What is the maximum APU ITT for starting the engine for 4 seconds?

The maximum APU ITT (Inter Turbine Temperature) for starting the engine for a duration of 4 seconds is 975°C. This temperature limit is critical for ensuring safe and reliable engine operation during the start sequence. Exceeding this limit during the brief startup phase could lead to thermal stresses that may compromise the integrity of the engine components.

The specified ITT value is derived from engine design and operational requirements to balance performance and safety. In the case of the Falcon 7X, adhering to this limit aids in preventing damaging effects like foreign object damage (FOD) and excessive wear on the turbine blades, which can occur at elevated temperatures.

It's essential to follow these operational limits carefully, as they are determined through rigorous testing and analysis to maintain optimal engine health and performance. While other temperature limits might exist, they pertain to different operating conditions or longer durations, thereby emphasizing the importance of context in interpreting engine parameters.