http://ntrs.nasa.gov/archive/nasa/ca...9960052267.pdf
The DC10 has 3 mechanical slat actuation assemblys.
Each of these assemblys moves a set of slats via cables (big, heavy cables).
You have:
1) An inboard slat actuation assy located at the aircraft centerline which move the L/H & R/H slats inboard of #1 & #3 engine
2)Left outboard slat actuation assy which moves all slats outboard of #1 engine.
3)Right outboard slat actuation assy which moves all slats outboard of #3 engine
Each of these mechanical slat actuation assys use 2 hydraulic actuators to move the cable drum on these actuation assys which moves the cables which in turn extend and retract the slats (Douglas and their cables).
Normally the slats are extended and remain extended due to hydraulic pressure trapped in the hyd. lines between the actuators on the actuation assy and its respective control valve (one control valve per actuation assy for a total of 3).
With AA in ORD, the catastrophic engine separation severed the lines going to the L/H outboard slat actuation assy causing the airload to retract the L/H outboard slats.
To protect against this possibility from reoccurring, the right and left actuation assys have "balanced relief valve" assys installed. Now, should a loss of hydraulic pressure occur with the slats extended, the balanced relief valve will close locking the respective slats extended by trapping fluid in the actuator regardless of hydraulic line damage. These balanced relief valves are on both outboard slat actuation assys but not the inboard assy(located within the fuselage).
The MD11 has a mechanical locking mechanism as well as the aforementioned balanced relief valves for use in case of mechanical failure of the slat actuation mechanism.
On either aircraft, the slats should remain extended in the event of hydraulic failure.