After a period in which a plane has been parked for a longer time without hydraulic pressure with slats and leading edge flaps in the extend position, it is possible that the LE flap/slat system is hydraulically locked by means of a closed retract fuse. This is a known phenomenon, causes by pressure differences in the actuators resulting in a leakage of hydraulic fluid along sticky seals. As a solution Boeing indicated that the fuse could be reset after waiting a while (according to the task 10 minutes, in reality frequently much longer), whereupon the LE flaps could be extended with stby power and then be retracted in the normal way. It is also possible to reset the fuse by loosening the lines on the fuse. On the boeing flight line, where the problem also frequently occurs, they have a much faster and very simple method for the fuse reset. THE TE flaps must be, after removing all hydraulic power, electrically cycled, which results in the resetting of the LE fuse through the leading edge control valve.
Release by alternate cycle:
Step by step:
1 – Switch off hydraulic systems A and B
2 – Disarm the hydraulic standby systems by pulling c.b’s P6-11: stdby hyd pump normal and P6-12: stbdy hyd pump alt.
3 – Switch ALT FLAPS switch in ARM position
4 – Make a complete flap cycle with the toggle switch (pay attention to maximum operating time of the elec. motor)
5 – Switch ALT FLAPS switch in OFF position
6 – Push in c.b’s stby pump norm. and alt.
7 – Switch on hydraulic system A and B. (Note: the flaps position must agree with the flap handle position.)
8 – Make a complete flap cycle, using normal hydraulic power
9 – If after these steps a slat is still stucked, repeat the whole procedure.