When I set the material’s alpha,the opacity is wrong

If I // the alpha,it’s correct, i can also set alphaThreshold to make it looks ok.

Please see the attachments
The plane’s left is looks wrong(see away3d.jpg,away3d2.jpg),the plane’s right is looks correct(see away3d3.jpg).

But I need the alpha,I have no idea how to make it see correct!So I need someones’s help!Thank you!

I used with away3d-core-fp11_4_1_1_Beta
......
[Embed(source=“assets/movie.png”)]
var Face:Class;

var viewport:View3D = new View3D(scene,camera,null);
viewport.backgroundColor = 0xffffff;
addChild(viewport);
viewport.antiAlias = 4;
......
var fogMethod:FogMethod=new FogMethod(0, 2000, 0xffffff);
var pp:TextureMaterial=new TextureMaterial(Cast.bitmapTexture(Face))
pp.repeat=true
pp.bothSides=true
pp.alpha=0.98
pp.addMethod(fogMethod);

var _plane:Mesh = new Mesh(new PlaneGeometry(1000, 50,20,1), pp);
_plane.pickingCollider = PickingColliderType.AS3_FIRST_ENCOUNTERED;
_plane.mouseEnabled = true; 
_plane.geometry.scaleUV(20,1)
scene.addChild(_plane);
......