What defines the top limit of the green arc on the airspeed indicator?

The top limit of the green arc on the airspeed indicator is defined by Vno, or maximum structural cruising speed. This is a critical airspeed because it represents the maximum speed at which the airplane can be safely flown in turbulent air. Flying above this speed increases the aerodynamic loads on the aircraft, which can lead to structural damage or failure.

The green arc itself indicates the normal operating range for the aircraft, and exceeding Vno means entering a speed category that could compromise safety, especially in adverse conditions. Therefore, the significance of Vno as the upper limit of the green arc lies in its role in maintaining structural integrity and ensuring safe maneuvering speed within the aircraft's operating envelope.

Other speeds, such as maximum flap extended speed or power-off stalling speed, are important for specific phases of flight but do not define the upper limit of the normal operating range indicated by the green arc. The never exceed speed is typically marked at the far right of the airspeed indicator and serves a different purpose, indicating a maximum limit that should never be exceeded under any circumstances.