What defines the 'critical phases of flight' where non-essential communication is restricted?

The phrase 'critical phases of flight' refers specifically to segments of flight where heightened attention and focus are necessary to ensure safety and minimize risks. This definition encompasses all ground operations involving taxi, takeoff, landing, and any flight operations below 10,000 feet Above Ground Level (AGL). During these phases, the crew must maintain clear communication and vigilance, as any distractions could significantly impact the safety and outcomes of these operations.

During taxi, takeoff, and landing, the aircraft is in more vulnerable states, and the potential for hazards increases. Therefore, restricting non-essential communication helps to promote a focused environment for the flight crew, ensuring they can manage all critical functions with minimal distractions.

While some of the other options suggest various altitude thresholds or operation modes, they do not capture the comprehensive nature of what constitutes critical phases of flight. This is why the second choice is the most accurate, as it specifically addresses ground operations and flight aspects that pose greater risks and require heightened attention.