What does a "lean" fuel mixture indicate in a turbine engine?

A "lean" fuel mixture in a turbine engine indicates that there is more air present than fuel in the combustion process. This situation arises when the air-to-fuel ratio is increased, allowing excess air to enter the combustion chamber relative to the amount of fuel being injected.

When a mixture is lean, it can lead to more efficient combustion because there is a better use of the available fuel with improved combustion temperatures and reduced emissions, especially in specific operating conditions. However, if the mixture becomes too lean, it may result in incomplete combustion, higher exhaust gas temperatures, and even the risk of engine damage.

It's important to maintain the proper air-to-fuel ratio to ensure optimal engine performance, as variations from this ideal ratio can lead to inefficiencies and potential operational issues.