In the United States, the transition altitude is 18,000 ft. Flight levels, which are approximated altitudes based on standard air pressure, are only used above the transition altitude, as it simplifies operations at high altitude. However, under 18,000 ft, the actual altitude as read using the correct local air pressure is used instead, and expressed in feet, not flight levels. Therefore, an air controller would not ask an aircraft to descend to flight level 65, but rather ask to descend to 6500 ft. Correction: This statement is true for the United States, but not every country uses 18,000 ft as the transition altitude. Germany for example uses 5000 ft, and in Switzerland it's 7000 ft. So, issuing a clearance to FL 065 is correct depending on the country. This accident happened over Germany.
