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 IAS To TAS
 Speedbird092 From Canada, joined Apr 2000, 169 posts, RR: 0Posted Fri Jul 27 2001 19:06:30 UTC (14 years 4 months 5 days 2 hours ago) and read 15524 times:

 Hey! Just a quick question, how can I calculate my true airspeed based on IAS, winds, etc Thanks Speedbird092
 DerekF From United Kingdom, joined Feb 2001, 919 posts, RR: 0 Reply 1, posted Fri Jul 27 2001 20:05:05 UTC (14 years 4 months 5 days 1 hour ago) and read 15489 times:

 You need to know several things like altitude, temperature and aircraft pressure errors. Basically TAS = EAS divided by the square root of atmospheric density ratios (hence using temp and altitude). EAS is derived from IAS by knowing the aircraft pressure erros to arrive at CAS the CAS to EAS by the scale altitude law or compressibility correction. If you need any more info let me know. Hope this helps! DerekF
 Whatever.......
 Ralgha From United States of America, joined Nov 1999, 1614 posts, RR: 5 Reply 2, posted Fri Jul 27 2001 22:48:47 UTC (14 years 4 months 4 days 23 hours ago) and read 15466 times:

 Or you could just use a flight compter  . Also, many airplanes have a TAS ring on their airspeed indicator that you can adjust for pressure altitude and temperature, you can then read your TAS right off the airspeed indicator.
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 XFSUgimpLB41X From United States of America, joined Aug 2000, 4294 posts, RR: 36 Reply 3, posted Fri Jul 27 2001 22:52:27 UTC (14 years 4 months 4 days 23 hours ago) and read 15463 times:

 Isn't it density altitude and not pressure altitude?
 Chicks dig winglets.
 N400QX From , joined Dec 1969, posts, RR: Reply 4, posted Fri Jul 27 2001 23:23:12 UTC (14 years 4 months 4 days 22 hours ago) and read 15460 times:

 Here's the equation I've got on my desk: TAS= [(IAS x 2%) • (ALT/1000)] + IAS So an example-- IAS= 300 at FL200, 300 • 0.02 • 20 + 300= 240TAS If I'm wrong, please correct me by all means.
 Bio15 From Colombia, joined Mar 2001, 1089 posts, RR: 7 Reply 5, posted Mon Jul 30 2001 18:46:52 UTC (14 years 4 months 2 days 3 hours ago) and read 15435 times:

 N400QX, at higher altitudes TAS is greater than IAS. But you got some of the math: IAS decreases by 2% every 1000ft you climb. IAS 280 at FL330 ---> [280 x (33 x 0.02)] + 280 = TAS 464.8 Winds affect ground speed mostly. -bio
 Max Power From , joined Dec 1969, posts, RR: Reply 6, posted Tue Jul 31 2001 02:38:19 UTC (14 years 4 months 1 day 19 hours ago) and read 15418 times:

 Get a E-6B or other circular slide rule and read the book. YUCK
 Dg_pilot From United States of America, joined Sep 1999, 856 posts, RR: 2 Reply 7, posted Tue Jul 31 2001 07:34:34 UTC (14 years 4 months 1 day 14 hours ago) and read 15411 times:

 (2% for every 1000 feet) X CAS, then plus CAS = TAS ==EXAMPLE== CAS: 90 kts Altitude: 6000 ft (6 x .02) x 90= 11 11 + 90= 101 kts TAS And XFSUgimpLB41X, it is pressure altitude I'm pretty sure...
 Sabenapilot From Belgium, joined Feb 2000, 2743 posts, RR: 45 Reply 8, posted Tue Jul 31 2001 16:02:51 UTC (14 years 4 months 1 day 5 hours ago) and read 15410 times:

 Here's the one and only simple thumbrule to convert IAS into TAS. (sadly only valid above FL100) TAS = IAS + half of your flight level To prove how accurate it is, I've used the same example as Bio15 so you can compare the results: IAS = 280kts. FL330 TAS = 280 + 165 = 445
 Jetpilot500 From United States of America, joined Nov 2000, 78 posts, RR: 0 Reply 9, posted Tue Jul 31 2001 17:41:25 UTC (14 years 4 months 1 day 4 hours ago) and read 15411 times:

