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RPM Flux

Hydraulic Problem or Not

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Got a utility hydraulic system that shows normal level until the engines are running. The level comes down quite a bit, several inches. However, when the engines are shutdown and system is un pressurized, the fluid level comes back to the same as before. Where is the hydraulic fluid going? What is causing this drastic drop in hyd fluid level?

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Hi I have had this before, more often in the booster system (which is easily tap to help seat when out in the field), but i beleive that you are looking at the primary check valve failing to seat correctly when the EDP\'s and suction boost pump are off allowing the hyd oil to drain back into the reservoir. From memory along the rear spar, fwd of the life raft bins, not so easy to tap.

Cheers Kevin B)

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Yep just sounds like bleed back through the priming check valve (24 hour check valve). Some bleed back is normal over time but if it fails completely you will have a huge mess on and in your floor:angry:

Now here is a real poser, many planes will show normal levels flying UNTIL you get up to altitude and the quantity will slowly go down - sometimes way down- until you descend and the quantity will come back up. Anybody?

Dan

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Well whisky man Dan. I wouldn\'t be waisting my time posting if I was kidding!! The FE said more than likely it may either be the one way check valve located near the life raft tubs or the resevoir vent filter has become saturated with hydraulic fluid. He didn\'t seem too concerned since no hydraulic fluid was being lost. I don\'t quite understand and I can\'t find anything in our books that talks about this, so I\'m asking an expert apinion from you older guys. Thanks.

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I have had lots of discussions with FE\'s about the vent line filter. I have had guys tell me that they can tell the moment that the filter has become contaminated with hyd oil, because during peak flow requirements the low pressure light (suction boost pump) will flicker due pump cavitation. Conversly I know lots of guys who would just clean out the drip tray and send her on her way, huge discussion points between aircrew and maintenance.

Regarding the oil level at altitude - cant say it\'s one that i have come across.

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I would also piggy back on the 24 hour check valve. It sounds like it might be slow acting to reseat after engine shut down allowing hydro in the wings to flow back to the reservoir. Unfortunately the Utility check valve is a little harder to get to. You may have to pull the inboard L/R tub, but I have seen others with skill get that thing out w/o taking the tub. Also, like Dan said earlier, when that thing fails totally then you\'ll have more than a pan full of Hydro.

One thing I\'ve seen from this board that the posters usually have legitimate questions. That is one of the reasons I like coming here because people are helpful and don\'t drop smart a$$ remarks. If you can help than give your two cents, if not then don\'t, it\'s that simple. My two cents, later on everyone!!

DaveB)

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A couple quick questions?

1st- After the hydraulic system sits for a day, where is the reservior sitting at?

2nd- What happen\'s to the reservior fluid level when you turn on the suction boost pump after the system sat for a while?

If your reservior is overserviced after the aircraft sat for a while and when you turn your suction boost pump on, and the fluid level drops your suction check valve in the wing is bad.

Is there anything else going on with the hydraulic system... i.e. having to keep servicing any accumulators? Air in the system can play H*ll on a hydraulic system.

I don\'t mean this to be silly, but have you checked the vent filter for contamination? A clogged filter can cause the reservior to pressurize and talk about the mess in the cargo compartment.

Eric

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Hydro brings up a good point...

If the utility system is serviced to capacity it would hold right at 20 gals total.... if the system is serviced correctly the reservoir would hold 6.5 gals....if the check valve was bad and the system is fully serviced the reservoir would over fill and make quite a mess.

So with that said, The check valve may be bad and the system not fully serviced. The fluid in the lines leading to the engine pumps is draining back into your reservoir making it look full.

(What is the time frame for the fluid level to return to normal???)

Another option is ...

Hyd fluid is not compressible for this discussion....

Air on the other hand is...if there is air in your system then it will compress, fluid will replace the area vacated by the air and the reservoir will go down... then with a bad check valve the fluid returns to the reservoir and everyone is happy cause it looks full..

The vent filter allows air to flow in and out as required... if it is install backwards it will be a check valve....if it is saturated it can act as a check valve and block the flow of air and pressurize the reservoir and once that happens you may see a drop in fluid level because now the pressurized air is displacing it...unscrew the vent line on top of the reservoir (before you start engines or suction boost pump) and see if the level stays somewhat normal... if it does you have an issue in the vent line if it drops when you turn on the suction boost pump then you are refilling the lines to your engine pumps and you have a bad check valve and your system is not serviced correctly....

