Xml data not showing in my XML file

I call this method and result show in alert box correct but not show in xml file...

public function writeFile():void
            {
                var newXML:XML = <author>Bob</author>;
                var f:File = File.documentsDirectory.resolvePath("new_authors.xml");
                 var fout:FileStream = new FileStream();
                fout.open(f, FileMode.WRITE);

                var output:String = '<?xml version="1.0" encoding="utf-8"?>\n';
                output += newXML.toXMLString();

                fout.writeUTFBytes(output);

                Alert.show(output);

                fout.close();

            }