kuobin
...Manual said it is not allowed to operate Flap between 1 and 5 during fuel jettison. Does Anybody know why.?....
I believe there is a requirement that, during fuel jettison, all the discharged fuel must be discharged clear of any part of the aircraft.
From memory, this question was asked on another aviation website some years ago.
One reply stated that during flight testing (of the slightly different wing on the B744 compared to earlier models) a transient disturbance in airflow was observed, as the flaps moved moved between 1 and 5, which drew some of the discharged fuel into contact with the wing trailing edge, hence the restriction.
Best Regards
Bellerophon