My a/c has been in the hangar for at least two weeks whilst we worked on an oil cooler problem.

 

When we closed it up and were doing the preflight I noticed the "Down" pressure gauge was showing zero but the pump refused to run.  On closer inspection I realised the needle was pegged against the stop and was registering 1000psi ++

 

I opened the emergency gear extension cock a hair, bled the pressure down to 400psi and the pump kicked in and brought it back to 550psi.  Pressure switch works ok obviously.  The hangar is heated to about 55F.

 

We flew the a/c and the gear worked well, no problem.  Where this pressure comes from beats me as with the gear down the HP side of the system is at zero and the gear select is in the down position.  The safety switch I have will not allow the pump to run to retract the gear.

 

Curiouser and curiouser to quote Alice in Wonderland.

Ian,

 

Let's see, 'twas a full moon recently - high sun spot activity also - Hmmmmmm........

 

Consider this:  Say that the system is leak free, The nose wheel well is well sealed from the entry of any engine heat, the gear is extended and locked during approach after a nice cold flight on a cold day and there is no slop in the linkages (hi pressure is maintained against the overcenter links rather than the cylinder stops).  The plane is then moved into a heated hangar.  Now, let's suppose that the fiberglass structures, hydraulic lines (both hard and flexible), cylinders and fluid all expand in self-compensating ways such that they don't contribute much to any change in pressure.  Finally, how much did the three steel actuator rods expand in their temperature rise?  Was it enough to push back on the actuator pistons to raise the pressure?

 

This is why I suggest the down stop be set first with the actuators unattached to the linkages and the system pressurized - then each securing bolt is snugly fit thru the linkage and rod end.  No slop, but enough "play" to accept some pressure variance without putting unintended consequential force on the linkages and attach points.  Of course, then the up-stops have to be adjusted.

 

Electro-hydraulic systems were designed by the Mad Hatter - We just missed the party where their operation was described by the Queen of Hearts.  Curious indeed!