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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?

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    $\begingroup$ Related: Did this aircraft illegally exceed 250kts below 10,000ft? $\endgroup$ – Bianfable Oct 8 '20 at 18:32
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    $\begingroup$ While no experton the Cessna CJ4, it makes sense to me that the cause would be due to the fact that it is an FAA regulation not to exceed 250 KIAS below 10,000 feet MSL. After all, you have stated that the airspeed indicator has Vne at 300 knots. $\endgroup$ – Dean F. Oct 8 '20 at 18:33
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    $\begingroup$ If you’re talking about MSFS 2020, I wouldn’t worry too much. There are way too many flight model errors in that game. $\endgroup$ – Carlo Felicione Oct 8 '20 at 19:29
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    $\begingroup$ @ob318 - Thanks for the feedback. This is a global website. And, aviation is a global endeavor. These facts do not in any way make the regulations of any civil aviation authority of any country irrelevant. Another poster’s insights regarding EASA or CAA regulations will enlighten us all and are just as valid if they wish to post them. Take into consideration my response to the question was posted as a comment and not an answer. Feel free to voice your own interpretation of the reasons for the overspeed warning. $\endgroup$ – Dean F. Nov 8 '20 at 1:16
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    $\begingroup$ @ob318 - Bear in mind that the questions’s original poster has a profile stating that he is from Virginia. The Cessna Citation CJ4 (Model 525C) was manufactured in Kansas. Its first flight was in Kansas. Its original Type Certification was issued by the FAA. Microsoft (the manufacturer of the flight simulator software) is headquartered in Washington. And, the use of feet and knots in the question somewhat (but not entirely) narrows the geographic location. These all make my rough guess not completely out of the ballpark. $\endgroup$ – Dean F. Nov 8 '20 at 1:29
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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? :-)

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  • $\begingroup$ Re travelling only below 10,000 FT MSL, maybe east of the Mississippi. Further west, that's not always such a good idea. Lots of mountains taller than that. FTM, there are a couple of airports where 10K ft isn't even pattern altitude: boldmethod.com/blog/lists/2014/08/… $\endgroup$ – jamesqf Oct 9 '20 at 4:24
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    $\begingroup$ Are you suggesting that the overspeed warning would come on at 250kt below 10,000ft in the real aircraft? I seriously doubt that. My guess is that this is a feature of the simulator. The warning system description says "Overspeed annunciations have no visual counterpart warnings. The voice or tone warnings are continous while the airspeed is above VMO/MMO on the copilot's airspeed indicator." There is nothing mentioned of 250kt below 10,000ft. $\endgroup$ – Bianfable Oct 9 '20 at 7:11
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    $\begingroup$ Agree with @Bianfable What if ATC instructed you to fly at 260 knots? You would just have to live with the warning? Cockpit warnings generally only relate to the limits of the aircraft, not regulatory limits (which can change at any time and are not always applicable) $\endgroup$ – expeditedescent Oct 9 '20 at 7:14
  • $\begingroup$ Airplanes departing U.S. west coast airports on trans-Pacific flights often get permission to exceed 250 kts if the fuel load requires the excess airspeed to achieve reasonable climb performance. I think the special airspeed approval is part of Clearance Delivery. Would be really annoying to have that warning go off. $\endgroup$ – Pete P. Oct 11 '20 at 8:40
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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.

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