During which phase of flight must personal electronic devices be set to airplane mode?

Setting personal electronic devices to airplane mode during the taxi and critical phases of flight is crucial for ensuring the safety of the flight. During these phases, the aircraft is often operating in environments where communication signals could interfere with the aircraft’s navigational and communication systems.

Critical phases of flight typically include takeoff and landing, as well as taxiing on the runway and taxiways. This is when the flight crew is most focused on maintaining control of the aircraft and ensuring a safe operation. When devices are in airplane mode, they do not attempt to transmit signals to cell towers, thereby reducing the potential for interference with the aircraft’s instruments and systems.

While the other options may reference important aspects of electronic device usage during flights, they do not encompass the comprehensive requirement needed during these phases. For example, while cruising altitude may allow for the use of devices, it's after ensuring that they are initially set to airplane mode. Thus, it is critical to adhere to the guideline of setting devices to airplane mode during taxi and all critical phases to uphold safety protocols.