The "Complex" TAS calculation

Although TAS is not as useful for glider pilots, as IAS and GS, some recent posts seem to sugget that TAS is not a clear concept for many pilots. Some people even consider it a 'Sophisticated calculation', which is really astonishing.

The advent of GPSs with the easy read out of Ground Speed has led to further missunderstanding of what TAS really is or how it's calculated. There's nothing 'complex' about it, pilots have been doing it for decades, way before electronic cockpits, using a simple piece of plastic called a circular E6B computer (under 10 bucks in any pilot shop).

Simply slide the circular calculator so that in the TAS window your current Altitude is directly above the Outside temperature. The TAS can then be read on the outer scale, associated with the IAS in the inner scale.

Don't have an OAT (Outside Air Temperature) reading ? Try and get it over the radio from someone who does, or calculate it based on the temperature on the ground below you (stardard lapse rate).

There's also a rule of thumb to 'estimate' TAS without considering temperature, which is : Add 2% to the IAS for every 1000ft of altitude above sea level. Its just a rough estimate, and I don't recommend using it for wind calculations, since small innacuracies here can cause some crazy wind vectors.

From a Glider pilot's perspective, TAS is used mostly for calculating the Wind in flight, whether or not you have a GPS.

Modern gliders usually have a Thermometer in the dash somewhere, with the temperature sensor installed in some ventilation opening (out of direct sulinght). On-board flight computers such as ILEC, Zander, Cambridge, already come with the temperature sensor, so that they can calculate TAS by themselves, and thus know what the real glider's performance is, as well as what the real wind is.

Without TAS they can't calculate wind accurately, although some use Thermalling drift to update the wind vectors, which works well if you thermal a lot. Long cross-countries in mountain wave may involve very little circling, and that's when you need a better method.

How about instrument error, position error and ram air temp rise?

For the first two, consult the Pitot/static error chart or table in your glider's POH. That'll give you CAS (Calibrated Air Speed). The difference between IAS and CAS has to be less than 3% or 5kt in a certified aircraft. Typical installations are actually very accurate, within 1kt at typical flying speeds.

Hence, the use of IAS for calculating TAS is prfectly acceptable, in fact done everyday in thousands of cockpits around the world. 1 or 2kt of accuracy is usually more than what you can actually see in the instument face.

Ram air temp rise, and compressibility are not a factor for the speeds and altitudes we fly. Try a 1% error at 180Kt @ 30000ft.

But if you applied that correction to TAS, the result would be called EAS, for Equivalent Air Speed.