I don't believe the UPS pilots have ever gotten a contract without threatening a strike. Under US law airlines fall under the Railway Labor Act. Collective Bargaining Agreements don't expire, they become amendable after a certain date. At that time management and labor start negotiating. Small things get settled but big items like compensation and scheduling remain. After a few years a Federal mediator is called in. Some more items get settled but eventually the mediator says the talks are at an impasse. Management can lock out the old employees and hire new staff and labor can strike. I think UPS is three years into the process of getting a new contract.