Sponsor Message:
Aviation Technical / Operations Forum
My Starred Topics | Profile | New Topic | Forum Index | Help | Search 
IAS To TAS  
User currently offlineSpeedbird092 From Canada, joined Apr 2000, 169 posts, RR: 0
Posted (13 years 2 months 6 hours ago) and read 11159 times:

Hey!

Just a quick question, how can I calculate my true airspeed based on IAS, winds, etc

Thanks
Speedbird092

11 replies: All unread, jump to last
 
User currently offlineDerekF From United Kingdom, joined Feb 2001, 912 posts, RR: 0
Reply 1, posted (13 years 2 months 5 hours ago) and read 11124 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.......
User currently offlineRalgha From United States of America, joined Nov 1999, 1614 posts, RR: 6
Reply 2, posted (13 years 2 months 2 hours ago) and read 11101 times:

Or you could just use a flight compter  Big thumbs up. 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.

 Big thumbs up



09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0
User currently offlineXFSUgimpLB41X From United States of America, joined Aug 2000, 4200 posts, RR: 37
Reply 3, posted (13 years 2 months 2 hours ago) and read 11098 times:

Isn't it density altitude and not pressure altitude?


Chicks dig winglets.
User currently offlineN400QX From , joined Dec 1969, posts, RR:
Reply 4, posted (13 years 2 months 2 hours ago) and read 11095 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.


User currently offlineBio15 From Colombia, joined Mar 2001, 1089 posts, RR: 7
Reply 5, posted (13 years 1 month 3 weeks 6 days 6 hours ago) and read 11070 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


User currently offlineMax Power From , joined Dec 1969, posts, RR:
Reply 6, posted (13 years 1 month 3 weeks 5 days 22 hours ago) and read 11053 times:

Get a E-6B or other circular slide rule and read the book. YUCK

User currently offlineDg_pilot From United States of America, joined Sep 1999, 856 posts, RR: 2
Reply 7, posted (13 years 1 month 3 weeks 5 days 17 hours ago) and read 11046 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...


User currently offlineSabenapilot From Belgium, joined Feb 2000, 2714 posts, RR: 46
Reply 8, posted (13 years 1 month 3 weeks 5 days 9 hours ago) and read 11045 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



User currently offlineJetpilot500 From United States of America, joined Nov 2000, 78 posts, RR: 0
Reply 9, posted (13 years 1 month 3 weeks 5 days 7 hours ago) and read 11046 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


User currently offlineSabenapilot From Belgium, joined Feb 2000, 2714 posts, RR: 46
Reply 10, posted (13 years 1 month 3 weeks 5 days 5 hours ago) and read 11036 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...)


User currently offlineN400QX From , joined Dec 1969, posts, RR:
Reply 11, posted (13 years 1 month 3 weeks 4 days 15 hours ago) and read 11024 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  Big grin


Top Of Page
Forum Index

Reply To This Topic IAS To TAS
Username:
No username? Sign up now!
Password: 


Forgot Password? Be reminded.
Remember me on this computer (uses cookies)
  • Tech/Ops related posts only!
  • Not Tech/Ops related? Use the other forums
  • No adverts of any kind. This includes web pages.
  • No hostile language or criticizing of others.
  • Do not post copyright protected material.
  • Use relevant and describing topics.
  • Check if your post already been discussed.
  • Check your spelling!
  • DETAILED RULES
Add Images Add SmiliesPosting Help

Please check your spelling (press "Check Spelling" above)


Similar topics:More similar topics...
Convert From IAS To Mach posted Fri Nov 9 2001 03:30:40 by Wardialer
Calculating TAS From IAS posted Thu Oct 20 2005 23:45:32 by Bio15
TAS And IAS Changes With Altitude. posted Tue Sep 14 2004 11:04:47 by GunFighter 6
TAS To CAS Question posted Fri Nov 9 2001 23:23:58 by Nicolaki
RE. TAS And IAS Post posted Tue Mar 27 2001 20:31:30 by Top Gun
Oasis Hongkong: Permission To Use Russian Airspace posted Fri Dec 8 2006 22:07:23 by A300605R
Boeing Equivalent To Take Off Flex posted Wed Dec 6 2006 23:53:21 by Aak777
Calculating Ground Speed To Obtain ROC? posted Wed Dec 6 2006 14:56:12 by Wardialer
Which Way To Go? posted Tue Dec 5 2006 21:56:33 by CFTOA
Where To Get Selcal Encoders/transmitters? posted Tue Dec 5 2006 18:15:02 by RobK
Calculating TAS From IAS posted Thu Oct 20 2005 23:45:32 by Bio15
TAS And IAS Changes With Altitude. posted Tue Sep 14 2004 11:04:47 by GunFighter 6
TAS To CAS Question posted Fri Nov 9 2001 23:23:58 by Nicolaki
RE. TAS And IAS Post posted Tue Mar 27 2001 20:31:30 by Top Gun
How Long Does It Take To Pay An Airliner Off? posted Wed Mar 26 2014 07:52:23 by ChrisNH
Boeing Updates Acap Sheets To Include 788 posted Sat Mar 22 2014 11:31:16 by sunrisevalley
Costs Of Flying 777 From LHR To JFK posted Sat Mar 22 2014 00:27:49 by ukoverlander

Sponsor Message:
Printer friendly format