What is the maximum distance from the airport that the field altimeter setting should be set?

The field altimeter setting should generally be set to the local altimeter setting within a maximum distance of 100 nautical miles from the airport. This is crucial for ensuring accurate altitude readings, especially during approach and departure phases of flight. Within this radius, the local atmospheric pressure can significantly influence altitude measurements, and maintaining an accurate altimeter setting is essential for safe navigation and obstacle clearance.

As pilots operate within this 100 nautical mile range, they are typically able to receive updated weather information and altimeter settings from air traffic control or automated weather reporting stations. Beyond this distance, the influence of local weather changes diminishes, and the altimeter setting can safely transition to standard pressure settings (29.92 inches of mercury), which are used at higher altitudes where the atmospheric pressure has less variability from local conditions.

This systematic approach to setting the altimeter helps mitigate the risk of altitude discrepancies, which can be especially critical in mountainous or congested airspace areas, thus promoting overall flight safety.