From how far from the airport should the field altimeter setting be set?

The field altimeter setting should typically be set within 100 nautical miles of the airport. This distance allows pilots to receive an accurate altimeter setting that reflects the current atmospheric pressure at the airport, which is crucial for ensuring safe vertical separation and altitude reading during approaches, departures, and throughout flight.

Setting the altimeter accurately within this radius helps to account for changes in atmospheric conditions, which can vary over distance and lead to altimeter discrepancies if using outdated or irrelevant pressure settings. By using a reference within 100 nautical miles, pilots can significantly reduce the potential for issues related to altitude interpretation leading to altitude deviations, which could affect safety in operations especially in busy or complex airspace environments.

In contrast, the other distances—50 nautical miles, 75 nautical miles, and 150 nautical miles—do not provide the same level of accuracy and reliability as 100 nautical miles for setting an altimeter. The shorter distances may not accommodate sufficient atmospheric pressure changes, while the longer distance could lead to outdated pressure being utilized, both of which could impact flight safety.