I'm flying the Cessna CJ4. I get overspeed warnings at 250kts when below 10,000ft, and the speed indicator has red lines at 300kts. Are these warnings triggered because it'll damage the plane to go that fast, or because it's illegal to go that fast that low? What's the distinction between 250kts and 300kts?
Most likely Cessna introduced a speed switch that is controlled by an aneroid. Anytime you are below 10,000 FT MSL, the aneroid activates that switch. If you go above 250 knots, the overspeed warning will sound.
As far as why that is the case, in the United States you are limited to a speed of 250 KIAS below 10,000 FT. I would imagine this is also the case for most other countries. The FAA realizes that most small airplanes with a much slower airspeed travel only below 10,000 FT MSL so the FAA is providing a safety margin to prevent in-flight collisions.
The structural limit of the airplane would not be a factor at 250 KIAS hence the red line at 300 KIAS. I would imagine 300 KIAS deals more with structural stability than anything else.
My question... can a slow-tation go that fast? :-)
As V(MO) is approached in a jet aircraft, the overspeed warning is activated to protect the structural integrity of the aircraft and ensure it remains inside of the acceptable structural envelope. Pilots need to react without hesitation to an overspeed warning - usually by reducing the throttles to flight idle and therefore bringing the speed outside of the warning envelope. As Dean F mentioned in his comment, the FAA prohibits flight over 250kt below 10,000 ft MSL as a measure for traffic control. That can only be exceeded when the controlling ATC agency issues a speed clearance, often in the form of a "high speed climb out approved" clearance. In addition, manufacturer guidance needs to be followed when approaching the high end of airspeed in jet aircraft as to how to recover from an overspeed or approaching V(MO)/V(NE) airspeed.
See https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/airplane_handbook/media/17_afh_ch15.pdf page 15-9 for more information.