What does the red radial line on the engine RPM gauge indicate?

The red radial line on the engine RPM gauge indicates the maximum allowable RPM limit for the engine. This marking serves as a crucial reference point for pilots, as exceeding this limit can lead to engine damage or failure. Pilots are trained to monitor the engine RPM closely to ensure that they operate within acceptable parameters during flight. Abiding by this limit is vital for maintaining engine health and optimizing performance, ensuring safe operation of the King Air 350. The other options represent different RPM thresholds or ranges, but they do not indicate the critical maximum limit necessary for safe operation, which is clearly marked by the red radial line.