What must be retained for at least 30 minutes in a cockpit voice recorder?

The correct answer is that flight crew audio must be retained for at least 30 minutes in a cockpit voice recorder. This requirement is in place to ensure that critical communications between pilots and air traffic control, as well as any conversations between members of the flight crew, are preserved during a key portion of the flight. These recordings are vital for investigating incidents or accidents, as they provide insight into the decision-making process and situational awareness of the crew leading up to an event.

In general aviation operations, the ability to review the audio recordings helps enhance safety by identifying potential areas for improvement in flight crew performance and communication protocols. The retention is specifically designed to ensure that any critical information is available immediately following an event without being overwritten by subsequent recordings.

The other options do not have the same critical retention needs. Navigation data is important but serves a different purpose and is typically retained by avionics systems rather than voice recorders. Maintenance logs are documentation of aircraft condition and history but are not stored within a cockpit voice recorder. Cargo information is likewise crucial for operational reasons but does not pertain to cockpit voice recordings. Thus, the requirement for audio retention specifically highlights its significance in flight safety and incident investigation.