Posted for "Ian B. Crowe" <ian.crowe@sympatico.ca>:

I have just started flying my plane LNC2/360 under the 25 hour test period.  The other day we kept on getting an intermittent low frequency vibration that according to the engine gauges was not attributable to the engine or propellor.  It would occur and then quickly disappear after a second or two.  Eventually we realised that it was occuring in lockstep with the hydraulic pump cycling.  This was evidenced by the high pressure gauge returning to the pressure limiting switch pressure and then cutting off.  However what was interesting was that we were seeing 150psi or so on the low pressure gauge.

So, as evidenced by the high pressure gauge, we are losing high pressure and the gear doors were partially opening and causing the vibration we felt. That seemed to be the logical explanation. When the plane is in the hangar it will hold its low pressure over a number of days.  At no time have I ever found the gauge at zero after a weeks lay over, so there is little or no leakage from the low pressure side to the high.  It is still a mystery to me how the pressure can migrate from the high to the low if it will not do it the other way.  I know that there is a higher pressure present but for all that  there should be some leakage from the low to the high when the high pressure side is at zero.

Prior to this flight I had changed out the nose wheel hydraulic hoses.  The hoses supplied with the kit became too rigid under the low temperatures we have been seeing lately in Canada. Whilst the leakage was minor I felt it was an accident waiting to happen. Even though I had bled the system under pressure I still got some "burping" due to the low pressure side losing pressure.  I opened the emergency valve and ran the pump and this seemed to cure the problem as the burping stopped.  So before we took off the system was not exhibiting any irregularities.  I also performed 4 retractions and extensions with the aircraft on the jacks.

My opinion is that there is air in the system somewhere which is giving the problem as there had been no problem once the system had settled down after the initial charging.  Having said that I still feel that none of us really have put our fingers on what is going on.  Gauges help but the only way is to go over the system piece by piece and verify each piece. Maybe one day.

Regards

Ian Crowe


[Don't forget that there is linkage between the high and low sides also through each cylinder... the pistons are sealed to their shafts and their peripheries are sealed to the inner cylinder walls by o-rings and that leakage past those o-rings can also account for various and sundry 'weirdnesses' in the system.  I hope that air working your way out of your system turns out to be the culprit, but if it continues please don't forget about the cylinders.  <Marv>     ]