What is the common procedure to follow when trimming an engine?

When trimming an engine, the procedure typically includes turning all accessory bleed air off. This is because accessory systems, including those that draw off bleed air, can introduce variables that may skew the engine performance readings and adjustments. By isolating these systems, technicians can focus on the engine's performance under more controlled conditions, ensuring that the trim adjustments are accurate and reflective of the engine's inherent characteristics without interference from auxiliary systems.

In a real-world scenario, if accessory systems remain operational during trimming, they might affect the engine's response, leading to misleading data or improper adjustments. This emphasizes the importance of maintaining a controlled environment while making precise modifications to engine performance parameters.