* A Power Transfer Unit - Source: YouTube *
If you've ever flown as a passenger aboard an Airbus A320 aircraft then there's a high chance you've heard a rather distinct "barking dog" sound which has also been likened to a workman with a saw. If you've ever wondered exactly what that sound is then you'd not be alone. In fact, it's probably up there among the most commonly asked question from both passengers and members of the public alike - especially those individuals who have a fear of flying and/or suffer from anxiety or heightened-nerves in the flying environment because, granted, it's a peculiar sound.
Those intermittent barking or sawing noises you can hear are actually coming from a hydraulic motor/pump known as the "Power Transfer Unit", or PTU for short. Due to the PTU's location on the aircraft you'd be most likely to hear this sound should you sit in close proximity to the wings of the aircraft. Unlike traditional motors/pumps in our lives - which are likely electrically powered - the PTU onboard the A320 is a hydraulically powered unit that exists to restore a minimum pressure across two of the aircrafts' hydraulic systems.
In this blog post I aim to explain exactly what the PTU is to provide some context to the noise you hear on your travels. If you're not sure what sound i'm talking about, I've captured it during a recent trip. Click the button below to play it.
I could literally jump in and say "Oh the PTU does this.." but I feel that doesn't actually answer the question properly. From my writing above we already know the noise comes from a pump, but why do we need that pump in the first place and why does it make the noise at the times it does? That's what I am wanting to answer with this blog post. To do this though I need to go into a little more detail.
Note - My aim isn't to overcomplicate things but to provide enough detail that a nervous flyer can piece the respective parts of the puzzle together and begin to understand why noises occur when they do etc. The saying is "knowledge is power" after all. So bear with me with the rest of the post. The answer will then make a lot more sense.
The Airbus A320 is heavily dependent on the presence of hydraulics to power a wide variety of things on the aircraft. The hydraulic system provides 3000psi of pressure to it's respective services. To help you picture just how many aircraft services require hydraulics let me list some:
You can see that with the aircraft being so dependent on hydraulics that it becomes paramount there be redundancy in place. Thankfully the A320 achieves this in two ways. I explain these in turn below.
* Map of A320 hydraulic systems to flight controls. Source: quora.com *
It would have been very easy to design the aircraft with one hydraulic system, one source of pressure and a single tank of hydraulic fluid, however if something was ever to happen to said system then none of the above listed items could function and the plane would essentially be inoperable. I think you and I can agree that in the modern world that's impractical and so Airbus's designers added some redundancy. They didn't just add one additional hydraulic system though... they added two. These systems are known as the Green, Blue and Yellow systems. Hence the colouring in the above graphic.
Taking that redundancy a step further it would also have been incredibly short-sighed to allow all of the flight control surfaces on a single wing to be powered by one hydraulic system. The same could be said about the rudder on the tail too. If that hydraulic system was to lose all of it's fluid, or had a pressure loss etc then they'd once again be inoperable. To prevent this scenario from becoming a reality Airbus engineers split most hydrulic services between the various hydraulic systems on the aircraft. For example, on the wings the five spoilers are split in the following order Green, Yellow, Blue, Yellow, Green. Where the rudder is concerned, all three systems can power it if needs be as it's that critical to safe flight.
Hopefully I've now started to build a basic overview in your head as to how the aircraft hydraulics system is designed. I won't go into a great deal more detail on that front as that in itself can take a whole separate blog post. The next bit of this post discusses how the hydraulic systems get their power.
* Engine-driven pumps power the Green & Yellow system *
In normal operation the hydraulic systems are pressurised in the following ways:
Now... let's think about redundancy again for a moment. It's all well and good splitting the various aircraft services between three hydraulic systems but it would be fairly silly to then only have one method of pressuring each of those respective systems, right? The answer to that is of course, a massive YES! Airbus engineers therefore came up with methods to restore normal operation in the event of a primary supply either failing completely, or not failing to provide enough pressure.
Hydraulics can therefore be restored by what is known as auxiliary methods:
* The A320 Hydraulic System Display showing the PTU in operation *
Earlier on in this blog post you will have read that the three systems on the aircraft operate at 3000psi. The engine driven hydraulic pumps are more than adequate to provide this amount of pressure to their respective system but what if there happened to be a loss of pressure? Or what if the engine-driven pump were to fail or the engine itself failed? This is where the PTU comes into play. In a nutshell the Power Transfer Unit's primary function is to restore the required 3000 psi in either the Green or Yellow systems where one of these systems experiences a drop in pressure of 500 psi or more.
