In terms of flight restrictions, some airlines do have weight restrictions in place for heavy instruments like keyboards. Delta, for example, has a hard and fast rule that prohibits carrying on keyboards that weigh over 165 pounds. Some airlines even require you to purchase a seat specifically for your bulky instruments. In terms of the weight of a keyboard inside of a given case, this is more of a case-by-case (no pun intended) basis type of question that doesn’t have a blanket answer. Some cases won’t be able to support the weight of super heavy keyboards.