you don't need ss to fly... social security is used for one thing and one thing only... social security insurance / retirement / disability

everything else is just a gimmick and used only to verify your identity (like your name or date of birth)
now two people can have same name and DOB so they need to add another security measure and it's SS

The government will not care who / how your number is used as long as it does not interfere with it's real purpose... social security insurance

Now when you work they NEED your SS number because your employeer and you are responsible for making payments to your social security insurance. Kind of like making a payment to any other insurance service.

Hope this helps...