View Full Version : Speed Error?

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?


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.


02-04-2007, 02:28 PM
Those numbers are for cruise altitude only! At 1500' MSL your max speed should be 250 kias...

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.

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

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.

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.


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?

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.

02-05-2007, 07:39 AM




02-05-2007, 12:59 PM
The basic technique for jet airliner operations is to climb at a fixed airspeed and then a fixed mach number. The numbers will vary by aircraft type and by airline.

Typical climb is at around 300 knots until at a height of approximately 28 to 30 thousand feet when the transition is made to mach number climb.

Local air traffic restrictions and particular airline procedures often limit the speed at lower levels (for example, the 250kt below 10000 ft limit in the USA).

An example (approximate numbers), climbing at 300kt indicated airspeed:

10,000 ft, 300 kt IAS, gives 345 kt TAS, mach 0.45
20,000 ft, 300 kt IAS, gives 400 kt TAS, mach 0.67
28,000 ft, 300 kt IAS, gives 470 kt TAS, mach 0.80
now climb at mach 0.80 - so
30,000 ft, mach 0.80, gives 290 kt IAS, 470 kt TAS
35,000 ft, mach 0.80, gives 260 kt IAS, 465 kt TAS

Maximum permitted indicated airspeeds for modern airliners are typically 350 to 370 kt. At low levels it is easy to exceed these speeds.

02-05-2007, 11:25 PM
so, is the mach speed indicated on jets and the AP the true mach speed? like when they say mach.80/447kts, is the .8 mach actually equal to 447kts or is that just what will be indicated? And do the jets measure mach the same way as knots?

02-06-2007, 05:16 AM
Mach number on the machmeter is always "true".

The speed of sound, and hence the true airspeed at a given mach number, will vary with temperature. My previous example assumed standard temperature. If the air at 35,000 feet is warmer than the standard -55deg C then M0.80 will be faster than 465kt and vice versa.

When a manufacturer quotes a cruise speed of M0.80/447kt he probably assumes full load economic cruise speed at the optimum height.

The subject is a little complex. Have a read of either an online encyclopedia, or better still your local library.

02-06-2007, 06:26 AM
There is not such thing as "true mach speed"

Mach is dependent on temperature.

Indicated airspeed (IAS) in dependent on pressure.

True airspeed (TAS) is indicated airspeed corrected for pressure altitude.

If you climb at a constant IAS, your TAS will increase: 250 IAS = 250 TAS at sea level; but 250 IAS = 300 kts TAS at 10,000 ft. Your mach speed will also increase

If you factor the wind into your TAS, you will get your groundspeed.

A 747 can fly at 0.8 mach at 35000 feet. This equates to a TAS of about 460 kts, but the airspeed indicator will read about 270.

If you tried to fly a 747 at 0.8 mach at 1000 feet, this will be a TAS of 527 kts. The airspeed indicator will show 525 kts, which is waaay faster than you can fly a 747 without bits falling off.

The transonic zone around Mach 1 generates shockwaves that break aircraft not designed to fly at the speed of sound.

So at any altitude, you have to fly slower than the never exceed speed, but also slower than the transonic zone. If you climb at a fixed indicated speed, your mach speed will increase until you reach a point where if you went any higher at that IAS you would enter the transonic zone, from then-on you climb at the mach speed. An eg might be 290/.75