N1234A has been assigned a descent clearance from 12,000 feet to cross MHZ VORTAC at 6,000 feet. When can N4567B be assigned 10,000 feet?

In air traffic control, ensuring safe vertical separation between aircraft is crucial. In this scenario, N1234A is descending from 12,000 feet to cross MHZ VORTAC at 6,000 feet, and N4567B needs to be assigned a descent to 10,000 feet.

The correct answer indicates that N4567B can be cleared to 10,000 feet when N1234A reports leaving 9,000 feet. This is because there needs to be a minimum vertical separation of 1,000 feet between aircraft to maintain safety, especially since both aircraft will be at altitudes near 10,000 feet. By the time N1234A descends past 9,000 feet, it has entered the safe buffer zone below N4567B, allowing the latter to safely climb to 10,000 feet without risks of conflicted airspace.

The other choices do not provide the same level of separation. For instance, if N4567B were cleared at 10,000 feet when N1234A reports leaving 10,000 feet, there would be a moment where both aircraft could be at the same altitude if N1234A had not yet initiated its descent. Similarly