• What is the PTU or Power Transfer Unit

If you have flown by Airbus, you have probably heard a peculiar noise before take off or after landing. Depending on who you ask, answers as to what this noise reminds them of will vary from it being similar to a dog barking to the noise made by a saw. Yes, it’s that weird.

Although it could seem that something is wrong or broken, nothing is further from the truth. This sound is produced by the PTU or Power Transfer Unit, and is an indicator that everything is working perfectly well.

The hydraulic systems of an aeroplane

Before getting down to the nitty gritty, it’s important to revise a little what hydraulic systems of an aeroplane are.

In this case, we will concentrate on the Airbus A320, the most common passenger aeroplane in Europe for short and medium haul. The A320 comprises three hydraulic systems: green, yellow and blue. That’s right, they are named after colours.

The hydraulic system’s purpose is moving different aeroplane parts: flaps, ailerons, landing gear, brakes, etc.

These systems are independent from each other and only share functions if any of them stop working.

It is also important to know that the systems that share the most functions are the green and yellow ones. As both of these have engine driven pumps or pumps that are activated only when the engine is on.

Did you know?

Car tyres have a pressure of 34 PSI, heavy goods vehicles, 80 PSI. Did you know the hydraulic system of an A320 works at a 3000 PSI?

So, what’s a PTU then?

Now you have a general view of what a hydraulic system is, let’s return to PTU or Power Transfer Unit.

PTUs, also termed energy transfer valves, connect the green system to the yellow one, activating it if a difference over 500 PSI is reached.

Before each flight, PTUs carry out a self check which is normally done after starting the second engine. That’s why it’s so common to hear it while the aeroplane is taxiing. Also, the noise will be heard whether the valve is working or not.

How the aeroplanes PTU works

From within the hydraulic systems, the blue one obtains pressure from an electrical pump and if this fails, it has a RAT or Ram Air Turbine, as a backup measure.

RATs are small helixes that come out of the fuselage and as air moves through them, they feed the blue system. But do not worry, in one of our future posts, we will further explain this.

The green and yellow systems are a little different. Their principal pressure source is a pump that only works when the engine is on; whereas the buck up system is the actual PTU.

If the PTU detects a difference in pressure over 500 PSI, it will come on automatically without needing further action from the crew.

When it’s on, it sends the required energy to whichever system needs it and receives hydraulic flux from each engine, but independently.

Now you know why PTUs sound like a bark

When PTUs are carrying out their self tests, the valve is operating in a vacuum, That’s why it sounds like the bark of a dog.

Some people are scared of flying although it is the safest means of transport there is. In aviation, everything is thought of and calculated to the limit, there’s always a plan b and nothing is left to chance. Today, we proved this with the example of PTUs.

We hope you can find this post useful and interesting and if you’d like to find out more about the different systems which make up commercial aeroplanes, we recommend you continue navigating our aviation blog. You’ll be surprised by the amount of topics we have already discussed!

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