I am flying to EGLL at FL390 in the PSS 744. The displayed TAT on the Upper EICAS is -39C as compared with a rather higher (I mean, it's usually around -27C) TAT on other flights. Perhaps this is because I set the air temp to -2C at KIAD before liftoff.Anyway, I observe that while the commanded Mach speed under FMC control is still 0.862, the displayed TAS on the ND reads only 472. Normally it reads 490-495 at this FL on this type of flight. IAS of 266 apears to be normal.Why does this much lower TAT that I currently see on the EICAS have this kind of affect on TAS?Appreciate posts, thanks!JS

Hey Jonathan.Alright, I'm gonna delve into the technical portion of this explanation, so bare with me. If you don't understand, give me a shout and I'll try to clarify.Mach speed, as you know, is a ratio of your speed and the speed of sound. The speed of sound depends o three variables: Gamma (specific heat ratio, basically a number that dependson what gas we are talking about. In our case, air, it is 1.4), the gas constant (again, depends on the gas. Air's is 287 J/Kg/K, but don't worry about the units), and the temperature, in degrees Kelvin (whatevr temperature in

Fabio:Many thanks. I think I get your general drift (quite a bit up here at FL390!). I shall probe further since this is an area of weakness in my FS knowledge.Cheers!JS

>Hey Jonathan. >>Alright, I'm gonna delve into the technical portion of this...>>Mach speed, as you know, is a ratio of your speed and the >speed of sound. The speed of sound depends o three >variables: Gamma (specific heat ratio, basically a number >that dependson what gas we are talking about. In our case, >air, it is 1.4), the gas constant (again, depends on the >gas. Air's is 287 J/Kg/K, but don't worry about the units), >and the temperature, in degrees Kelvin (whatevr temperature >in

Hey Ron.Thanks for the complements, it is a more complete answer now.And thanks for correcting me on the directly proportional statement, I had missed that.Interesting you should say that the atmosphere is the 1976 standard, I think I am going to make some tests, have you tried any tests before?

>Hey Ron. >>Thanks for the complements, it is a more complete answer >now. "Theta2", SQRT(T/To) is an important parameter for turbine engines. Also, Mach Number and Gamma.>Interesting you should say that the atmosphere is the 1976 >standard, I think I am going to make some tests, have you >tried any tests before? Many have verified the FS atmosphere. I know it is the 1976 Std Atmosphere since there are two 'Inverse Atmospheric Pressure' numbers for some of the turbine tables that correspond to exact altitudes, (I think 35,000 and 70,000 ft). Using the 1961 Std Atmosphere those altitudes came out at odd altitudes. It was only when I found they corresponded exactly to the above altitudes that I was sure turbine entries were as stated. Of course, one can set 'real WX' or other non-standard values. For flight testing I use the FS default, which sets 15 C at SL and 29.92" Hg. Incidently AFSD.exe (in the libary) displays deviation for ISA in one window. That is always 0 degrees if the default atmosphere is set. I think the Concorde panel displays the same deviation from ISA; it's important in how high and fast the AC can be flown. If the cruise level temperature is above ISA too much one has to reduce Mach number to keep within the 127 C limit. Nor, can one climb as high at the same weight.Ron

JS,I quite can't understand why people would reply to your post with giving you all these math formulas for speed of sound, etc. Unless you design avionics or work for Boeing you really don't have to know all this stuff. The simple explanation is that pitot tube - the primary instrument that measures the IAS is very sensitive to air density and air density changes significantly with temperature (that is why planes in winter need less runway for takeoff - air is more dense). So temperature must be taken into account when reading the pitot tube.Michael J.

Hello Michael.Thank you for your insight.I just thought some people would like to understand the physics behind it. You don't have to work for Boeing or design avionics to be interested in what is going on in the atmosphere.

Dear All:Thanks for your posts.I like both science and intuition. True, I do not have the real knowledge scientifically, but I enjoy science in a basic way. It motivates me to find out more.I take simming seriously although I'll never be a scientist or a real pilot. But I like to study now and then and it helps me to understand something about WHY things happen because then I can figure out more for myself as I go further into this amazing hobby.Anyway, I really appreciate the posts and the spirit of members here. Thanks again!JS

>I just thought some people would like to understand the >physics behind it. You don't have to work for Boeing or >design avionics to be interested in what is going on in the >atmosphere. Yes, but to understand physics (on the elementary level) no equations are usually necessary. They often obscure simple phenomena and scare people. It is actually much easier to grasp the essense what's going on (in this case) without the formuals. If we were talking about general relativity or particle physics that might be different.I am a physicist myself but I try to put myself in the shoes of someone who would rather understand something without an overdose of math.Michael J.

>JS, >>I quite can't understand why people would reply to your post >with giving you all these math formulas for speed of sound, >etc. Unless you design avionics or work for Boeing ... Or, are trying to understand and set up the MSFS flight and powerplant models. ;) >you really don't have to know all this stuff. The simple >explanation is that pitot tube - the primary instrument that >measures the IAS is very sensitive to air density and air >density changes significantly with temperature (that is why >planes in winter need less runway for takeoff - air is more >dense). So temperature must be taken into account when >reading the pitot tube. >Michael J. Except the question related to Mach Number vs TAS, not IAS. Expaining how compressability affects IAS/CAS is a bit more complicated than how Mach number varies only with absolute temperature. Michael, there are quite a few technical people here, including yourself. Yes, it's best to give simple, clear explanations without too much detail -- at the start of a reply. But, it doesn't hurt to go into more detail as long as that doesn't confuse the basic matters. I knew a_o (speed of sound at specific conditions) varied with Tabs^1/2 but not that R and Gamma came in the same way. Anyway, I'm an amateur at aerodynamics and gass dynamics. My degrees are in Electrical Engineering. Then again, it seems most of Bruce Artwick's guys were also EE's, most with PPL's. - Ron

Fabio,With a temperature of -39

Hello Stamatis.Very good catch! You are, of course, right. TAT is the OAT increased by the friction of the air against the skin of the aircraft. These two temperatures, at such high speeds, will be very different. (-39