Glossary
What is Class A airspace?
Controlled airspace beginning at 18,000 feet MSL over the United States. Everything flown in it operates under instrument flight rules on an air traffic control clearance, so a pilot flying visually does not enter it at all.
That is the whole entry for now. The longer version, with the mechanics and worked examples, is still to be written. See autorotation for how those read once they exist.