Final answer:
A transition altitude is where aircraft switch from local altimeter settings to a standard setting to ensure vertical separation. In the NAS, this altitude is consistently 18,000 feet.
Step-by-step explanation:
What is a Transition Altitude?
The transition altitude is a specific altitude in the field of aviation whereby aircraft must switch from using the altimeter setting referenced to sea level pressure (QNH) to the standard altimeter setting of 29.92 inches of mercury (1013.25 hPa), also known as standard pressure (QNE). This change is essential for maintaining vertical separation between aircraft as they ascend and descend through different air pressure environments.
Transition Altitude in the NAS
In the National Airspace System (NAS) of the United States, the transition altitude is uniformly set at 18,000 feet. This means that when an aircraft climbs through 18,000 feet, the pilot must adjust the altimeter to the standard pressure setting. Conversely, when descending below 18,000 feet, the pilot must change the altimeter back to the local pressure setting provided by Air Traffic Control.