Yes, that's one possible outcome if you don't have an agreement. And if no one cares about the plane beyond his 50%, and the partners agree on the value, that might be a fine outcome.
But what if your partner sells his half to some stranger. Or someone you know to be unsafe. Or your nemesis. Or a flight school. And just saying that if one wants to sell, you sell, doesn't solve the problem. What if your partner needs cash ASAP to pay his gambling debts, and will take 50 cents on the dollar to get it? Should he be able to force you to take a haircut? Should you be able to force him to hold out for top dollar? What if he just liens up his half, or croaks leaving medical bills, and you wind up with the bank or the bankruptcy trustee as your partner?
What if you just really like the plane, and can't replace it with your 50% of the cash but can't buy him out (isn't that why you have a partner?) so don't want to sell at all?
What if I told you there were well-known ways to avoid all of these issues?