What is the first important action when starting to move an aircraft while taxiing?

The first important action when starting to move an aircraft while taxiing is testing the brakes, as this ensures that the aircraft can be safely controlled during movement. The brakes are critical for the pilot to have immediate stopping capability to avoid potential collisions or unsafe situations on the taxiway or runway. By performing a brake check prior to taxiing, the pilot confirms that the braking system is fully operational. This verification is essential for ensuring safety and control, particularly in busy airport environments or when navigating tight spaces.

While testing the flaps, checking fuel levels, and setting the navigation system are important pre-flight tasks, they do not directly relate to the immediate safety and control of the aircraft during taxi operations. These actions can be addressed at different phases of pre-flight preparation, but ensuring the brakes function correctly stands out as a primary safety concern right before taxiing.