You can start Fly mode from the 3D Navigator by clicking the Fly Mode control 
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Application-Specific Information
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You can also press the F key to start fly mode.
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After you enter Fly mode, the 3D Navigator no longer displays. Use your mouse and keyboard to move around through the model.
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To exit Fly mode, right-click, press F again or press ESC. The cursor changes back to its original icon and the 3D Navigator displays.
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In Fly mode, your cursor changes to a crosshair that is located at the center of your view window.