I must have flown hundreds of times in an a319/320/321 in my life (used to fly up to 40 times a year for work) and always enjoyed (is that the word?) the famous barking sound as the PTU engaged and the second engine started up.

I wanted to ask though whether a similar sound is heard in other Airbus aircraft. The a330 (which I have been in just a handful of times), or the a340 (once or twice).. or a350 (never, as far as I recall)? Or a380 - in which I have flown a few times but have no memory of the PTU sound, if any. I am sure the answer is basically 'no', but a fuller description (comparison) of what one hears from the cabin vis-a-vis the PTU would be appreciated (for reasons I could explain, if anyone asks, but won't bore folks with right now). Any real-world sound recordings anyone knows of??

Thank you.

Here is a very long recording of an A320 PTU:

https://youtu.be/XEG1ZfPHkVo

Just search for „barking dog“ on YouTube, there are plenty of videos with sound.

Never heard it on any other plane. The A320 hydraulic system is different than the one of the newer models. By the way: there is also no PTU-Sound you can hear on the NEO.

Thanks. Well, it is audible on the a319, but don't recall a flight on the a321 to be able to say... surely must be there too, the three a/c being so similar? Other than a319/320.321, doubtless absent (why is that though? what is the difference in the PTU system to account for that?).

By the way: there is also no PTU-Sound you can hear on the NEO

Oh, that I didn't know. Is that for sure?

The green hydraulic system is pressurized by an engine driven pump, the blue system by an electric pump (and can be pressurized by the RAT in an emergency), the yellow system is usually driven by an engine driven pump but is also equipped with an electric pump for redundancy/ground operations (the yellow system moves the cargo doors through the electric pump). The PTU is able to power the yellow system through the green system and vice versa. It automatically operates when the difference in pressure between the yellow and green systems exceeds 500PSI. Without the PTU the failure of engine Nr. 1 or the loss of the green engine driven pump would result in the loss of the green system.
The reason you can hear the PTU during engine start is that it performs a self-test during the second engine start. If you hear it inflight it probably means that a hydraulic pump or one engine failed.
By the way, the yellow electric pump is also quite audible in the cabin. It's a quite high pitched whining sound - it can usually be heard during cargo door operation or during single engine taxi in (for single engine taxi, engine 2 is shutdown and to keep the yellow system powered without using the PTU the yellow electric pump is switched on).

Martinlest wrote:

Thanks. Well, it is audible on the a319, but don't recall a flight on the a321 to be able to say... surely must be there too, the three a/c being so similar? Other than a319/320.321, doubtless absent (why is that though? what is the difference in the PTU system to account for that?).

By the way: there is also no PTU-Sound you can hear on the NEO

Oh, that I didn't know. Is that for sure?

Pretty sure there was a mod that Airbus did to put the dog out of its misery and make the PTU much quieter.

Martinlest wrote:

I must have flown hundreds of times in an a319/320/321 in my life (used to fly up to 40 times a year for work) and always enjoyed (is that the word?) the famous barking sound as the PTU engaged and the second engine started up.

I wanted to ask though whether a similar sound is heard in other Airbus aircraft. The a330 (which I have been in just a handful of times), or the a340 (once or twice).. or a350 (never, as far as I recall)? Or a380 - in which I have flown a few times but have no memory of the PTU sound, if any. I am sure the answer is basically 'no', but a fuller description (comparison) of what one hears from the cabin vis-a-vis the PTU would be appreciated (for reasons I could explain, if anyone asks, but won't bore folks with right now). Any real-world sound recordings anyone knows of??

Thank you.

The A330/A340, the A380, and the A350 have different hydraulic architectures from the A320 and no PTU.

In summary. Any corrections welcome.

- A320: The green and yellow systems are run by engines 1 and 2 respectively. The blue system is run by an electric pump which runs when either engine is running. The PTU allows transfer of hydraulic power from green to yellow or vice-versa. The RAT provides emergency hydraulics by powering the blue system.

- A330: The blue and yellow systems are run by engines 1 and 2 respectively. The green system has two pumps, one in each engine. So if you lose an engine you retain two hydraulic systems. The RAT provides emergency hydraulics by powering the green system..

- A340: I can't find any diagrams or other info, but given it is basically the same aircraft as the A330 I would guess the architecture is similar but with more pumps since it has more engines.

- A380: There are only two hydraulic systems, green and yellow. Green is run by 2 pumps in each of engines 1 and 2. Yellow is run by 2 pumps each in engine 3 and 4. Backup hydraulics are supplied by self-contained hydraulic actuators, so there is no third "primary" system.. The RAT does not supply hydraulic power, only electric power.

- A350: There are only two hydraulic systems, green and yellow. Both systems have two pumps, one in each engine. So if you lose an engine you retain all hydraulics. Backup hydraulics are supplied by self-contained hydraulic actuators, so there is no third "primary" system. The RAT does not supply hydraulic power, only electric power.

Starlionblue wrote:

- A340: I can't find any diagrams or other info, but given it is basically the same aircraft as the A330 I would guess the architecture is similar but with more pumps since it has more engines.

Yes, the general layout is basically the same, but with the same amount of pumps. Eng 1 and Eng 4 drive the green HYD pumps, Eng 2 is pressurizing the blue system and Eng 3 the yellow.

glen wrote:

Starlionblue wrote:

- A340: I can't find any diagrams or other info, but given it is basically the same aircraft as the A330 I would guess the architecture is similar but with more pumps since it has more engines.

Yes, the general layout is basically the same, but with the same amount of pumps. Eng 1 and Eng 4 drive the green HYD pumps, Eng 2 is pressurizing the blue system and Eng 3 the yellow.

