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A320 PTU Inhibition

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A320 PTU Inhibition

Old 7th Apr 2013, 16:35
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A320 PTU Inhibition

Hi Guys,

Need some help understanding something regarding PTU and when it is inhibited.

I am reading the airbus diagram in the FCOM, and the AND/OR/AND drawing is hard to figure out.

My understanding is the PTU is inhibited no matter where you are if:

- park brake is on; and also when
- nws is in towing position
- cargo door is operated (+40secs)

I am trying to get someone to explain to me the other conditions to do with the nose shock absorber, and the eng master switches.

The scenario i am thinking is after an RTO, engine 1 failed, the ptu runs. After stopping captain sets the parking brake (PTU stops). Then ready to taxi in he releases the parking brake, however this time as 1 eng master is off, the PTU will still be inhibited and so we cannot taxi back in as the NWS is on the green system.

I may be going off on a tangent, if someone can explain it to me it would be great.
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Old 7th Apr 2013, 17:33
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Once the rh engine is running will the inhibition not be cancelled and the ptu can operate? Only a spotter so sorry if I'm way of target,the fact the lh engine master switch is set to off should not prevent the. Ptu from operating

Last edited by spottilludrop; 7th Apr 2013 at 17:41.
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Old 7th Apr 2013, 17:48
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Firstly, that AND/OR drawing is called a Boullian Expression.

Secondly, the PTU is only inhibited when:

1. Y pump PB is off AND
2. Cargo Door OPER

And as a side note the PTU continues to be inhibited for 40 seconds after the cargo door closes.

The other conditions show when the PTU runs, these are

1. PTU pb switch in auto AND
2. Diff pressure between G & Y >500 AND

Either of:

3a. NOSE L/G shock absorber extended (i.e. airborne)
3b. Both ENG Masters off
3c. Both ENG Masters on (These two ensure PTU doesn't run after only one engine is started)
3d. Park Brk off AND NWS not in towing position


You are right about the three inhibitions, as far as I can tell. But your assumption in RTO is slightly wrong. We need to focus on conditions 3a to 3d to understand what will happen, conditions 1 and 2 will be met following a failed engine with the PTU pb in its normal position

When the engine fails the PTU will kick in once diff pressure drops less than 500.

Condition 3a is not met since you are on the ground.

When you stop, set the p.brake, you probably will still have both ENG Masters set to ON, when you finish ECAM actions you'll have one ENG Master off, so both conditions 3b. and 3c. will no longer be met,

The only condition left that will allow for the PTU to run is 3d. Well the NWS is not going to be in towing position, so that condition is met, the only thing left is the p.brake, i.e. when its off the PTU will run, when it's on the PTU will not run. So as soon as you are ready to taxi again and switch the P.Brake off the PTU should start running and will give you pressure again in the Green or Yellow systems.

This is based on how I understand the diagram.

Last edited by Airmann; 7th Apr 2013 at 17:52.
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Old 7th Apr 2013, 18:24
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airmann

airmann thank you very much for the help!!

your explanation has also taught me how to read/understand these diagrams thank you very much !!
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Old 7th Apr 2013, 18:50
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Firstly, that AND/OR drawing is called a Boullian Expression.

Secondly, the PTU is only inhibited when:

1. Y pump PB is off AND
2. Cargo Door OPER

And as a side note the PTU continues to be inhibited for 40 seconds after the cargo door closes.

The other conditions show when the PTU runs, these are

1. PTU pb switch in auto AND
2. Diff pressure between G & Y >500 AND


Either of:

3a. NOSE L/G shock absorber extended (i.e. airborne)
3b. Both ENG Masters off
3c. Both ENG Masters on (These two ensure PTU doesn't run after only one engine is started)
3d. Park Brk off AND NWS not in towing position


You are right about the three inhibitions, as far as I can tell. But your assumption in RTO is slightly wrong. We need to focus on conditions 3a to 3d to understand what will happen, conditions 1 and 2 will be met following a failed engine with the PTU pb in its normal position

When the engine fails the PTU will kick in once diff pressure drops less than 500.

Condition 3a is not met since you are on the ground.

When you stop, set the p.brake, you probably will still have both ENG Masters set to ON, when you finish ECAM actions you'll have one ENG Master off, so both conditions 3b. and 3c. will no longer be met,

The only condition left that will allow for the PTU to run is 3d. Well the NWS is not going to be in towing position, so that condition is met, the only thing left is the p.brake, i.e. when its off the PTU will run, when it's on the PTU will not run. So as soon as you are ready to taxi again and switch the P.Brake off the PTU should start running and will give you pressure again in the Green or Yellow systems.

