Emergency descent on autopilot - speed window management
When descending on autopilot the MCP speed window will transition from Mach to Vcas when passing through the Mmo/Vmo altitude. For the most rapid emergency descent the crew will have dialed the window up to Mmo. At the Mmo/Vmo altitude the speed window will transition from Mmo to a Vcas value that represents a nominal descent speed (less than Vmo). In order to continue a rapid descent profile, the crew will have to dial the speed window up to Vmo.