After how many minutes of no radio communication should an aircraft's activity be considered possibly suspicious?

In air traffic control practices, a period of 5 minutes without communication from an aircraft is the standard threshold that raises a flag for potential suspicious activity. The 5-minute marker is based on the need for timely and responsive monitoring of aircraft operations, particularly in contexts where a lack of communication may indicate an emergency situation, equipment malfunction, or other abnormal scenarios.

This time frame allows controllers to balance the necessity of monitoring with the understanding that temporary communication losses can occur due to various reasons, such as radio failures, changes in altitude or location, or unintentional disconnects. If an aircraft does not respond within this 5-minute window, it prompts further investigation to ensure safety and security, allowing controllers to take appropriate measures, such as attempting re-establishment of communication or alerting other relevant authorities.

Considering the options, 2 minutes is generally too brief to conclude there is an issue, as occasional minor interruptions in communication are not uncommon. Similarly, 10 and 15 minutes are often viewed as excessive, potentially leading to unnecessary concern, as they allow too long a gap before action is prompted. Hence, the 5-minute threshold is a well-considered standard in aviation operations for ensuring safety while recognizing normal variations in communication.