Which agency primarily controls the altimeter settings in flight?

The primary control of altimeter settings in flight is managed by Air Traffic Control (ATC). This responsibility is part of ATC's broader function to ensure safe and orderly air traffic management. During flight, ATC provides pilots with current altimeter settings to account for local atmospheric pressure changes, which are crucial for maintaining accurate altitude readings and ensuring safe vertical separation between aircraft.

Pilots rely on the altimeter setting provided by ATC to calibrate their altimeters before takeoff and while in flight. This consistency is essential because it helps prevent discrepancies in altitude readings, especially when flying near busy air traffic or in varied weather conditions. By adhering to the altimeter information given by ATC, pilots can maintain safe altitude levels as they navigate through controlled airspace.