What altitude must the altimeter be set to for a cross-country flight?

For a cross-country flight, the altimeter must be set to the altimeter setting at an airport within 100 nautical miles of the aircraft's current position. This practice ensures that pilots maintain accurate altitude information, which is critical for safe navigation, especially when transitioning through different airspace structures and elevations.

Setting the altimeter to an appropriate setting, which reflects local atmospheric pressure, helps pilots avoid unintended altitude deviations that could lead to obstacles or other air traffic. The 100 nautical mile guideline reflects a balance between being adequately close to a relevant pressure reference while still allowing the aircraft some distance from the departure or destination airport to accommodate for the flight's progression.

This concept is crucial for ensuring that the altimeter is adjusted for changing atmospheric conditions, particularly as the aircraft ascends and descends, hence promoting safety and navigation accuracy during the flight.