For my university project, I am designing the fuel supply of an aircraft using a Williams International FJ44-3A engine.

Studying the related FAA TCDS, I read "Fuel from the motive flow port on the fuel control unit may be extracted to drive jet or turbine pumps in the airplane fuel system". About these pumps:

  • What is a turbine pump?
  • What is it used for in this engine?
  • Why would this pump be driven by the motive flow?
  • $\begingroup$ I did look into that question before, but it doesn't answer my question of why does the TCDS said that the motive flow is used on the turbine pump and not the primary fuel boost that answer mentioned. It also didn't answer my question of what is this turbine pump. $\endgroup$
    – Faito Dayo
    Commented Oct 29, 2021 at 3:40
  • $\begingroup$ The exact sentence in the certificate is: "Fuel from the motive flow port on the fuel control unit may be extracted to drive jet or turbine pumps in the airplane fuel system", the linked question explained what is a jet pump. Now you would like to know what a turbine pump is, so let me edit and simplify your questions. Feel free to roll back to the previous version if you prefer your version. You may want also to update the title to reflect your current needs. $\endgroup$
    – mins
    Commented Oct 29, 2021 at 15:45

1 Answer 1


The certificate does state such motive flow is available at a port of the FCU and it can be used to drive pumps, either jet (ejector) or turbine pumps. It doesn't say these pumps are already in place, but if they are installed, and if their design matches, they can be powered by tapping the related port.

This question: What is motive flow and why is it used in some aircraft hydraulic systems? explains both the motive flow and jet pump concepts and what are the main uses of jet pumps (e.g. fuel boost and scavenge pump).

So the only missing element is what is a turbine pump. It's a pump which is driven by a turbine. A turbine in general is a device converting fluid energy into rotational motion, like in a turboengine. Here the fluid would be fuel from the motive flow. It would turn the pump. The pump itself can be used to move a fluid, likely fuel, like in the jet pump.

The choice to drive pumps by the motive flow is to rely only on the engine for providing the energy. Once started, this system engine-pump is autonomous.

This is similar to using fuel as a power source for other elements in an engine, e.g. moving variable blades.


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