Both the plane and its contents experience the same acceleration due to gravity. Unless the plane is accelerating, the occupants will experience weightlessness

Both the plane and its contents experience the same acceleration due to gravity. Unless the plane is accelerating, the occupants will experience weightlessness (e.g. vomit comet). Of course the plane is accelerating - in straight and level flight this acceleration counteracts the acceleration due to gravity. This acceleration (lift) is provided by the wings, which don't move, so it is always in the same direction (in the plane's frame of reference). So if you kept flying upside down the tea would eventually fall, right? No. Eventually the plane would hit the ground, since it is not only falling but also generating "lift" in a downwards direction.