What is the general practice when preparing to trim an engine?

When preparing to trim an engine, the general practice involves turning all accessory bleed air off. This is important because accessory bleed air can affect the engine's performance and fuel metering during the trimming process. Trimming usually involves fine-tuning the fuel flow to ensure optimal combustion and efficiency. By shutting off any sources of bleed air, you minimize variables that could lead to inaccurate readings or adjustments, enabling a more precise and controlled environment for the fuel system's adjustments. Ensuring that accessories do not interfere allows the technician to focus solely on the performance of the engine itself.

The other options, while related to engine management, do not represent a standard preparatory step for trimming. Adjusting fuel flow settings is a part of the trimming process, but it comes after ensuring that the engine is in the correct state, including the status of bleed air. Checking for leaks is always good practice but typically occurs as part of routine maintenance rather than specifically before trimming. Turning on all accessory bleed air would be counterproductive, as it would introduce additional factors that could disrupt the trimming process.