The pressure altitude (flight level) at which the IAS (indicated air speed) climb schedule ends and the climb Mach number schedule starts. For example, an aircraft whose climbing speed is 288 knots/0.8 M will climb at 288 knots until the Mach number at this speed is 0.8. This will be reached at flight level 310 (31,000 ft). The aircraft will now climb at 0.8 Mach instead of 288 knots, and flight level 310 becomes the changeover level. This level will remain constant irrespective of the outside air temperature.
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved