When must a turbofan engine be re-trimmed?

A turbofan engine must be re-trimmed primarily after significant changes to its fuel control system, if the engine is not developing maximum thrust, or if there is excessive throttle stagger. This process is essential to ensure that the engine operates efficiently and delivers the required performance.

Fuel control systems are critical for managing the fuel flow to the engine, and replacing this component can affect the engine's operation and performance characteristics. If the engine fails to develop its maximum thrust, it indicates a need for re-trimming to optimize the fuel delivery and correct any discrepancies. Additionally, excessive throttle stagger—where there are differences in engine responses—can impact the aircraft's performance and handling. Re-trimming helps to synchronize the engine's performance with the expected values, ensuring safety and operational effectiveness.

Other options such as performing the trim after oil changes, requiring it after major airframe modifications, or doing it as a routine check every 100 flight hours do not directly correlate with the operational performance markers of the engine. While these actions might be part of maintenance procedures, they do not specifically necessitate the re-trimming of a turbofan engine unless related to the fuel control or performance issues.