REFLECTOR: Hydraulic system pressure

[Scott Baker]

Hello John,
The recommended gear DOWN hydraulic pressure of 550psi is already 
significantly lower than the recommended gear UP pressure (something in 
the neighborhood of 1150psi).
While true, the main landing gear does not benefit from hydraulic 
pressure when in the gear DOWN and LOCKED position - the nose gear 
linkage is aided by hydraulic pressure to keep it in the over-center 
(Down/Locked) position.
Somewhere in the Reflector archives is a video focusing on the nose gear 
mono shock at the moment of landing.  When the nose wheel touches the 
runway, the top of the nose strut wants to move rearward - and in fact, 
it does move a small distance, which is counteracted by the mono-shock.  
Without the benefit of hydraulic pressure to help keep the nose gear 
linkage in the over-center position, the nose gear could easily collapse 
at the nose wheel touches down.
So how much gear down hydraulic pressure is "enough" to keep the nose 
gear linkage in its overcenter position?  Dono.  Again the factory 
recommends 550psi - which is not a lot of pressure for a hydraulic ram 
to handle.
My thoughts.
Scott B.

On 9/5/2013 7:43 AM, John Dibble wrote:
> I checked and found that the main cylinder is leaking at the forward 
> end which is under pressure when the gear is down.  Hoping to delay 
> the repair for cooler weather, I have been able to stop the leak by 
> pulling the breaker once the gear is down and then cracking the dump 
> valve to relieve pressure once on the ground. Now I'm wondering why 
> the pressure needs to be so high?  The gear will come down by itself 
> with the dump valve open.  The pressure has no benefit in keeping the 
> main gear down.  When the nose gear is over center, pressure from the 
> nose strut actually pulls on the cylinder rather than pushes, so the 
> hydraulic pressure has no benefit there either.  If the linkage 
> somehow got under center, then the nose gear would push on the 
> cylinder.  However, with the dump valve closed and little or no 
> pressure from the pump, the hydraulic fluid, being non-compressible, 
> would resist compression from the gear.  So I'm thinking that 
> significantly reducing the hydraulic pressure in the down position 
> could greatly extend the seal life.  Comments appreciated.
>
> John
>
> On 7/17/2013 2:38 PM, Scott Baker wrote:
>> Hello John,
>> Unless your aircraft has something other than standard factory 
>> equipment, the speed brake is operated by an electric linear actuator 
>> (no hydraulics).
>> The speed brake actuator opening in the floor of the aircraft is 
>> serving as an exit place for a leak in the hydraulic system.
>> Remove the keel cover(s) and keel inspection plates - grab a 
>> flashlight, mirror, and clean rag - and begin searching for puddles 
>> and drips.
>> If you are lucky, it will be a loose hose fitting.  If the cause is 
>> bad cylinder seal, it is reasonably easy to fix.  This will likely 
>> involve removing the cylinder from the aircraft for service.
>> Good luck!
>> Scott B.
>>
>> On 7/17/2013 2:52 PM, John Dibble wrote:
>>> A small amount of hydraulic fluid is leaking at the back side of my 
>>> speed brake.  Looking on the inside it seems to originate where the 
>>> "cylinder" connects to the brake.  Does the speed brake use 
>>> hydraulic fluid?  If so, does it come from the retract system, or is 
>>> it separate?
>>>
>>> Thanks,
>>>
>>> John
>>> _______________________________________________
>
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