What altimeter setting should an aircraft use below 18,000 feet MSL?

An aircraft should use the current altimeter setting along its route from a station within 100 nautical miles below 18,000 feet MSL. This is essential for maintaining accurate altitude and separation from other aircraft. By using the local altimeter setting from a nearby station, pilots can ensure that their altitude readings correspond to the actual height of the aircraft above mean sea level, which is crucial for safe navigation and avoiding terrain and other obstacles.

While the standard pressure setting (29.92 inches of mercury) becomes applicable at and above 18,000 feet MSL, below this altitude, it's important for aircraft to use a local altimeter setting for precise navigation. This approach reflects the variations in atmospheric pressure that can occur at different locations, which would not be captured if everyone were using the standard setting. Additionally, the local airport elevation setting would only be relevant if within a specific airport environment, which doesn’t guarantee safe operation for en route flights.