When a person or corporation files for bankruptcy protection they *must* list all assets and liabilities as part of the filing. Assets are those things which the person or corporation owns. Liabilities is a listing of all of the money that is owed to others, (persons or businesses). United undoubtedly owes money to Air Willy. They also undoubtedly owe money to ACA, Sky West, their employees, caterers, fuel suppliers, Boeing, GE Capital, etc., etc.
All liabilties are to be declared in the bankruptcy pleadings because the court will add up all of the outstanding debts and will pay each of the debtors a pro-rated portion of the money owed to them when the bankruptcy is resolved. This keeps the person or corporation who is filing bankruptcy from playing favorites and paying those debtor who they like, while stiffing those debtors who they may not like. Generally, everybody who is owed money from United will get only part of the money owed to them, but at least the payments will be proportionately even.
The only exception to the pro-rata payments is the government. They get any taxes owed to them, straight off the top, at 100% fo the amount owed. It's the golden rule. He who has all the gold makes the rules.