Poor flap "reserve" hydraulic pressure after master off

artairman's picture


When I first flew my LIV-P almost 25 years ago, after turning the master off, I could raise and lower the flaps 2 or 3 times before the pressure would bleed down enough to stop travel.

Over the last couple of years, however, that no longer happens--when I shut the master off, my flaps will lower if I put the handle down, but will not retract. I have replaced the "bomb" accumulator several times, without change. I have checked the flap linkage and there is no binding that I can find. The gear works perfectly.

My good friend and LNC2 driver Luke Alcorn has suggested that perhaps there is a sticking shuttle valve in the hydraulic pack (base upon the pack never having been opened or touched over the last 25 years. Does that sound like a winner? If so, how big a deal is it to remove that pack from a perfectly operating airplane (or can it be serviced in place)?



Another option is that

Another option is that hydraulic fluid may be bypassing between the ports in the manifold that contains the flap handle.

From memory (23 years ago) these ports have orings sealing the up and down ports as the rotary element attached to the handle move between each position.

I've had that valve apart due

I've had that valve apart due to some leaking/bypass where my hydraulic pressure would drain down fairly soon after powering down the system.  I found a tiny scratch in the drum.  I sanded it out with 1,000 grit sand paper and it helped a lot.......for a while.  Still hard to believe your losing pressure that fast.

I would be curious what your hydraulic pressure is doing.  I have gauges on my system to monitor and it's been a great tool for troubleshooting hydraulic issues.  Do the flaps work fine if you leave the power on?  If so, then when the flaps have traveled as far as they will after the master is shut off, what happens if you turn the master back on?  Do you hear your hydraulics building up pressure again like it would when the hydraulic system has no pressure?  Installing a gauge would be one of the best (IMHO) additions you could add to your plane.  Do you get a panel annunciation when your hydraulic pump kicks in?  If so, have you noticed it running very often in flight?

Bottom line, knowing what the system pressure is doing will narrow down the problem to something actually flap and controls related, or overall hydraulic system not maintaining pressure which could lead back to numerous other issues.  

Tom Sullivan

Hi Tom

Hi Tom

Curious to know where you attached the pressure gauges? You also have a drain valve to ease servicing?


I have two gauges.  One on

I have two gauges.  One on the up (for the gear) side of my hydraulics, one for the down side. As a lifetime mechanic, I can’t imagine operating one of these planes without that information at my disposal while flying. 


bknotts's picture

I had that problem.  Replaced

I had that problem.  Replaced the accumulator and that fixed it.  However, I needed to get a high pressure nitrogen source to pre-charge the accumulator.  My memory is that it took 600 psi to pre-charge the accumulator.  When you changed the accumulator did you pre-charge it?