overspeeds at cruising altitude

[BeukStylez03]

Ok, I have this problem with every single aircraft I use.  I finally get to her level cruise altitude...for example, I'm using a Cessna Citation II and she cruises at 35,000 feet at a max speed of about 420 mph...why is it that I can't get her even close to that without hearing my controls go crazy and seeing "overspeed" in the corner of the screen?  I just don't get it.

help?

[Ivan]

Use IAS instead of TAS

[BeukStylez03]

Use IAS instead of TAS

I have the IAS button engaged on the autopilot.  

[congo]

Use TAS.

Indicated airspeed shows as much slower than the True airspeed at altitude, because the air is not as dense up there, fewer air molecules are striking the pitot tube, ie. lower air pressure = lower reading on the ASI.

While cruising, change briefly back to True Airspeed for a look.

Structural limits on the aircraft are more closely related to Indicated Airspeed, not true airspeed, hence we retain the use of IAS, it provides us with an easy reference so we don't stress aircraft.

If you are setting the Indicated Airspeed hold to 500kts at altitude, then you are trying for supersonic, that's why you are getting speed warnings.

[OTTOL]

Use MACH!


Above FL310 you should be using Mach, for the purposes of ATC querries and Autopilot settings. At this Altitude, IAS AND TAS are for planning and reference ONLY! You may see magazines and Jane's books and manufacturer brochures expressing cruise speed in Knots(or MPH(especially in the case of the manufacturer!