FAA Powerplant Written Practice Test

What is typically turned off when trimming an engine?

1. All accessory bleed air

2. Main fuel supply

3. Cooling systems

4. Electrical circuits

The correct answer is: All accessory bleed air

When trimming an engine, it is typically important to turn off all accessory bleed air. Bleed air is the high-pressure air taken from the engine's compressor section, used for various purposes such as cabin pressurization and engine starting. Disabling the bleed air during the trimming process allows for a clearer assessment of the engine's performance and reduces the likelihood of disturbances that could affect measurements. This is particularly crucial when fine-tuning parameters or diagnosing issues, as any bleed air flow could mask or complicate the analysis of the engine's operational state. Turning off the main fuel supply could interrupt the engine's operation entirely, making it impossible to trim while the engine is running. Cooling systems are generally maintained during operation to manage engine temperatures and ensure safety, and electrical circuits often remain engaged for necessary monitoring and control functions. Thus, turning off all accessory bleed air is the best practice when it comes to engine trimming to ensure optimal performance evaluation.