What altimeter setting is required for an aircraft operating at 18,000 feet MSL?

When an aircraft is operating at 18,000 feet Mean Sea Level (MSL), it is required to use a standard pressure setting of 29.92 inches of mercury (Hg) for altimetry purposes. This altitude is unique in the air traffic system as it represents the transition altitude, where aircraft switch from local altimeter settings to a standard setting. Below this altitude, pilots must set their altimeters to local air pressure, which can vary based on weather conditions.

At and above 18,000 feet MSL, all aircraft operating under instrument flight rules (IFR) must set their altimeters to 29.92" Hg to ensure uniformity across the airspace. This standardization is crucial for maintaining safe vertical separation between aircraft and mitigating the risk of collisions. By using a constant reference, air traffic control can effectively manage and provide guidance to multiple aircraft operating at high altitudes. Thus, utilizing the standard pressure setting is the correct approach for altimeter settings at that altitude.