The first aircraft emissions standards were proposed by the U.S. Environmental Protection Agency (EPA) in Proposed Standards for Control of Air Pollution, in 1972. Landing-takeoff cycles (LTOs), aircraft engine emissions factors, and times-in-mode (TIMs) were the basis of computing aircraft emissions. Discrete values were assigned to each TIM. No formal record of the derivation of these TIMs is available. Thirty years later, the same TIMs presented by EPA in 1972 are still used in developing aircraft emissions inventories. Today's aircraft perform much differently than did aircraft from the past. A study was attempted to validate the TIM values currently used with radar data. A sample emissions inventory will be developed for Chicago O'Hare International Airport to show the impact of the proposed changes of TIMs.