I was just wondering what the actual usage of these are?

I know that about the transponder... I'm more interested in what it's exactly used for though, and if some radars only pick up certain transponder codes or something.

And sorry, by TAS I mean True Air-Speed.


Thanks! 

Think of true airspeed like this.

You know that indicated airspeed is the speed you read from the airspeed indicator. for example 120 knots.

The airspeed indicator works by detecting the speed of air as it "blows" into the pitot tube.

You also know that as you increase altitude, air density decreases. That its... the air molecules spread out further apart from one another.

because of the reduction in air density, fewer tiny air molecules enter the "pitot tube" and therefore fewer air molecules "feed" the airspeed indicator.

This causes the airspeed indicator to show a speed which is slightly slower than how fast you are actually moving through the air.

The higher you climb, the more pronounced this error is.

"True airspeed" defined as "the aircraft's actual speed through an air mass" is what you come up with when you use various methods which correct for this "error" of fewer molecules entering the pitot tube.

Modern airliners and business jets use "Air Data Computers" which take inputs like altitude and airspeed and groundspeed etc and they are able to calculate corrections ofr you instantly. Even some small single engine aircraft have a calibrator on the airspeed indicator which can be turned manually to correct for this error given certain known variables like temperature and altitude.

However, the DC-3 or B-17 for example, use air driven gauges all the way up, and therefore the indicated airspeed will be slower and slower as you climb higher and higher.

for example. assume on a day with absolutely no wind... a B-17 at 10,000 feet might indicate 140 knots, while a B-17 at 23,000 feet might indicate 114 knots. the issue is, that neither B-17 is necessarily "outrunning" the other... because through the air mass, both planes are going the same "true airspeed" of say 143 knots.

"True airspeed" defined as "the aircraft's actual speed through an air mass" is what you come up with when you use various methods which correct for this "error" of fewer molecules entering the pitot tube.

in terms of time speed distance and fuel consumption problems, true airspeed is used as a "base number" for calculating ground speed, our ultimately important speed number.

Sweets