Possible Clues:

• Given instruction when on terra firma
• Not on the air, having been through the mill?
• Good, perhaps, but not in the air
• Good — well-developed and sensible
• Close to being plump, not allowed to fly
• Training finally complete in all aspects, but not allowed in cockpit
• Punished for the weekend, perhaps
• Fundamentally instructed to be confined to one's room
• Good description of a corporation perhaps, firmly established
• Sensible and well-balanced