What must be done after replacing the fuel control unit on an aircraft gas turbine engine?

After replacing the fuel control unit on an aircraft gas turbine engine, it is necessary to retrim the engine. This procedure involves adjusting the engine's fuel flow and other operational parameters to ensure that it operates efficiently and within its designed performance specifications. The fuel control unit plays a critical role in regulating the amount of fuel delivered to the engine, and any change in this unit can significantly affect the engine's performance.

Retrimming ensures that the new fuel control unit is properly calibrated for the specific engine characteristics and operational requirements. This is essential to maintain safe and reliable engine operation, as it helps to optimize fuel flow, prevent issues such as over-fueling or under-fueling, and ensure that the engine produces the intended power and efficiency.

The other options may seem relevant but do not directly address the immediate requirement following a fuel control unit replacement. For instance, reinstalling turbine blades is unrelated unless they were removed for the replacement process, and while calibrating the engine monitoring system can be important, it is not the primary step immediately after replacing the fuel control unit. Similarly, replacing the fuel pump would not be necessary unless there were indications of a failure or compatibility issue with the new fuel control unit.