What N1 percent must be reached for aural tones to be audible during engine start?

The correct answer is based on the established thresholds for aural tones during engine start in specific aircraft protocols. For aural tones to be clearly audible, the N1 (fan speed) must reach a certain percentage. In this context, 50% N1 is the threshold that ensures that the sound emitted by alerts can be properly heard and distinguished by the crew.

At this level, the engine noise and other ambient sounds become sufficient to allow engine start warnings or alerts to be perceptible. Below this percentage, the noise levels may overpower the alert sounds, making it difficult for the crew to receive important notifications regarding engine performance or safety.

Therefore, achieving 50% N1 is crucial for ensuring that alert systems are effective during the critical phase of engine start, thus enhancing safety and facilitating better monitoring of the engine's status.