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Roger Mazengarb

cruise speed at high altitude

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Recently I flew in an a340-600. It flew at FL400 at 565kts. To recreate the same speed I took a flight in FSX with default 744, but the simulator gave me overspeed warning when I crossed 370kts even when I was above 10000feet. Then when I started climbing through around FL250 towards FL400 the speed started to drop inspite of 100% throttle and I ended up being at 250kts at FL400. The speed didnt increase even when I leveled at the cruise altitude. The same was the speed of the cruising ai planes aswell. But this is not what should happen. Is something wrong with the simulator?

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565 kts @ FL400 is the GROUND SPEED, not the KIAS necessary for the aircraft to fly.The higher an aircraft flies, the greater the difference between the Indicated Air Speed in the cockpit and the ground speed.Check the ground speed in the GPS - doing 250 kts @ FL400 was pretty close to 575 kts, depending upon winds.(Yes, I know I'm avoiding the whole ground speed / KTAS discussion - on purpose - since it is clearly beyond the OP question/ experience)

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but the details in red that appear on top of the screen on pressing 'shift+z', do they also show the IAS and not TAS?And if such is the difference how is the pilot able to rely on the autopilot on maintaining the cruise speed?

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simply stated (with some generalizing) -the ground speed isn't a concern (exception ... see ATC comment below). aircraft try to fly the most efficient speed.groundspeed is also increased/decreased by the direction of the wind (jetstream). in USA aircraft heading east fly routes 'faster' than aircraft going west --- usually because the jetstream is going west to east giving the planes some extra knots of 'free' groundspeed. (this can be 100+ knots at times during the winter resulting in aircraft flying KLAX to KJFK, for example, arriving almost an hour early on the east coast).indicated airspeed is what the autopilot uses. jet aircraft begin to use mach at about 27000' (+ or -). mach is a better measure of speed when air density decreases.the actual speed set depends on airline SOP (which in this day of high oil costs is whatever is most fuel efficient) and the need of ATC (based on traffic).what is the most efficient airspeed (mach)? all depends on the aircraft, altitude, temp, length of trip, etc.--edit: clarifying statement--

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The numbers at the stop of the screen will show KIAS if you have that set in your aircraft realism settings.A pilot is most concerned with the KIAS number, the KTAS or ground speed number is secondary.KIAS is the speed of the wind flowing over the wings - and it must be within a certain range or the aircraft cannot fly. Too high, the aircraft will over stress.While real aircraft will not fold up after 60 seconds like FS - they will build continual over stress damage over time and become unsafe.If the winds are adverse, or the routing takes the pilot out of the way, he can speed up the aircraft only so much to try and maintain the schedule.If the winds are very favorable, and good routing, a pilot may slow down the aircraft to stay on schedule. Quite often at busy airports arriving very early can be as disruptive to the schedule as arriving very late.Any auto-throttle system in an autopilot is set on KIAS speeds, not KTAS cruise speed or ground speed.

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Compressibility effects make it impossible to use I.A.S. as a means of determinging the aircrafts speed at such values as you state.You should be using Mach number as the aircrafts speed reference.As previously noted the displays in the pax cabin are ground speed.Roger

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