Help with TAS inaccuracies

[simonflight]

Hi Guys,

I'm hoping some one can put me straight on how to read the True airspeed from the airspeed indicator.

As I understand it you rotate the knob on the ASI so that your current pressure altitude is aligned with the current outside air temperature. This rotates the TAS scale along the bottom edge so you can read off the True Airspeed ?

If I match a pressure altitude of 2000ft with an OAT of +20degC I can read that an IAS of 110knots gives a TAS of around 117knots. Comparing with an electronic E6B I get 115knots which is sort of close.

But when I read off an IAS of 130Knots it shows a TAS of 141knots but the E6B shows 135knots !

Is this the usual level of accuracy for these instruments or are there other factors to take into account ?

I always get this when flying with some errors greater than others depending on the altitude. But strangely enough I can fly with FSX weather or ASN and the computed GS on the F2 popup, GPS or E6B are always within agreement, give or take a knot or two.

Any ideas ?

Regards
Simon

[lonewulf47]

Simon,

The truth is always a bit hidden in the manuals . See the extract below from the C182T manual:



What you read from the ASI is IAS, to calculate the proper TAS value with your E6B you need to use what is called CAS (calibrated airspeed) which differs from IAS by the position error. So if e.g. you see 140 KIAS on your ASI you should use (according the above table) the corresponding CAS value of 136 CAS in your E6B. This will give you more accurate results.

Another good example to show what the TAS calculator in the ASI does:
Enter the standard atmosheric conditions for Sea Level (-> 15° C at Zero Altitude). Theoretically this should indicate equal values for IAS and TAS - but it doesn't. the reason being again the positon error.

[simonflight]

The truth is always a bit hidden in the manuals . See the extract below from the C182T manual:

Image

Hi Oscar,

Thanks for putting me straight, I new I was missing something. The numbers in your extract tie up with my current flight so I'd better find a copy of that manual.

Thanks once again

Regards
Simon

[chaders]

Enter the standard atmosheric conditions for Sea Level (-> 5° at Zero Altitude)..

Isn't ISA 15° at sea level??

[lonewulf47]

Enter the standard atmosheric conditions for Sea Level (-> 5° at Zero Altitude)..

Isn't ISA 15° at sea level??

Yes, of course. Sorry, that's what happens if one types too quick ... 15° C is correct. I corrected it on my initial post. Thanks for advising!