* The A320 PTU schematic - Source: aircraftsystemstech.com *
The PTU is known as bi-directional which means that it can restore the required pressure to either of the systems where required. I'm not an engineer so won't go into the PTU in that level of depth but generally speaking the PTU makes use of the side with the highest pressure to drive a a motor which then in turn drives a pump for the other systems hydraulic system. Notice here that there is never a transfer of actual hydraulic fluid between the two systems, merely a transfer of energy from one system to another. This prevents a total loss of all fluid should there be a leak.
Example: Yellow's pressure drops to 2400 psi then the PTU would demand the Green system drive the unit to provide sufficient pressure output to the Yellow system subsequently ensuring 3000 psi is always available.
Now you know how it works, let's turn our focus to the noise it produces. You might think that a pump providing an output of 3000 psi would make a continuous noise however the PTU achieves the creation of pressure in quick and successive cycles. As soon as pressure gets to 3000 psi in the receiving system it no longer needs to operate and as such switches off. If the reason for the drop in pressure isn't removed from the equation then the pressure will soon start to drop again and the PTU will once again start. That creates the somewhat ON OFF ON OFF ON OFF cycle and associated barking sound.
As mentioned previously the PTU is used when the engines are not able to provide an adequate output. In-flight you'd seldom require the PTU to run at all. That's not to say it wouldn't ever kick in, but even if it did there's a high chance engine sound would drown it out. Most passengers say they can hear the PTU when on the ground shortly after push-back, during taxi. Let's explore why.
Shortly After Pushback
During aircraft push back the PTU is typically inhibited. By this I mean it would not make sense for the PTU to operate at this stage of the flight when the engines have yet to be given a chance to start up. This prevents the PTU running wild after the first engine starts due to one system being at 3000 psi and the other at zero. It remains inhibited until both engines are running and the engine ignition switch has been set back to the Normal position. At this point you'd expect both engines to power their own respective hydraulic systems with the required pressure and so the PTU wouldn't run then either. What is actually causing the PTU to operate during pushback is its' self-test function.
Up until engine start the PTU is off which means that the aircrafts' warning computers haven't had the opportunity to check the PTU is functioning. To ensure the PTU is working correctly the aircraft trigger PTU operation for a short period during the starting sequence of engine two. The test itself typically lasts only a few seconds but can last up to two minutes. Even a matter of seconds is a sufficient length of time to produce the barking sound which as a passenger you will almost certainly hear. If the aircraft warning computers detect it likely the PTU is not functioning correctly then it will notify the pilots and action would be taken accordingly.
During Taxi for Departure and Arrival
With the cost of aviation fuel only ever increasing and our awareness of the environment also on the up, it is quite common for many airlines to taxi out to the runway or to the gate after landing by using only one engine. A single engine taxi departure would see us taxi towards the runway on one engine before starting engine two prior to departure. During these scenarios we want to prevent the PTU from running and as such enable the electric pump associated with the yellow system.
When taxying out to the runway for a departure, the delayed start of engine two would initiate the PTU self-test mentioned earlier. The major difference now is that the test occurs during the taxi as opposed to the pushback. This explains why you may hear it on the way out to the runway.
When taxying back in to the gate after landing we would shutdown engine two and turn on the yellow electric pump to once again prevent the PTU from running. However, when we arrive at the gate we would switch the yellow electric pump off prior to shutting down engine one. The period of time between the yellow electric pump being turned off and the engine being switched off is minimal but there is a small chance pressure could drop, with subsequent PTU activation being witnessed. This explains why it can sometimes be heard when pulling onto stand. Another reason you might hear on taxi, although slim, is that the pilots might have forgotten to switch on the yellow electric pump and as such the PTU kicks in for the taxying period. I say this is slim as the electric pump activation is part of the single engine taxi procedure, but at larger airfields with ATC talking to you and with complex taxi routings the forgetting to enable said pump couldn't be ruled out.
So there you have it: The PTU Explained.
I'm sure it can now be seen that the PTU is a fairly important bit of a kit on the aircraft; it's just a tad unfortunate it makes a racket during operation. Nevertheless, hopefully through this post you have expanded your own knowledge of the Power Transfer Unit and if you're a nervous flyer you'll feel a tad more at ease when hearing said noise aboard an Airbus A320 aircraft.
If you have any further questions about aviation or the A320 family of aircraft then let me know in the comments section below and I'll add it to the list of blog posts to add to the blog going forward.
All the best,