Hydraulic Flaps Won't Hold Pressure and Stay Down

[buckfever06]

All,

1965 M20C. My flaps will not hold pressure. Aerodynamic forces, or even spring pressure, will retract the flaps with the switch in the down position. My preferred aircraft mechanic is planning on looking at it in the next week. According to the maintenance logs, this issue has been addressed at least twice before. In the meantime, does anyone have any experience with this issue? Brakes work fine. Flaps extend and slowly retract. If I extend them prior to takeoff, they are fully retracted sometime between power application and  before I am off the ground (I don't watch them while I'm taking off.) If I put them down in the pattern, they are retracted almost immediately. If I can't get this fixed, how extensive/expensive it is to change to electric flaps? Thanks in advance for any insights!

[47U]

I’d suspect the retract cable needs cleaning and lubrication.  Pull the belly panel and make sure the flap retract lever inputs sufficient travel to the arm on the flap pump.

[tony]

The flaps and brakes are two different systems that share a reservoir.  After making sure you have enough hydraulic fluid then I would suspect that the flap valve is leaking and needs to be rebuilt.   The first place I would go,  is to call Dan at LASAR to see if they rebuild them.  

Edited May 3, 2022 by tony

[M20F]

51 minutes ago, tony said:

The flaps and brakes are two different systems that share a reservoir.  After making sure you have enough hydraulic fluid then I would suspect that the flap valve is leaking and needs to be rebuilt.   The first place I would go,  is to call Dan at LASAR to see if they rebuild them.  

This is the most likely suspect.  Easiest way to verify is the reservoir which will be low from leaking. 

Note the 2 springs look very much the same but are different (one weak and one strong).  They need to go back the same way.

[Fly Boomer]

1 hour ago, M20F said:

Note the 2 springs look very much the same but are different (one weak and one strong).  They need to go back the same way.

No idea how much work is required to get to those but, if that's the problem, it sure would be an easy fix.

[N201MKTurbo]

Take the pump out, put it in a vice and smack the ball that lets the flaps go up real hard with a punch. It will re-coin the valve seat and fix your problem. There are no parts that will fix your problem, just this procedure.

[Andy95W]

As others have noted, your flap system just needs a little work to get it right.  The only problem is that there aren’t many people left around that really understand how to do it.  @Shadrach is one, @N201MKTurbo is another.

Basically, it comes down to just a few possibilities:

-leaking ball seat(s) in the pump (Rich mentioned this, above)

-leaking o-ring at the actuator (this would be accompanied by hydraulic leakage back by the flaps)

-cable for the on/off lever not rigged correctly

Edited May 3, 2022 by Andy95W

[steingar]

My mechanic is a real fix it guy.  When this happened to me he took the put apart and fixed it somehow.  I don't recall what he did, but he said it took a week and he was pretty chuffed about it.

[Shadrach]

4 hours ago, tony said:

The flaps and brakes are two different systems that share a reservoir.  After making sure you have enough hydraulic fluid then I would suspect that the flap valve is leaking and needs to be rebuilt.   The first place I would go,  is to call Dan at LASAR to see if they rebuild them.  

They share a common reservoir but A leak in one does not affect the operation of the other.

[Shadrach]

I would start with the the up/down cable first. It operates a cam that actuates a plunger that unseats the ball on the return circuit. Disconnect the cable from the armature and ensure the cam is not actuating the plunger (See attached image). Pump the flaps down. If the stay down, the cable needs to be adjusted/indexed upon reassembly. If that fails, then there is likely a check valve issue that will need to be addressed. Dirt and or a valve seat issue is the likely cause. If the fluid is decades old, gelling can cause problems. The pump will require disassembly to correct these issues. Unfortunately, new problems can be introduced when performing a rebuild. If your local MX pro is not familiar with the system, it might be cheaper and easier to send it out to Maxwell for a rebuild.

@Andy95W has covered all of the possibilities. However, if the malfunction is caused by a leak at the actuator, you would have an obvious mess and the fluid level would need constant replenishment. You may find small leaks that need to be addressed but are not the actual cause of the malfunction. 
 

Edited May 3, 2022 by Shadrach

[Rjfanjet]

On mine a spring actually broke inside the pump. When my mechanic and I removed the pump, we could see pieces in the bucket we drained the fluid into. I sent it to Lasar for a rebuild and haven't had any issues since.

[carusoam]

Go MS!

Could be as easy as a cable adjustment…

Could be as complex as OHing the pump…

First challenge… the important stuff is hiding behind the belly panels…

 

Just remember…

The steel balls like to get away….

Think defensively while taking the pump apart…

 

 

There are some parts to acquire… O-rings and a teflon seal?

If your hydraulic fluid has leaked into the belly… it could be seals… or the low pressure hoses…

Old hoses get brittle and do a fair amount of leaking before failure…

 

The flap system stops working before the brakes do….

There are a lot of small details that Ross has covered nicely… around here somewhere…

My M20C leaked its hydraulic fluid just before annual one year… it got low pressure hoses and seals to get it working properly again…

PP thoughts only, not a mechanic…

Best regards,

-a-

[buckfever06]

Thank you to everyone who replied! I really appreciate all the info!

If I can find anything of value to post about the outcome I will do so.