 Class A airspace extends from mean sea level MSL to FL600 (approximately MSL) throughout the contiguous United States and Alaska.Unlike the altitude measurements used in other airspace classes, the FLnnn flight levels used in class A airspace are pressure altitudes referenced to a standardized altimeter setting of 29.92" Hg and thus the true altitudes depend on local atmospheric pressure variations.Unless otherwise authorized by ATC, all flight operations in class A airspace must be under ATC control, and must be operating IFR, under a clearance received prior to entry.