Class A airspace starts at 18000 MSL, not “above FL180″

Page 43: While the definition given in Chapter 3 is simple to understand, it’s not technically correct. Class A airspace starts at 18,000 feet MSL. [Note: This is why FL180 is sometimes unavailable. "Flight level" refers to what the altimeter reads (in hundreds of feet) when set to standard pressure (29.92). When ambient air pressure is lower than this, flight level 180 goes below 18,000 feet, thereby not providing 500-foot separation from VFR aircraft flying at 17,500.]