look at!

[Uplinker]

All the hydraulic systems have multiple accumulators to smooth out the pressure delivery. Accumulators also assist the main pumps by supplementing the system pressure when a large demand is made - such as the landing gear being moved.

If the accumulators are leaky, or not charged with the correct nitrogen pressure, the Green Hyd pressure might well momentarily drop below the PTU triggering pressure during gear movement, or indeed multiple demands, so it cuts in briefly to help.

In the cabin one can certainly hear the PTU blipping on very briefly in some aircraft during gear deployment. I don't think one or two second's operation is long enough for an ECAM memo to be triggered though?

[LandASAP]

As already said in the first video they are doing an Retraction / Extension Test only with the use of the Yellow Elec Pump and no Ground Cart. Thus the PTU needs to operate to pressurize the Green System which is in use for Retraction / Extension.

If i have a Ground Cart available i would avoid the use of only the Elec Pump / PTU because, as you can see, they are able to do the Job but having a hard time. Personally i would like to have a smother operation. But there is nothing wrong doing it only with the Elec Pump.

In the second Video, during operation of the LDG Gear there is a very high demand in the Green Hydraulic System. Big Actuators needs to move a lot of weight simultaneously. This leads to an pressure drop in the Green System which in turn will trigger the Threshold Limit and the PTU starts to Operate. Perfectly normal situation. There is no Logic in the A320 which will turn on the PTU automatically during LDG Operation. It's just because of the pressure difference between G and Y during high demand operation.

In the A330 e.g. you have a logic which will turn on the Elec Pumo during LDG Operation to "support" the EDPs.

[MD83FO]

Been flying an airplane where you can hear the PTU sound constantly, but there’s no indication in the cockpit. Wonder what’s going on
left it written on the tech log.