B738 Hydraulic Quantity Inflight
 

Last night flying at FL370, system A hydraulic quantity fell to 67%. When I first checked the hydraulics it was at about 75% and was falling 1% every 3-5 minutes.

I have never seen this before. Usually I check the hydraulics inflight and it is usual around the 90% mark at all times.

Once landed we checked the quantity which rose to around 90%.

I've looked in our FCOM and the only relevant info I can find was a cold soaked wing can cause fluctuations. Has anyone got any other information or anything from Boeing with inflight hydraulic levels for the B737?

Thanks in advance!

i once had the following:
a/c came from maint. air filter that filters the air for the hyd reservoir pressurization had been washed. but obviously not properly dried.
during flight filter froze, liquid started foaming, both pump low press lights came up. descended. back to o.k.

I know on the 737NG's there are issues with the seals on certain actuators (cant remember which ones) getting cold soaked during extreme cold weather/ longer flights resulting in fluid loss. Certainly from an engineering point of view during the winter months we are constantly topping up the hyds.... really noticeable compared with summer months, although i know this doesnt explain what you describe....

It's the fluid hiding of hydraulic system A and is perfectly normal. The landing gear is coupled to system A and much of the hydraulic fluid is trapped inside the gear system and cannot be monitored during flight.
That's why you saw quantity rise again after landing.

I hope that helps :)

What about the related qty sensor on the reservoir.could it be a snag.As a reservoir pressure loss would affect all three systems indications.
regds
MEL

Memo from Boeng
 

My company sent out a memo from boeing which I don't have to hand so can't get the exact figueres but stated that;
With gear retraction (system A fluid is now in the actuators) a fall of aprox "xxx" % can be expected.
Under certain conditions, especially long cruise below ISA (to be expected this time of year) thermal contraction of the fluid will result in a further decrease of up to "xxx" %.

Sorry no exact figures but I do recall that the quantity can get very low without being a non normal condition, however QTY's should obviously be checked after landing.

Hope this helps, perhaps someone out there has the same memo to hand to share the figures.