I've always had a problem when the weather warms up at all. The gear won't
come down unless I crack open the dump valve for a second. I don't like doing
that but if I don't I can't get the pressure out of the low switch to start
the pump. I've tried adjusting the pressure switches a little each way to see
if it would help but it doesn't. Maybe a bypass relay to start the pump in the
down position would be the answer.
Randy Stuart
LNC2


[Randy, it almost sounds like you're fighting an internal pump issue.  If you've got the pressure switches set to their lowest possible settings to achieve consistently reliable operation at _some_ particular temperature, unless there is something restricting the internal flow there is almost no reason that temperature should be able to be removed from the equation (ie, temp increase = pressure increase, which should be dealt with already).  Isn't this one of the
symptoms we hear about so often with the reversed spool valve?  Greyhawk...???    <Marv>

Consider that if the spool/shuttle valve is stuck in some bad spot and the pressure is equally very high on both sides so that the dump valve cannot release the pressure, there may exist problems with the relief valves.  Besides that, some of us have symmetric spool valves and some of us have offset ones.  It is hard to install the symmetric ones backwards.

 

Peculiar pressure build ups associated with heat expansion may also be associated with the pump relief valves.  Mike Reinath claims that his relief valves operate differently when the pump runs.  He has adjusted his to be close to the pressure valve action.

 

The pump manufacturer claims that the relief valves should be set about 400 psi higher than the circuit requirement.  Theoretically, the manufacturer has set the relief valves for about 1000 (600 lo pressure) and 1500 (1100 hi pressure).  Also remember that pressure switches have a dead band - I.E. it may be triggered (to open) at the high side of its pressure setting and the pump may continue to run long enough to raise the pressure even more.  Then, the pressure switch may not close again until the pressure has dropped several hundred psi below its setting.  Or, maybe not.  Hydraulic logic is not precise in this application.

 

In my case, gear up operation frequently pauses before another blip finally tucks away everything nice and tight.  Why?  Because a spike opens the pressure switch prematurely and a few seconds later,  pressure has degraded enough to re-activate the switch.  A leak, you say?  No, because that ultimately sucked up gear will remain up tight for days whilst the plane is on jacks.  'Splain that Lucy?  Or, is it because that wild and crazy nose gear gas spring behaves differently at the end of a fast retract, then relaxes a bit after it is compressed for a few seconds and that causes a slight drop in the "hold it all up" pressure?  Is it my Karma?

 

Perhaps you should set your gear down pressure switch to a bit higher pressure.  Perhaps you should install a pressure switch bypass switch as some have done.  I dunno.  The current dump valve resolution doesn't seem too bad when the occasional problem occurs.

 

Grayhawk