If N1234N is descending from 12,000 feet to 6,000 feet, when can N4567B at 14,000 feet be assigned 12,000 feet?

The correct answer is based on the concept of vertical separation between aircraft operating at different altitudes. In this scenario, N1234N is descending from 12,000 feet to 6,000 feet, and you want to assign N4567B, which is climbing from 14,000 feet, to 12,000 feet.

When N1234N reaches an altitude of 10,000 feet, N4567B can be assigned to 12,000 feet because there will be a minimum of 1,000 feet of vertical separation between the two aircraft. This is crucial for maintaining safety and compliance with FAA regulations regarding vertical separation standards.

However, the key moment is when N1234N reports leaving 12,000 feet. Until N1234N passes 12,000 feet, it occupies that airspace. Only after it reports leaving this altitude can N4567B safely be assigned 12,000 feet because at that moment N1234N has already moved out of that airspace and N4567B can enter it.

Therefore, waiting until N1234N reports leaving 12,000 feet ensures that the vertical separation required by FAA standards is maintained, allowing for the safe operations of both aircraft