Plain English Meaning
A warning on the autopilot panel telling the pilot that the climb-and-descend-at-a-set-rate function isn't working.
Full Definition
An autopilot annunciator message indicating that the vertical speed mode has failed or cannot be engaged, typically because the system has detected a fault, lost a required input signal, or cannot capture or hold the commanded rate of climb or descent.
Context Anchor
Seen on an autopilot or flight display when using, selecting, or monitoring vertical speed mode during instrument flight.
Why Pilots Care
The pilot must immediately take manual control or select another autopilot to prevent unintended changes.