"Michael 182" wrote in message
news:V3Bnb.36695$mZ5.183695@attbi_s54...

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?


Because there would be less than 1000 feet separation between aircraft at
17,000 MSL using 28.87 as an altimeter setting and aircraft at FL180 using
29.92. In this case, FL190 is also unavailable for the same reason.