loss of hydraulic b system

[calvinlewington]

I have been chasing a hydraulic leak for a few weeks now. I am loosing up tp 40% of B system ,but only on flights greater than 5 1/2 hours. The plane can fly for weeks at a time and not loose a drop until a long enough flight will cause it to plummet. I have check tightened lines and changed a couple of suspect actuators,but still the problem continues.ANY suggested are welcome...

[sjap]

Are you sure you don't loose it anywhere externally?
Maybe at the Engine driven Pump, the hydr oil can be withdrawn from there into the air, and not be seen externally.
some text from Boeing:

"Thermal contraction during cruise causes an additional drop in reservoir level. Operators have reported 5 to 10% further drop in system reservoir level after some time spent in cruise, and Boeing's calculations indicate that thermal contraction after prolonged cruise at low temperatures could cause as much as 20% drop. "

But a 40% drop is very very much, and you have to refill it too?

Another possibilty, very strange i think but not impossible: a leak in your hydr heatexchanger inside the fueltank, causing casedrain fluid leaking into the fuel, and then be burned by the engine? But why only after 5 1/2 hours of flight? I dont know, anyone?

[calvinlewington]

Last refill I added 11 qts,on each occurence of this 8-14 qts have been lost. Nothing has been noted leaking around the edp but it is running a quite a high pressure (3180 at cruise) just within limits.

[sjap]

I asked around at work and discussed some issues.
Did you already find out what i was?
We still think cold soaking and associating leakage of seals can be the cause of the loss. Especially since all the slat actuators are on the B system maybe its leaking there. A consumer from one of the flight controls can be the cause as well, when there is no "rated internal leakage" in one of the actuators, they become frozen and can cause a leak.
Also if the B hydr reservoir overfill/overpressure relief is stucked/frozen open, lots of fluid can be drained thru the drainmast, leaving no evidence of hydr fluid loss. Check your reservoir pressurization module air-holes and openings. They are often corroded or clogged.
Is it a classic or a NG ?

Best regards
sjap

[calvinlewington]

The aircraft in question is an NG. On our last long haul there was no excessive fluid loss,and the only thing that had been changed was a MLG actuator that was found with a puddle under it prior to another (shorter flight) Cold soaking has always been my first thought, but I like your check valve theory and will look into that if the problem occurs again. I doubt very much that the mlg actuator was the source of my grief as pressure to that should be bypassed as soon as the pilot selects the gear to "off" after take off. Thanks for your input ,I will post further if I have any more problems

[airtech_59]

You can check,for example,LE slat actuators for fluid leakage.We had such a leakage on one LE slat RH actuator,only a lot of fliud leaked over the wing.Check the upper wing surface for fluid leakage.On the other acft a cause of leakage was LH hydraulic driven pump.The leakage was found during the engine motoring to find this leakage.Refilling was 7qts...