I was unable to quote what I wanted to, so ignore the quote above. Can anyone provide info on these self contained hydraulic actuators?

sailsail wrote:

glen wrote:

Starlionblue wrote:

- A340: I can't find any diagrams or other info, but given it is basically the same aircraft as the A330 I would guess the architecture is similar but with more pumps since it has more engines.

Yes, the general layout is basically the same, but with the same amount of pumps. Eng 1 and Eng 4 drive the green HYD pumps, Eng 2 is pressurizing the blue system and Eng 3 the yellow.

I was unable to quote what I wanted to, so ignore the quote above. Can anyone provide info on these self contained hydraulic actuators?

I'm guessing here, the term actuators is incorrect. Usually self contained back up systems have accumulators. It's a tank that has a diaphragm in it. The side open to the system has hydraulic fluid while the other side has a nitrogen charge that provides a limited back up pressure to the system. They also act as system shock absorbers during brief high demand.

Most aircraft have these on very small scale subsystems like brakes.

By definition actuators use hydraulic pressure to apply a force, not create hydraulic pressure. But since I'm not familiar with the A380 system, maybe Airbus has some funky naming going on here.

Starlionblue wrote:

- A350: There are only two hydraulic systems, green and yellow. Both systems have two pumps, one in each engine. So if you lose an engine you retain all hydraulics. Backup hydraulics are supplied by self-contained hydraulic actuators, so there is no third "primary" system. The RAT does not supply hydraulic power, only electric power.

I'm sure I'm missing something obvious here but, in an extremely unlikely dual-engine failure scenario: Would the APU be responsible for hydraulic power? And if in a Gimly Glider fuel-exhaustion scenario (the odds of which are next to none), would the aircraft be controllable?

By definition actuators use hydraulic pressure to apply a force, not create hydraulic pressure. But since I'm not familiar with the A380 system, maybe Airbus has some funky naming going on here.

The A350 and A380 have EHA (electro hydrostatic actuators )fitted to all flying controls as the back up hyd system.
These are small power packs powered by electricity, and include a hydraulic pump and a tank and an actuator all self contained in one unit.

This is nothing new, The Vickers VC10 had a very similar system in 1964. But the VC10 needed you to go round and manually check the levels and top up the tanks. I think on a weekly check?. The A380 has it all built in.

rjsampson wrote:

Starlionblue wrote:

- A350: There are only two hydraulic systems, green and yellow. Both systems have two pumps, one in each engine. So if you lose an engine you retain all hydraulics. Backup hydraulics are supplied by self-contained hydraulic actuators, so there is no third "primary" system. The RAT does not supply hydraulic power, only electric power.

I'm sure I'm missing something obvious here but, in an extremely unlikely dual-engine failure scenario: Would the APU be responsible for hydraulic power? And if in a Gimly Glider fuel-exhaustion scenario (the odds of which are next to none), would the aircraft be controllable?

In case of loss of both hydraulic systems, flight control is provided by the EHAs (Electro-Hydrostatic Actuators) and EBHAs (Electrical Backup Hydraulic Actuators). EHAs use electrical power to generate their own hydraulic pressure, using a self-contained reservoir. EBHAs use "main" hydraulics in normal operation, but act like EHAs when main hydraulics are lost. See diagram below.

This architecture means the aircraft is controllable with both engines flamed out. That's a certification requirement anyway. You'd be left with the inner ailerons, rudder and elevators powered by EHAs, as well as one pair of spoilers powered by EBHAs.

The A350 APU does not provide hydraulic power. And neither does the RAT. The A350 RAT generates electricity only. Contrast with the A330 RAT, which powers the green hydraulic system, which in turn powers the emergency generator if needed.

The APU will start automatically in case of dual engine flameout (assuming you have fuel).

sailsail wrote:

Can anyone provide info on these self contained hydraulic actuators?

See diagram.

For more information, check out the "Flight Controls" section in this excellent briefing: https://www.smartcockpit.com/docs/a350- ... pilots.pdf

Dalmd88 wrote:

sailsail wrote:

glen wrote:

Yes, the general layout is basically the same, but with the same amount of pumps. Eng 1 and Eng 4 drive the green HYD pumps, Eng 2 is pressurizing the blue system and Eng 3 the yellow.

I was unable to quote what I wanted to, so ignore the quote above. Can anyone provide info on these self contained hydraulic actuators?

I'm guessing here, the term actuators is incorrect. Usually self contained back up systems have accumulators. It's a tank that has a diaphragm in it. The side open to the system has hydraulic fluid while the other side has a nitrogen charge that provides a limited back up pressure to the system. They also act as system shock absorbers during brief high demand.

Most aircraft have these on very small scale subsystems like brakes.

By definition actuators use hydraulic pressure to apply a force, not create hydraulic pressure. But since I'm not familiar with the A380 system, maybe Airbus has some funky naming going on here.

As you say, most aircraft have this kind of thing on smaller systems only. Apparently the EHAs and EBHAs are much heavier than normal actuators, which explains why the A380 and A350 still have most surfaces powered by main hydraulics. However, the extra weight of having some EHAs and EBHAs is probably more than compensated for by not having a third hydraulic system.

glen wrote:

Starlionblue wrote:

- A340: I can't find any diagrams or other info, but given it is basically the same aircraft as the A330 I would guess the architecture is similar but with more pumps since it has more engines.

Yes, the general layout is basically the same, but with the same amount of pumps. Eng 1 and Eng 4 drive the green HYD pumps, Eng 2 is pressurizing the blue system and Eng 3 the yellow.

Thanks for info!