 You are all providing interesting rules of thumb, but there are a lot of factors involved to come up with an accurate answer for all speeds and altitudes. As someone else mentioned, use an E6B. Here is the correct method found on this website of aviation formulas: http://www.best.com/~williams/avform.html Mach numbers, true vs calibrated airspeeds etc. Mach Number (M) = TAS/CS CS = sound speed= 38.967854*sqrt(T+273.15) where T is the OAT in celsius. TAS is true airspeed in knots. Because of compressibility, the measured IAT (indicated air temperature) is higher than the actual true OAT. Approximately: IAT=OAT+K*TAS^2/7592 The recovery factor K, depends on installation, and is usually in the range 0.95 to 1.0, but can be as low as 0.7. Temperatures are Celsius, TAS in knots. Also: OAT = (IAT + 273.15) / (1 + 0.2*K*M^2) - 273.15 The airspeed indicator measures the differential pressure, DP, between the pitot tube and the static port, the resulting indicated airspeed (IAS), when corrected for calibration and installation error is called "calibrated airspeed" (CAS). For low-speed (M<0.3) airplanes the true airspeed can be obtained from CAS and the density altitude, DA. TAS = CAS*(rho_0/rho)^0.5=CAS/(1-6.8755856*10^-6 * DA)^2.127940 (DA<36,089.24ft) Roughly, TAS increases by 1.5% per 1000ft. When compressibility is taken into account, the calculation of the TAS is more elaborate: DP=P_0*((1+0.2*(IAS/CS_0)^2)^3.5 -1) M=(5*( (DP/P+1)^(2/7) -1) )^0.5 TAS= M*CS P_0 is is (standard) sea-level pressure, CS_0 is the speed of sound at sea-level, CS is the speed of sound at altitude, and P is the pressure at altitude. These are given by earlier formulae: P_0= 29.92126 "Hg = 1013.25 mB = 2116.2166 lbs/ft^2 P= P_0*(1-6.8755856*10^-6*PA)^5.2558797, pressure altitude, PA<36,089.24ft CS= 38.967854*sqrt(T+273.15) where T is the (static/true) OAT in Celsius. CS_0=38.967854*sqrt(15+273.15)=661.4786 knots [Example: CAS=250 knots, PA=10000ft, IAT=2C, recovery factor=0.8 DP=29.92126*((1+0.2*(250/661.4786)^2)^3.5 -1)= 3.1001 " P=29.92126*(1-6.8755856*10^-6 *10000)^5.2558797= 20.577 " M= (5*( (3.1001/20.577 +1)^(2/7) -1) )^0.5= 0.4523 Mach OAT=(2+273.15)/(1 + 0.2*0.8*0.4523^2) - 273.15= -6.72C CS= 38.967854*sqrt(-6.7+273.15)=636.08 knots TAS=636.08*0.4523=287.7 knots] In the reverse direction, given Mach number M and pressure altitude PA, we can find the IAS with: x=(1-6.8755856e-6*PA)^5.2558797 ias=661.4786*(5*((1 + x*((1 + M^2/5)^3.5 - 1))^(2/7.) - 1))^0.5 Have fun trying to figure this out! JetPilot500
 Sabenapilot From Belgium, joined Feb 2000, 2743 posts, RR: 45 Reply 10, posted Tue Jul 31 2001 20:02:17 UTC (14 years 4 months 1 day 1 hour ago) and read 15401 times:

 All are correct (although I haven't really spend time checking them over...) However, the question was: how can I quickly get an idea of my TAS based on IAS? I don't think any of these formulas are helping you any further. -------------------- BTW, since you talked about it: here's a quick formula to find OAT from IAT: OAT = IAT - 20 times the speed in mach e.g.: indicated temp = -25°C M = .70 OAT = -25 - 14 = -39°C --------------------- And another very usefull notion. Machnumber equals distance travelled per minute. e.g.: At M.70 you travel about 7NM/minute. Ok, both might be off somewhat at extreme winds, speeds altitudes or temperatures, but they are more then accurate enough for flight follow-up and are often used in the cockpit of planes without FMS, like the B737-200. (I started on that one at Sabena...)
 N400QX From , joined Dec 1969, posts, RR: Reply 11, posted Wed Aug 1 2001 09:45:48 UTC (14 years 4 months 12 hours ago) and read 15389 times:

 OK... I understand now. Bio-- the reason the TAS in my math shows 240 is because of a typo... I believe I meant to put in 340. oops
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