Minor gremlin in hydraulic flaps

[DXB]

'68C with hydraulic flaps.  I've posted on this before a few years ago - still haven't found a clear solution:

My flaps mostly work well - two full pumps for half flaps, four full pumps for full flaps.   The issue is that after putting in two pumps on the ground for takeoff flaps, the pump handle will sometimes SLOWLY come back up part way on its own, and the flaps will accordingly come up slightly.  It's now part of my final before takeoff flow to make sure the flap handle is all the way down along with pushing mixture to full and boost pump to on.  This problem never happens when the flaps are loaded by air pressure on the take off roll or in flight, so it's a pretty minor issue because they work perfectly in the air.  I assume there is a check valve in the system that is not getting enough pressure to close a the ball valve fully until there is enough pressure in the system - provided by air pressure on the flaps themselves.   At last annual, some fresh hydraulic fluid was flushed through the system with the thought that there was some gunk on the seat of the valve that needed to be cleared, but doing so made no difference. Supposedly the flap pump in the belly was rebuilt by Airmods in NJ in 2015 a year after I got the plane due to a small leak at the pump, but of course I can't confirm what they actually did because I wasn't there.  However the problem appeared a few months after that annual. I don't want to reflexively pull that pump again without a perfect idea of what's going on.  I suspect it could only be one discrete issue in the eyes of someone who really knows that hydraulic system?  The parts manual schematic of the hydraulic system is below.

 

 

 

[DCarlton]

2 hours ago, DXB said:

'68C with hydraulic flaps.  I've posted on this before a few years ago - still haven't found a clear solution:

My flaps mostly work well - two full pumps for half flaps, four full pumps for full flaps.   The issue is that after putting in two pumps on the ground for takeoff flaps, the pump handle will sometimes SLOWLY come back up part way on its own, and the flaps will accordingly come up slightly.  It's now part of my final before takeoff flow to make sure the flap handle is all the way down along with pushing mixture to full and boost pump to on.  This problem never happens when the flaps are loaded by air pressure on the take off roll or in flight, so it's a pretty minor issue because they work perfectly in the air.  I assume there is a check valve in the system that is not getting enough pressure to close a the ball valve fully until there is enough pressure in the system - provided by air pressure on the flaps themselves.   At last annual, some fresh hydraulic fluid was flushed through the system with the thought that there was some gunk on the seat of the valve that needed to be cleared, but doing so made no difference. Supposedly the flap pump in the belly was rebuilt by Airmods in NJ in 2015 a year after I got the plane due to a small leak at the pump, but of course I can't confirm what they actually did because I wasn't there.  However the problem appeared a few months after that annual. I don't want to reflexively pull that pump again without a perfect idea of what's going on.  I suspect it could only be one discrete issue in the eyes of someone who really knows that hydraulic system?  The parts manual schematic of the hydraulic system is below.

 

 

 

My '67F flap setting will creep up as well waiting to take off.  Been that way for a long time.  Interestingly, I've never noticed whether the handle comes up too.  I usually just give it a short pump if needed before takeoff.  It's part of my routine.  If you find a defect or repair, please let us know.  Thanks.  

[N201MKTurbo]

Restake ball 24 and your problems will go away.

[Shadrach]

I remember discussing this before but I don’t remember the part about the flaps retracting with the handle. Given that information, I believe it’s likely that the down stream (out flow) check valve is slowly leaking back into the the pump. The upstream (in flow) valve is holding so the only place for the fluid to go is into the cylinder which slowly pushes the piston as it fills the cylinderthereby raising the handle. I’ve labeled what I believe to be the offending valve with a yellow arrow in the image.

[N201MKTurbo]

1 minute ago, Shadrach said:

I remember discussing this before but I don’t remember the part about the flaps retracting with the handle. Given that information, I believe it’s likely that the down stream (out flow) check valve is slowly leaking back into the the pump. The upstream (in flow) valve is holding so the only place for the fluid to go is into the cylinder which slowly pushes the piston as it fills the cylinderthereby raising the handle. I’ve labeled what I believe to be the offending valve with a yellow arrow in the image.

But, it would have to leak past the 3/16 ball to get the pressure to the pump.

[Shadrach]

Just now, N201MKTurbo said:

But, it would have to leak past the 3/16 ball to get the pressure to the pump.

Are you referring to the valve in the return circuit? If so, I believe you are mistaken.

[Ragsf15e]

I hate to ask this, and I never want to have to open up my flap hydraulic system, but where are these parts from @Shadrach’s picture?  Under the floorboard?  Accessible from the belly panels?

[Shadrach]

Just now, Ragsf15e said:

I hate to ask this, and I never want to have to open up my flap hydraulic system, but where are these parts from @Shadrach’s picture?  Under the floorboard?  Accessible from the belly panels?

Yes. Will be second panel back from the rear of the nose wheel well if you have the stock belly.

[DCarlton]

Has anyone rebuilt or overhauled their hydraulic flap system successfully?  I’ve put up with my flaps slowly creeping up to avoid opening a can of worms that I couldn’t successfully close.  Do any of the MSCs do it routinely?  So far it’s just an inconvenience.  Thanks.  

[Shadrach]

30 minutes ago, DCarlton said:

Has anyone rebuilt or overhauled their hydraulic flap system successfully?  I’ve put up with my flaps slowly creeping up to avoid opening a can of worms that I couldn’t successfully close.  Do any of the MSCs do it routinely?  So far it’s just an inconvenience.  Thanks.  

I’ve done mine twice in last 20 years. Always has performed flawlessly. Just weeps fluid at the pump halves after a few years.

[47U]

16 minutes ago, DCarlton said:

Has anyone rebuilt or overhauled their hydraulic flap system successfully?

At my first annual (in 2010) everything in the flap system was leaking.  I overhauled the flap pump, actuator, and replaced all the hoses, which were still original, some 47 years old.  So far, so good, still holding tight.  

[N201MKTurbo]

2 hours ago, Shadrach said:

Are you referring to the valve in the return circuit? If so, I believe you are mistaken.

The steel ball on the aluminum seat holds all pressure applied to the flaps. The plastic seats are just involved in pumping fluid to that ball. If the flaps are going up and pushing the pump handle up, it has to get past that ball first. The described failure requires two valves to fail.

[Shadrach]

6 minutes ago, N201MKTurbo said:

The steel ball on the aluminum seat holds all pressure applied to the flaps. The plastic seats are just involved in pumping fluid to that ball. If the flaps are going up and pushing the pump handle up, it has to get past that ball first. The described failure requires two valves to fail.

I don’t have a pump in front of me at the moment. But my recollection is the the return circuit (3/16 ball) simply unports a machined pathway to the back side of the input check valve. 

[Rjfanjet]

I sent mine to LASAR for overhaul 2 years ago. No complaints.

[Bunti]

 My A+P removed the pump, cleaned everything and replaced all the O-rings and seals. The kit came from LASAR. Works great now.