I ran into another Lancairian at OSH that had the problem of occasional Hydraulic system lock-up with the pressure so high on both sides of the system that neither gear-up, gear-down or dump valve opening would solve the problem.  Of course, the relater will remain nameless as he must be classified as a lurker.  His solution was to wire a switch across the up pressure switch in order to short it out momentarily to get the system moving.

 

Now, this problem manifests itself most heinously by not allowing the gear to be retracted.  Let's consider how the condition can or cannot be reached:

 

0. The system is "perfect" with no apparent leaks. 

 

1. There is a leak in an actuator at the end of a down operation so that just before the pressure builds enough, the other side is also pressurized and both pressure switches are opened - there is no place for the pressure to bleed off.  But that should not be possible because the down side needs no more than 500 or 600 psi while the up side needs about 1100 or 1200 psi to tuck everything away.

 

2. The pressure switches are in backwards (hi on the down, lo on the up), the system would work and #1 occurs.

 

3. The pressure switches are poorly adjusted and #1 occurs.

 

4. In addition to #3, the relief valves are incorrectly set.

 

Hmmmm...  I currently have a minor leak on the down side (perfect seal on the up) and have never experienced this problem.  I think I'll just leave everything the way it is now.  I have become tired of finding leaking actuators, rebuilding or replacing them and adjusting (or replacing) pressure switches.

 

Let's remember that up pressure should be about 1100+ psi and the relief valve set at about 1500 psi.  The down side should be at about 600 psi and the relief valve at about 1000 psi.  The lockup condition should never occur because of properly adjusted pressure switches and relief valves. 

 

5. The infamous Shuttle Valve allows some fluid to pressurize both sides at the end of the down operation.  Just like #1 and the follow on.

 

I believe that upping the up-side pressure switch and/or lowering the down-side pressure switch should solve the problem in a perfectly sealed system.  That is, if the relief valves are also properly set.  Then, if there is a problem, a flick of the dump valve should solve it (allow pressure to drain from one side to the other.  Remember that it only takes about 500-600 psi to actually raise the gear, but about 1100-1200 psi to retain everything tightly.  It should take little pressure to extend the gear and hold the over center locks since even a free fall should accomplish that.

 

I have never heard of this happening the other way around - if so, the emergency gear extension could not take place.

 

Another solution could be to install a small reachable petcock on the up side line that drains outside the plane.  Just momentarily pop it when the problem occurs and forget all the pressure settings and other adjustments.