The user can change the device's orientation in real space at any moment and will likely expect the on-screen content to reflect this. AIR for iOS can detect these changes and automatically rotate the stage to match the device's new physical orientation.
This recipe will show you how to enable auto-orientation.
If you have completed the Setting the default aspect ratio recipe, then you can continue to work from its FLA. Alternatively, from the book's accompanying source code, open chapter8\recipe5\recipe.fla
into Flash Professional and work from there.
In either case, you will find a bitmap image of a fire truck sitting on the stage. The stage itself is sized to fit a landscape aspect ratio and the FLA's AIR for iOS settings reflect this.