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I was watching a refuel yesterday and I was wondering: what type of mechanism is in place, so that when the fuel hose is disconnected, no fuel spills out? (wondering since the hose is pointing upwards connecting under the wing). Some type of vacuum? What's this mechanism called?

Here's a picture to illustrate the situation I'm referring to:

refuel [skyapron.net]

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    $\begingroup$ Hello jlo, welcome to aviation.stackexchange.com. Could you please provide [image attribution] for the image in your question? $\endgroup$
    – DeltaLima
    Commented Nov 1, 2023 at 14:44
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    $\begingroup$ I wonder why I never wondered this. Great question, welcome! $\endgroup$ Commented Nov 1, 2023 at 17:39

1 Answer 1

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The system is based on two elements:

  • The underwing coupling is closed by a spring-loaded lid which can be pushed up to open the tank fuel line.

  • The refueling nozzle has a poppet valve which interrupts the fuel flow and when moved out allows the fuel to leave the nozzle, and pushes the tank coupling lid, allowing fuel to flow to the tank.

This is the general design and mode of operation:

enter image description here

Source.

Now if we focus on the nozzle:

enter image description here

Source (fuel exits at the bottom).

The poppet valve is at the level of the red annular bumper, and looks like this:

enter image description here

Source.

The poppet has number 15 below, and is resting on seal 43 until the nozzle is locked:

enter image description here

This ensures fuel cannot exit the nozzle prior the nozzle is locked, and after it is unlocked:

enter image description here

Source.

Handle 14 is used to lock the system and open the poppet with link 34 and shaft 32.


Another model:

enter image description here

Source.


In action

See that in action on this video. Note how the nozzle is first rotated with the ring handle after it is hung to the underwing intake, and then the smaller locking handle is operated to allow the fuel to flow. The same is done when disconnecting the nozzle, in reverse order.

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    $\begingroup$ If I understand the diagram, the poppet valve is in the filling nozzle. How does that keep fuel that might be above the poppet valve from being spilled when the nozzle is removed? $\endgroup$ Commented Nov 1, 2023 at 17:46
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    $\begingroup$ @WayneConrad: There is no space between the poppet in closed position and the wing coupling wall. The nozzle goes into the coupling and when the poppet is opened it pushes to open the tank fuel line. More visible on this diagram. $\endgroup$
    – mins
    Commented Nov 1, 2023 at 19:00
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    $\begingroup$ I think I understand: There's a spring-loaded valve in the wing that is pushed open by the filling nozzle, but closed before the filling nozzle is removed. And the design minimizes the space between the part of the nozzle that pushes the wing's valve open and the bottom of the wing's valve, leaving no place for fuel to be spilled from when the nozzle is removed. $\endgroup$ Commented Nov 1, 2023 at 19:03

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