Passengers should be aware that airlines do not guarantee their schedules when making travel arrangements. Although airlines work hard to deliver passengers to their destinations on time, a number of factors can and frequently do so. It can be difficult to predict and frequently out of the airlines' control when certain problems arise, such as bad weather, air traffic delays, and mechanical failures. When Delayed Flight Compensation United, airlines are not required to compensate passengers. Only when certain passengers are "bumped" from an oversold flight is compensation required by US law.