14.4 What is the difference between "indicated" (IAS) and "true" (TAS) airspeed?

From FlightSim

Jump to: navigation, search

A lot of people seem to have problems with what they consider to be unrealistic over speed warnings. I think the problem is they're not aware of the difference between indicated and true airspeed. Not to get too complicated, but indicated air speed is what we fly by--the number on the air speed indicator. True airspeed is the indicated air speed corrected for temperature and pressure and is only useful for flight planning purposes and airplane salesmen. You can switch between the two in FS. With your airspeed indicator showing 1180 knots at FL580 it sounds like you've got true airspeed turned on for the air speed indicator. As an example, a 747 cruises at FL350 at about 500 knots (true airspeed) but the indicated airspeed will only be about 270 knots. In this case you might get an over speed warning at 300 knots and say "Wait a minute-- I know this thing can go faster. Lousy flight model!" If you really want to what your true airspeed is, level off and fly directly towards a VOR with no winds set. Your ground speed in this case will equal your true air speed.

MDK


I am with you all the way. But, the last two lines are not clear. As long as no winds are set, the ground speed will be equal to TAS anyway. Why the condition " fly directly to the VOR ". May be I am missing a practical point/tip over here. Can you please elaborate! And how can we know the ground speed in the air! Are you using "DME Dist/ Time flown " or some such thumb rule! as for as thumb rules go there is another one we could use to find out the TAS from IAS, which is :- 

        At 10000 ft ......TAS= 116%  of IAS
        At 20000 ft ......TAS= 137%  of IAS
        At 30000 ft ......TAS= 164%  of IAS
        At 40000 ft ......TAS= 200%  of IAS.

Of course all this is only if you have no provision for TAS indicator or you don't want to keep toggling between IAS and TAS......

Bala


I should've said that your TAS will be equal to the GS display for the VOR. The GS readout is only accurate when you're tracking directly towards or away from the station.

MDK


When tuned in to and flying toward a VOR, the "speed of closure" is shown on the HSI right under the nms reading. This speed is not the exact TAS since it includes the altitude leg of the triangle, but at large distances, 50-100 NMs, it is close enough for county work.

Bob Fiedler


Thanks Bob. I learned a new one today. I have not come across an indicator like what you have described but obviously it exists and rightly so it will be representative of the TAS.

Bala


Or in aircraft not equipped with the HSI, you can read the ground speed readout on the DME, if you're tracking straight to or from it.

Larry N.

Retrieved from "http://forums.flightsim.com/fswiki/index.php/14.4_What_is_the_difference_between_%22indicated%22_%28IAS%29_and_%22true%22_%28TAS%29_airspeed%3F"
Personal tools