Crab and Bravo thanks for reply on subject...
I just read that Hydraulic Lock is actually used to hold down undercarriage in some light fixed wing a/c, perhaps in some helis too. Anyone know of their type's u/c design?
I have just dug up the ole notes.. (the power of search that Bravo taught me
):
It seems most Power Control Units consist of "a balanced double acting actuator where the body of the actuator is attached to the flying control surface input and the ram is attached to the structure of the a/c".
The pilots selector is linked to a valve, moving the selector will open one side of the actuator to pressure, the other to return, thus assisting the movement. Things equalise in a follow up action (not sure exactly how)and the control therefore assists only.
However in the event of fluid being lost (I assume after accumulator exhaustion) an interconnecting valve drops to break the 'hydraulic lock', allowing the control to be moved manually.
Of course most bigger helis are not really controllable manually. But it is one thing to be dumped with manual control and it is another for the interconnecting valve or equivalent to fail and hold the controls in place = hydraulic lock.
Now of course this is just me digging out exam material of old which was mostly fixed wing related and says no more... So I wonder could people familiar with their type/hyd systems e.g AS350/365 dual system and As332s etc please volunteer more detail? For example how does a dual system work in practice. Thanks Crab for that...
Cheers