Performance differences between the aircraft types are natural and they inevitably cause problems in the air traffic system, particularly when slower aircraft types are followed by faster types in a climb or cruise queue. This paper presents the excess fuel consumption impact of aircraft performance differences in the air traffic environment. For this purpose, both climb and cruise phases of a flight mission are analyzed. Revision of the airworthiness regulations and air traffic procedures are suggested. The need for a flight level - cruise performance categorization is also recommended.