"Frank Ch. Eigler" wrote in message news

"Michael 182" writes:

Was flying in the Denver area today and approach told a plane that FL 180
was unavailable die to low altimeter, which was 28.87 at the time. What is
the reason for this? Confusion with 29.92 readings?

FL180 is 18000 indicated feet with the altimeter set to 29.92. That
same airspace region is at 16950 ft indicated with the actual
altimeter setting. In order to keep traffic using the two altimeter
settings apart, FL180 is not used, nor is FL190 (17950) in this case.

To put it another way, flight levels don't exist below the transition altitude
(18,000 Feet MSL In the US). If FL180 would be at 16950, it's not available.