Speed Error?

**Cessnaedo** · 02-04-2007, 01:09 PM

I looked in the learning center for the cruising speed of the airbus A321, and it said .80 mach and 447 knots. I went into a free flight to test that. I flow at 1500 MSL so alltitude wouldn't be a factor in the IAS. The aircraft went over speed at about 345 IAS. Also i tryed putting .80 mach into the mach part of the AP and it said that was 500+ knots. I noticed problems like this in FS9. Is that learning center wrong or is there something wrong with the overspeed thing. What can i do to fix this?

thanks,
Erik

**JerryH** · 02-04-2007, 02:27 PM

Hi Erik,

The short answer is that everything is okay.

The learning center value of 447 knots at Mach 0.80 is knots TRUE airspeed (KTAS) at cruising altitude. The speed of sound is less at altitude than at sea level, so 500+ KTAS sounds right for Mach 0.80 at 1500 MSL.

The overspeed thing has nothing to do with True airspeed. Overspeed is concerned about excessive pressure on the airframe and control surfaces, for example. And 345 KIAS is not surprising.

It takes time to fully understand some of these things. Your aircraft operates in a pressure environment. It doesn't know how high it is or how fast it is going. It only feels the air pressure that provides the lift and produces the drag. At high altitude, it may actually be going like a bat out of hell (KTAS), but because the air is less dense it will feel only moderate pressure (KIAS).

This is only a brief intro to a complex subject. Hope it helped.

Regards,
JerryH

***n4gix*** · 02-04-2007, 02:28 PM

Those numbers are for cruise altitude only! At 1500' MSL your max speed should be 250 kias...

**Cessnaedo** · 02-04-2007, 03:00 PM

Thanks Jerry,

So what your saying is that at high alltitudes while that indicated airspeed may be only 300 KIAS, the true airspeed is more like 447 KTAS.

**NetJets_G200** · 02-04-2007, 04:49 PM

The performance specs of a jet are not based on flying it at sea level, they are numbers that you'll get at cruise, and where does a jet cruise (not at 1500 MSL).

Rule of thumb, TAS = IAS + 2% for each 1000' above sea level

At 30,000' 300 IAS * 1.6 = 480 KTAS

**NetJets_G200** · 02-04-2007, 05:00 PM

Some countries don't have that 250 below 10,000' speed limit like the U.S. does, so it's as fast as you want to go anytime, anywhere.

**JerryH** · 02-04-2007, 05:45 PM

That is correct, Erik. You can check those values by turning on the GPS. GPS may only display ground speed, which is true airspeed (KTAS) plus or minus the wind component. With no wind, ground speed and KTAS are identical.

The other responders also had some good comments.

Regards,
JerryH

**Cessnaedo** · 02-04-2007, 09:34 PM

Thanks for the replies. That tip from netjets is very usefull.
Do you have any sources where I could learn more about this?

**angels355** · 02-05-2007, 01:41 AM

If you have a good library in your neighborhood, describe what you want to learn about to the reference librarian and he/she should be able to help you out.