Fuel use in aviation is burdening the industry owing to its expense and worries about carbon dioxide emissions. With limited technological solutions for aircraft, small improvements in fuel efficiency are becoming increasingly important. Cost Index (CI) is a tool which controls the speed of an aircraft and therefore overall fuel use. Although the concept is simple, balancing fuel costs and time-dependent costs it is subject to misuse. This study aims to evaluate how the correct use of CI can affect fuel use and carbon emissions and assess the barriers and measures that are needed in finding the optimal CI value. A range of CI values are modelled for six different aircraft types to assess the difference in flight time and fuel use, before costs are modelled for a B767-300ER. Real aircraft data is used to validate results and provide an insight into the practical flight considerations that are required when using the CI. Results show that although changing CI values may only result in small changes in fuel use of around 1%, when this is applied over a route, an airline and the whole industry the fuel and carbon savings are significant. It is concluded that CI needs to be considered as an important tool and much better understanding of its use is needed along with factors, such as carbon pricing, which will affect its use in the future
