During maintenance it is easy to move the leading edges (Slats and Kreuger flaps) without moving TE flaps (which is often not possible due to removed components). There is a standard procedure in the AMM (27-81-00) to de-activate the leading edges in the UP (retracted) position. This must always be carried out if this is necessary, e.g. for the engines, to work with thrust reversers opened. It is sometimes necessary to move the leading edges to EXTEND (halfway) or FULL EXTEND (entirely down).
This can be done by moving the LE control valve piston in the flap control box together with hydraulic pressure B available. However, the valve must be disconnected from the input rod, by removal of the attachment bolt.
-If the valve is moved fully in (inboard in the direction of the wheel well) then the leading edges will move to FULL UP (this is the de-activated situation as described in the AMM).
-If the valve is moved out (direction outboard) then the leading edges will move to FULL EXTEND.
-If the valve is moved halfway then the SLATS go to EXTEND. The Kreuger Flaps will fold out, since they have 2 positions only, up or entirely down.
Movement of the leading edges is now independent of the trailing edge flaps and the position of the flap-handle in the cockpit. Pay attention on the position of the flap-handle when switching on the B-pump! Always look to the INBOARD thrust reverser doors, if they are down.