Hi Pilou - are the faces you are extruding non-planar? If so then that could be your problem since non-planar faces don't have a single perpendicular direction to them unlike a planar face.

Or are you possibly using the "Set dir" option to set a specific extrusion direction?

Ordinarily if you have a set of planar faces and you do a multiple selection on them, by default they will extrude along their own separate surface normals like this:





If the faces are non-planar then it won't behave in the same way though - when you extrude a non-planar face it will try to do a best fit plane through the curves or surface edges and then decide which of the cplane's x y or z axes are closest to that and extrude them all in that direction.

So if your faces are not planar you will have to do them individually, with setting a custom direction on each one (or just do one and make an array of the result if there is symmetry). To get the behavior you were looking for originally you would need to have planar faces that are being extruded.

- Michael