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The altitude up to which a given manifold air pressure (MAP) set can be maintained at a given RPM. The lower the manifold air pressure selected by the pilot, the greater is the altitude to which it can be maintained. In turbocharged engines, if an aircraft is at its rated altitude and still climbing, the butterfly valve of the carburetor will be fully open at its rated boost, which will begin to fall. At a lower boost, the butterfly will still be opening through the action of the aneroid capsule, and the pressure will be maintained until the butterfly can open no further at full-throttle height for that particular boost and RPM. Also called critical altitude (for a specified MAP).
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved