Re: True Air Speed?

Date:         01 Feb 99 02:36:49 
From:         Larry Stone <lstone@wwa.com>
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On 1/17/99 2:37 AM, Scott (scott@golfmike.demon.co.uk) said:
>Can anyone tell me how to work out TAS for IAS?
>What other info is required?

There's actually two conversions to be made. First is IAS (Indicated Air
Speed) to CAS (Calibrated Air Speed). This corrects for aircraft design
influences on the indicated speed through the range of speeds. This
correction tends to be small (no more than 2 or 3 knots max. in the
planes I fly) and a table or graph is published in the Pilot's Operating
Handbook (POH). There can also be a correction for instrument error but
since this is going to vary from plane to plane, you won't find it
pre-published in the POH.

Then there's the conversion from CAS to TAS (True Air Speed) which is a
function of pressure altitude and temperature. Unfortunately, I don't
have that formula. TAS=CAS as sea level at standard temperature (15C). As
you climb, CAS becomes less than TAS. A higher than standard temperature
increases the difference; a lower than standard temperature decreases the
difference.

Some examples (from my Beech Duchess manual) - note that these are IAS to
TAS since Beech has already incorporated the IAS to CAS conversion into
the the tables:
Press. Alt.       Temp.       IAS   TAS
Sea Level         Std.        152   152
Sea Level         Std. +20C   148   153
Sea Level         Std. -20C   156   151

8,000             Std.        146   164
8,000             Std. +20C   142   165
8,000             Std. -20C   150   163

16,000            Std.        120   153
16,000            Std. +20C   115   153
16,000            Std. -20C   124   152

-- Larry Stone
   lstone@wwa.com