All airlines typically receive payments for flights in advance of when they actually operate the flight. Very often, the receipt is well in advance of the actual flight.
The money received goes into revenue. The equivalent value is then held on the balance sheet as a Liability. It is not held as an asset and it is therefore not "capital". The whole thing is a simple and basic business practice with well established accounting rules.
Great!
So nothing would change if those payments in advance were received and held by someone else until the service is provided? That would relieve airlines of the terrible burden of having to hold the amounts as a liability on their balance sheet.