Passengers should keep in mind when planning a trip that airlines do not guarantee their schedules. While airlines strive to get passengers to their destinations on time, there are numerous factors that can – and sometimes do – make this difficult. Some issues, such as bad weather, air traffic delays, and mechanical issues, are difficult to predict and frequently beyond the airlines' control. When united airlines delayed flight compensation are not required to compensate passengers. Only when certain passengers are "bumped" from an oversold flight is compensation required by US law.