The A320 PTU has an in-line variable displacement pump on one side and a bent-axis fixed displacement pump on the other connected directly by a shaft.

what would be needed to know how fast the shaft turns during PTU operation?

  • 3
    $\begingroup$ From a pilot's perspective, the answer is "who cares" (either the PTU works or it doesn't, and that rotation rate isn't directly observed nor influenced from the cockpit). From the designer's perspective, it probably was an interesting number, years ago. From a mechanic's perspective, it needs to match whatever is prescribed in the maintenance manual, or at least the output needs to be within limits. Can you explain why such an esoteric number is of interest? $\endgroup$
    – Ralph J
    Mar 29, 2021 at 21:50
  • $\begingroup$ @Ralph J, I am no longer surprised, but more concerned, by the number of esoteric Airbus A320 specific questions we get here. Do the operators not train their people in where to go for needed information so that they have to turn to internet boards to do their job? Or is there something about the A320 that is just so fascinating that aviation enthusiasts crave trivia like this? $\endgroup$ Apr 1, 2021 at 18:02
  • $\begingroup$ @MichaelHall for all we know it could be a homework exercise or an exam question for A320 mechanics. $\endgroup$
    – DeltaLima
    Apr 1, 2021 at 18:24
  • $\begingroup$ @DeltaLima, I think you are agreeing with me... But either way, if it was homework shouldn't you seek the answer in approved manuals? $\endgroup$ Apr 1, 2021 at 18:50
  • 2
    $\begingroup$ @MichaelHall my theory is that an increasing number of people can't absorb information from a well structured document anymore, unless that document has been indexed by google. The "art" of reading a document index and finding relevant sections is getting lost. As a consequence people turn up here with questions to which the answer is written in the book next to their keyboard. That then begs the question, are Boeing maintenance manuals easier found on the internet than their Airbus counterparts. $\endgroup$
    – DeltaLima
    Apr 1, 2021 at 19:50

3 Answers 3


from Eaton published data
Source: eaton.ru (PDF)

From an A320 systems design document (own work from published on-line data):

Power Transfer Unit (PTU) Eaton MPHV3-115-1C

The PTU is a double hydraulic pump mounted on a common axis, one side connected to the Green hydraulic system and the other to the Yellow hydraulic system. This enables the Green system to be powered by the Yellow system and vice versa, without hydraulic fluid transfer between the two systems. The PTU enables the gear to be retracted upon engine 1 fail, and for the LH elevator to be powered upon engine 2 fail.

The PTU has the following characteristics:

  • Each pump can function as a hydraulic motor for powering the pump at the other side.
  • The Green side pump is a fixed displacement bent-axis type, delivering max. 90 l/min on the Green side and costing max. 120 l/min on the Yellow side. Delivered flow is a linear function of pump rotational speed.
  • The Yellow side pump is a variable displacement type, delivering up to 50 l/min on the Yellow side at a cost of 60 l/min on the Green side.
  • The PTU switches ON automatically when one side has a pressure that is 500 PSI lower than the other side. It switches OFF when pressures are close to equal.
  • PTU function can be inhibited from a switch in the cockpit.

enter image description here
Source: aircrafttechnic.com

The PTU shaft RPM can be determined from the following:

  1. Powering the Green side: a linear function of Green system delivered flow.
  2. Powering the Yellow side: also a function of delivered Yellow system flow, although not linear. The pump is a variable displacement one, which usually ramps up to its standard operating RPM and then regulates displacement from a pressure feedback loop.
  3. A sound measurement, in which a dominant frequency will occur of the pump pistons rotating. The PTU sound is a cue that is recorded and reproduced in Level D simulators, frequency analysis will reveal the pump piston frequency.

There will be dynamic effects in RPM to to changes in power delivery.

  • 1
    $\begingroup$ @ymb1 The text is own work, from a simulation software design report. $\endgroup$
    – Koyovis
    Sep 1, 2021 at 19:38

The shaft speed is dependent on the flow rate through the fixed displacement side of the PTU. I am not sure what the size of the A320 PTU pumps are, but it will have some fixed amount of volume per revolution, such as 0.9 cubic inches (14.7 ml) per revolution. Therefore, if you know the flow rate into that pump (Green Side) you can easily calculate to the shaft speed.

Again, I am not familar with the A320 system specifically, but the PTUs I am familiar will all have a flow regulator upstream to control the maximum flow rate (and thus speed) of the PTU.


If you are not allowed to attach the measuring device to it, the two unobtrusive methods are by sound or by video.

Measuring by video would require the device to be visible, have some surface features to determine some particular rotation and the possibility to make probably video recording while in operation. The recorded video can be split into individual frames in the simplest case just estimating the rotation visually, as the time interval between the frames would be known. Otherwise, tools like OpenCV or fast.ai may help to analyse the images automatically. But these methods are limited by the number of frames per second you can capture. Most of the cameras would only do 30 frames per second or about but much faster cameras are available if you look on purpose. But the method can also capture slow or irregular movements.

Another approach is by sound - record the sound and plot the windowed spectrogram of recording. This can be done with less access possible, and work with higher frequencies, as long as the rotation sound is loud enough. Of course it will be a strong band from the engines on the spectrogram but it may fall far outside the reasonable frequency range of this "barking dog".

  • $\begingroup$ Sounds like a credible answer, but do you have any references? Does this come from the AMM? $\endgroup$ Apr 1, 2021 at 18:08

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