How must the Alerter be used under ATC?

The correct choice emphasizes the importance of adhering to established protocols and procedures during Air Traffic Control (ATC) operations. Using the Alerter as per pre-ATC operations signifies that it should be operated based on the guidelines and practices that were set prior to the introduction or implementation of ATC.

In this context, the Alerter serves to ensure that all personnel are familiar with its use, relying on consistent methods that promote safety and efficiency within air traffic management. This ensures that communication remains clear and expected behaviors remain standard, avoiding misunderstandings or delays in response to alerts.

The other options, while they may seem relevant, do not align as directly with the structured and regulated nature of ATC operations. Increased sensitivity might lead to an overwhelming number of alerts, which can distract from critical information. Similarly, using the Alerter at twice the previous frequency could overload communication channels, making it harder to discern important information. Lastly, limiting Alerter usage only when safety warnings arise might lead to a reactive rather than proactive approach, which is not conducive to the preventative measures required in aviation safety.