What should be done with the parking brake before advancing the PCL at the EOR?

Before advancing the Power Control Lever (PCL) at the Engine Operating Range (EOR), it is crucial to set the parking brake. This action ensures that the aircraft remains stationary and secure, preventing any unintended movement that could compromise safety. Setting the parking brake provides stability during pre-flight checks and engine status verification.

In aviation, proper procedures are vital for operational safety, and ensuring that the parking brake is set is a key part of these procedures. This helps in maintaining control over the aircraft while performing critical engine checks. The other options focus on releasing or ensuring the brake is not set, which would not secure the aircraft effectively in that phase of operation.