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

When operating an aircraft at 18,000 feet Mean Sea Level (MSL), the standard altimeter setting is 29.92 inches of mercury (Hg). This altitude marks the transition from the altimeter setting of local barometric pressure to the pressure setting of 29.92 Hg, which is used in the United States and many other countries as a standard reference point for aircraft operating above this altitude.

At 18,000 feet, all aircraft are required to switch to the standard altimeter setting to ensure uniformity in altitude reporting among all aircraft, especially in busy airspace. Using 29.92 inches of mercury allows for consistent and reliable altitude measurement, which is crucial for maintaining separation between aircraft flying at high altitudes.

Other options reflect different levels of atmospheric pressure that might be present at ground level, but they are not applicable at 18,000 feet where the reference standard is firmly established at 29.92 inches of mercury. This standardization helps prevent collisions and ensures safe and efficient air traffic management.