# Aircraft speeds & Temperatures

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Hi,I know (or I think that I know it...lol!) the difference between IAS and TAS...but until now I thought that there wouldn`t be a (big) difference between TAS and GS.When I took off today in rough-weather (strong cross-wind component etc.) the ND in MAP-mode showed a huge difference: TAS 145 knots while GS at 162 knots! 1) Could you please explain in easy words what is the difference between TAS & GS and how it is calculated?2) Could you also give the difference between IAS and CAS? 3) I never saw (also not in a real cockpit) a cockpit instrument for the CAS. Can you guys explain why this speed isn`t displayed in the (usual) cockpits of modern airliners?Another topic:When you fly as a passenger the PNF usually makes a brief explenations about the actual routing, speed, ALT etc...Sometimes he/she also gives the actual outside air-temperature (OAT) while being enroute...it is usually between -45 to -60 degrees celsius (or so). I ask myself how the PNF knows this actual OAT? When I look at the PMDG cockpit (or other simulators) there is always "only" a display for the True Air Temperature (TAT). 4) How does PNF in real life read or calculate the OAT?5) Is there a possibility to show the OAT in the PMDG-simulator?Many thanks in advance! :)GreetingsSusan

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This is very simple. TAS is your true speed in relation to the air. GS is your speed in relation to ground. If there is no wind both will be identical. If there is a wind - that's the diference.And first "T" in TAT is not "True", it is "Total".Your questions have to do with so called 'basic' aeronautical knowledge. You should really go to some website, use google or pick up some manual that will explain it all.Michael J.WinXP-Home SP2,AMD64 3500+,Abit AV8,Radeon X800Pro,36GB Raptor,1GB PC3200,Audigy 2http://www.precisionmanuals.com/images/forum/747400.jpghttp://www.hifisim.com/images/asv_beta_member.jpg

Michael J.

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IAS is the indicated airspeed, and reflects the airspeed derived from the pressure difference between the pitot and static ports on the airplane.TAS is this indicated airspeed corrected for factors including altitude and temperature. It basically corrects for the fact that altitude and temperature change the density of the air flowing into the pitot tube, and can cause the IAS to read higher or lower than it would at sea level under standard conditions.BOTH of the above speeds are the speed of the airplane through the surrounding air mass. Groundspeed is the speed the airplane makes across the ground. So let's say the airplane is flying through an air mass that's moving in the same direction as the airplane - this is a tailwind, and the groundspeed will be higher. If flying into a headwind, where the air mass is moving in the opposite direction, then groundspeed will be lowered.CAS is indicated airspeed corrected for position error - basically the pitot tube on airplanes is not placed out in smooth undisturbed airflow. When they flight test airplanes you'll see a LOOOONG snout coming off of the airplane - this is to measure the difference between the reference pitot on the snout and the airplane's "normal" pitot tube - and determines a CAS table.TAT is used on jets because flying at high airspeeds compresses the air and causes heating.They may determine the OAT by looking at a table from the weather bureaus who sample the air temperatures, or by taking the TAT and applying a correction for speed.I'm away from my textbooks, so I'm sure about the first three paragraphs, pretty confident of the fourth, and making educated guesses on five and six.Best wishes,

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Tim,once again many, many thanks for your great answers! :)You write that the dispatcher or weather dep perform a "correction for speed" to the TAT.Do you have a kind of a practicabel "formula" how to (easy) calculate the OAT from the displayed TAT in the PMDG cockpit? It doesnt`t have to be accurate... :)Many thanks again...Kind regardsSusan

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The CREW might be able to take TAT and convert it to OAT - I don't know.Their other option would be to take the forecasted temperature from their dispatch weather briefing, and use that in their speech to the passengers. It may not be the exact temperature outside, but close enough.

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Pilots have normally little interest in OAT. Much more important is TAT since this is for example the temperature to watch when they fly polar missions and are worried about danger of fuel freezing in the wings.Michael J.WinXP-Home SP2,AMD64 3500+,Abit AV8,Radeon X800Pro,36GB Raptor,1GB PC3200,Audigy 2http://www.precisionmanuals.com/images/forum/747400.jpghttp://www.hifisim.com/images/asv_beta_member.jpg

Michael J.

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Susan,5) on the PROG page you have the OAT presented.Cheers,

Mats Johansson
PMDG Flight Test Dept

| Asus Z270-A | Intel i5-7600K @ 4.8 GHz OC/H2O | nVidia Geforce GTX 1070 8GB OC/O2|

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Hi Mats,>5) on the PROG page you have the OAT presented.But isnt`t that the "PREDICTED" OAT that *I* as the pilot have to enter in the Prog page of the FMC? I was searching for a display where the *actual* OAT is calculated non-stop during the flight dependant of the actual Alt etc... Thanks again! :)Have a great week-end!GreetingsSusan

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The readout you get on PROG 2/2 is your actual SAT (basically OAT),(SAT is static air temperature)You can't enter anything on the 2nd prog page...Cheers,Adrian

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>But isnt`t that the "PREDICTED" OAT that *I* as the pilot have>to enter in the Prog page of the FMC?Clearly as the name suggests "predicted" is not actual - it must come from whatever weather reports your dispatcher supplied you with.Michael J.WinXP-Home SP2,AMD64 3500+,Abit AV8,Radeon X800Pro,36GB Raptor,1GB PC3200,Audigy 2http://www.precisionmanuals.com/images/forum/747400.jpghttp://www.hifisim.com/images/asv_beta_member.jpg

Michael J.

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pratt engines like the jt9d and pw4100 require the use of nacelle anti-ice in visible moisture when the temp is between +10 tat to -40 sat, except in descent when it's require in visible moisture anytime the temp is below +10 tat, so sat is nice to have.i think tat has more impact on tank temp. i remember the 747 manual saying that if tank temp was approaching min temp, either descend (warmer air) or go faster (higher tat).

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