A330 Green hydraulic quantity increase on landing gear retraction
 

Hello guys,

Does anyone know the technical reason for the green hydraulic quantity increase on landing gear retraction?

“HYD : Fluid quantity. After landing gear retraction, the indicated fluid quantity of the green
system is higher than on ground.” PRONORSOP CRUISE

I would have thought when the gear was up the green fluid quantity would decrease as some fluid is trapped holding the gear up?

Not rated on the A330 at all, but a look at the retract mechanism suggests the retraction actuators are 'closed' when the gear is up, and 'extended' when the gear is down. As they are presumably double-acting hydraulic cylinders, the fluid displaced by the volume of the ram inside the cylinder itself is likely responsible for the increase in quantity in flight, notwithstanding the actual system quantity has not changed.

Something else I noticed, as regards the gear shortening mechanism - if this operates by displacing hydraulic fluid from the oleo, then that could also be partly responsible for the increase in indicated quantity, as it has to go somewhere...

As KRviator has explained, the fluid is displaced from the jacks and needs somewhere to go. In the case of the 737, fluid level actually goes down upon gear retraction.

Hydraulic double acting rams displace a dissimilar quantity of oil when opening and closing. The reason is that when expanding the whole diameter of the cylinder is filled with oil. When retracting the oil goes between the shaft and the outer diameter. The difference being the volume of the shaft.

This also means that the opening force can be a lot more than the closing force.

Thank you for your answers, I think in this case a picture would paint a thousand words. Does anyone have a pic?
Cheers

Check out the A330 FCOM Landing Gear section, on page 3 it shows the extended and retracted views.

As regards the why, this image below shows it at its' most basic. Here we have a double-acting hydraulic ram. The shaft itself displaces a certain amount of fluid when it is 'retracted'. The A330 uses the retraction actuator extended when the gear is lowered, or in the case of this image, pushing left-to-right.

To retract the gear, the shaft would move right-to-left, but in doing so, even though the ram has the same internal volume on both sides, the shaft itself displaces hydraulic fluid, as a function of its' diameter and how far it retracts into the cylinder. It's likely this displaced fluid is the cause of the increase in indicated quantity with the gear retracted.
http://www.jwhydraulic.net/images/do...ylinder-b1.jpg

Fantastic thank you very much that's very clear :)

One A330 MLG retract actuator expels 2.11 Litres of fluid upon landing gear retraction, because they retract by pressurising the annular (rod end) chamber only, the full area chamber is expelled to return. The dissimilar area rod volume is the reason for the fluid level increase.

On retraction the green sys tank reservoir therefore increases by 4.22 litres (fluid expelled from MLG actuators) and whatever the NLG actuator volume is.

It decreases by the same amount on extension, when the selector valve pressurises both chambers of the actuator at the same time, re-filling the rod volume as the differential area forces the actuator to extend (with a little help from the weight of the gear)

Any other 'unbalanced' actuators, or accumulators will cause reservoir fluid level fluctuations too, like spoilers, thrust resersers, but landing gear is the largest producer of 'delta volume' that the reservoir designer has to contend with.

Primary flight controls don't change the reservoir volume when actuated because they are designed to be 'balanced' (equal area on both sides of the piston, normally because the rod sticks out both ends)

If you're in the business of designing reservoirs, the spec for the process of accounting for all the level fluctuations is defined in AS5586.

Hope that helps?

Actually the more clever design is an actuator which contains more hydraulic fluid with the extended landing gear, like on the A330. In case of a hydraulic line blockage, it is easier to suck (cavitate) than to expell fluid into a closed tube (hydraulic lock). So gravity extension even works with a fully blocked hydraulic system.
You of course have the risk that somebody overfills the reservoir on ground, and it overflows when you retract the gear.

But the fluid on the "retract" side of the actuator still has to be expelled somewhere. I can't figure out how gravity could do that if the relevant line is fully blocked.

The line will not be blocked if you have selected 'MLG Down'.

The Mk1 Airbus, (A300) has the opposite effect when gear is retracted. The Green contents decreases.

Er, The Oleo is in no way connected to the hydraulic system. They are sealed cylinders partially filled with a completely different type of fluid.