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This issue has come up in our squadron once again. This time the FE serviced it in flight, told hydro that it may be over serviced after they landed. When day shift came in, they had a mess to clean up. It\'s gotten to the point we got MX Officers meeting with Operations Officers. I actually heard the MX Officer say that the pressure in the reservoir could have caused it to rupture damaging the airplane or personnel. Now, I have been flying for a long time and I have never heard this one. Any comments??:laugh:

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I haven\'t worked hydraulics for many moons, so forgive a bit of silliness - is it possible that the 9-port (ground check-out) valve is leaking through to the AUX system and temporarily \'hiding\' the fluid there? Even if the accumulators on the UTILITY system were not charged correctly, the amount of fluid being \'hidden\' doesn\'t make sense. Does this aircraft have a separate accumulator for emergency NOSE GEAR extension, off the UTILITY system? If so, is it charged properly?

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RPM Flux wrote:

This issue has come up in our squadron once again. This time the FE serviced it in flight, told hydro that it may be over serviced after they landed. When day shift came in, they had a mess to clean up. It\'s gotten to the point we got MX Officers meeting with Operations Officers. I actually heard the MX Officer say that the pressure in the reservoir could have caused it to rupture damaging the airplane or personnel. Now, I have been flying for a long time and I have never heard this one. Any comments??:laugh:

I have had a \"T\" model utility reservoir rupture while taxiing after maintenance cleaned out the drip pan from over servicing and I have had the top of the utility reservoir blow off after it also pressurized from the same situation but this time it was over serviced by an inflight mechanic that was riding along... So yes it can happen.

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Maybe I shouldn\'t be staying anything it\'s been 40 years but if you top off a system under pressure without knowing where the leak is don\'t that mean a clean up when the system is de-pressurized maybe I\'am wroung:unsure: :unsure:

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HerkPFE wrote:

RPM Flux wrote:

I have had a \"T\" model utility reservoir rupture while taxiing after maintenance cleaned out the drip pan from over servicing and I have had the top of the utility reservoir blow off after it also pressurized from the same situation but this time it was over serviced by an inflight mechanic that was riding along... So yes it can happen.

I forgot to add that he reservoir top that blew off was on an \"A\" Model. And yes, to service a system under pressure without knowing where the leak is taking a chance but the \'A\" models we were moving had some leakage but to top the reservoir up under pressure is bad.

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Thanks HerkPFE after Iwrote that it has been so long that I might have been wrong we had that happen one time at Sewart but it was on the ground. It\'s like putting brake fluid in when your brakes are wore out when you you put new pads on and reset the cyclliner all that fluid has to go somewhere:blush: :blush:

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\"If the utility system is serviced to capacity it would hold right at 20 gals total.... if the system is serviced correctly the reservoir would hold 6.5 gals....if the check valve was bad and the system is fully serviced the reservoir would over fill and make quite a mess.\"

So with that said, The check valve IS bad and your system was/is not fully serviced.

The fluid in the lines leading to the engine pumps was draining back into your reservoir making it look full.

I guess the time frame for the fluid level to return to normal is overnight..... (Priming check valve is bad!!!)

Sounds like your system was not serviced correctly due to the reservoir \"looking\" full due to the valve failure....

the bad check valve allows fluid to return to the reservoir and everyone is happy cause it looks full... your FE filled the reservoir with the engines running so everything was good and upon shutdown the failed check valve allowed fluid to drain back and caused the mess...

Change your Priming Check Valve and Reservoir vent filter, re-service the system and you should be good....

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Utility to Aux transfer is possible but it would not return to the utility system upon shutdown and the Aux would eventually be overserviced...

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Dan Wilson wrote:

Yep just sounds like bleed back through the priming check valve (24 hour check valve). Some bleed back is normal over time but if it fails completely you will have a huge mess on and in your floor:angry:

Now here is a real poser, many planes will show normal levels flying UNTIL you get up to altitude and the quantity will slowly go down - sometimes way down- until you descend and the quantity will come back up. Anybody?

Dan

You would have loved flying on the old whistling shithouse Dan. The hydro fluid was always playing “musical reservoirsâ€. Standard practice was after everything was cranked up and running the IO would mark the reservoirs (Utility and Aux) and check them often.

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