At what flight level does the FL180 not exist if the altimeter setting is below 29.92 inches?

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The reason FL180 does not exist when the altimeter setting is below 29.92 inches is primarily due to the relationship between pressure altitude and flight levels. Flight levels are determined based on a standard altimeter setting of 29.92 inches of mercury (Hg). At this setting, FL180 corresponds to an altitude of 18,000 feet.

However, if the altimeter setting falls below this standard atmospheric pressure, the actual altitude at which the aircraft operates will be higher than the nominal flight level. As such, under reduced atmospheric pressure, the altitude of FL180 effectively rises above 18,000 feet. Consequently, pilots and air traffic controllers must adjust their altitude reference to ensure safe vertical separation between aircraft.

In practical terms, this means that when the altimeter setting is below 29.92 inches, FL180 is no longer applicable in the sense that it may not align with the actual altitude where the aircraft is located, leading to the conclusion that FL180 effectively does not exist under those conditions.