This is based on how I understand the diagram.
Hi airman another question then:

so we have established that with the RTO condition the PTU will run once we release the parking brake.

Which brings me to my next question:

We are on a taxiway starting engine 2 after push back

NWS is not in the towing position; and
the parking brake is off

i have lime greened the conditions above so it appears the PTU should run, however if you check DSC-29-20 P2/8 PTU pb sw NOTE: the PTU is inhibited during the first engine start and automatically tested during the second engine start.

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Old 8th Apr 2013, 01:58
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No there are some errors in your statements

When on the ground with the engine master sw’s both not off or both not on the PTU will only run when the parking brake is on and the NWS is in not in the towing position!
The PTU will only run when the P Brake is set to OFF not ON, i.e. when your moving, your second statement seems to get this right.

Additionally with no engines running (both master sw OFF), if the yellow electrical pump is switched on, the green system will pressurise via the PTU, which is why ground crew must be advised before switching it on and the PTU can be set to OFF first to avoid this.
Precisely

The PTU is inhibited whenever the Y Pump pb is OFF, whenever a cargo door is operated until 40seconds after the door stops operation and during first engine start.
Be careful on this one, look at the diagram, notice the shape of the box that contains the lines of the inhibit conditions (Y Pump is off, cargo door is operated). Its an AND gate, not an OR gate, meaning that the inhibit condition is when the Cargo door is operated AND the Y Pump pb is off, not either or, both have to occur at the same time.
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Old 8th Apr 2013, 08:50
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Maybe this picture makes it more clear:


Last edited by IFixPlanes; 3rd Apr 2016 at 16:46. Reason: Old Link didn´t work anymore
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Old 8th Apr 2013, 14:27
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Additionally with no engines running (both master sw OFF), if the yellow electrical pump is switched on, the green system will pressurise via the PTU, which is why ground crew must be advised before switching it on and the PTU can be set to OFF first to avoid this.
Unless you're just posing hypotheticals, you wouldn't, or at least shouldn't, see this situation at all.
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Old 13th Nov 2015, 14:16
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PTU inhibited...
during first engine start (self test on second engine start)
40 seconds after cargo door operation for second engine start (will get ecam)
parking brake on and engine master switches split (one-on, the other-off)
parking brake on and nose wheel steering disconnect memo on e/wd
If either green or yellow hydraulics have low quantity, low air pressure or overheat in reservoir.
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Old 14th Nov 2015, 04:19
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"NWS is on the green system"... Must be a CFM powered model...
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Old 14th Nov 2015, 05:46
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"NWS is on the green system"... Must be a CFM powered model...
We have an all V2500 fleet, some have NWS on yellow, some on green. Doesn't seem to be an age thing. Anyone know why?
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Old 14th Nov 2015, 09:24
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The NWS hydraulics changed when Airbus started adding an ABCU. In our fleet it seems to have cutover around MSN 2500.

We have an entire fleet of V2500, just under half with NWS off the yellow.
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Old 14th Nov 2015, 09:29
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The changeover came with the A320 Enhanced.
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Old 14th Nov 2015, 19:17
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Our oldest and newest have NWS on yellow, with some in the middle being on green. Doesn't seem to be a simple change at a certain serial number.
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Old 6th Oct 2020, 01:57
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Originally Posted by Airmann View Post
No there are some errors in your statements



The PTU will only run when the P Brake is set to OFF not ON, i.e. when your moving, your second statement seems to get this right.



Precisely



Be careful on this one, look at the diagram, notice the shape of the box that contains the lines of the inhibit conditions (Y Pump is off, cargo door is operated). Its an AND gate, not an OR gate, meaning that the inhibit condition is when the Cargo door is operated AND the Y Pump pb is off, not either or, both have to occur at the same time.
Hi Airmen,

To understand, when the YELLOW ELEC PUMP switch is selected to OFF, selection is indicated when no lights are illuminated and the electric-driven pump is de-energised. It is automatically energised when the forward or aft cargo door lever is operated. So assuming if there is no electrical power available, (Yellow pump not energised) cargo door operation is done using the hand pump, in this case PTU is inhibited.

Also why did airbus remove the PTU inhibition diagram from the FCOM's.
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Old 17th Oct 2020, 19:32
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Originally Posted by experiencedbusdriver View Post
PTU inhibited...
during first engine start (self test on second engine start)
40 seconds after cargo door operation for second engine start (will get ecam)
parking brake on and engine master switches split (one-on, the other-off)
parking brake on and nose wheel steering disconnect memo on e/wd
If either green or yellow hydraulics have low quantity, low air pressure or overheat in reservoir.
One engine master switch ON , parking brake OFF , nosewheel steering disconnected ( pushback configuration)
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