In the Boeing 737 versions, the temperature is shown as TAT (Total Air Temperature) which is related to engine-performance.Does a formula exist to convert TAT into SAT (Surrounding Air Temperature) or into OAT (Outside Air Temperature)?TAT doesn't mean anything to me, since I am not a flightengineer. Paddy.

SAT is actually Static Air Temperature not Surrounding Air Temperature though I suppose the basic meaning is about the same. There is a formula somewhere which shows the kinetic effect of the movement of the aircraft through the air which increases the temperature but I haven't got it to hand. However you can assume the difference is about 27 degrees C at cruise alt.

Thanks Rob, for your reply. I'll take note of it!Cheers,Paddy.

TAT = Total Air Temp and is SAT + the ram rise of the air due to compression against the temp sensor. The formula to get an approximate SAT (Satic Air Temp same as OAT - the ambient temp) is:TAT = SAT + (TAS/100)^2

Hi,Thank you for explanation and formula.Much obliged!Cheers,Paddy.

>TAT = Total Air Temp and is SAT + the ram rise of the air due>to compression against the temp sensor. The formula to get an>approximate SAT (Satic Air Temp same as OAT - the ambient>temp) is:>>TAT = SAT + (TAS/100)^2Sorry but that formula will not do for a B737 as it is a severe approximation above 100 kn.TAT =SAT*(1+0.2*MN^2)where MN = Mach NumberMN = TAS/(661.4785(in kn)*sqrt(SAT/288.15 K)Ian

Thank you very much Ian, for your correction and for stating the exact formulas.(They look dead serious!)Perhaps you are willing to tell us in short why the temp reading in the B737 indicates in TAT and not in SAT or OAT. What is the benefitof this TAT-reading for the pilot? Is it indeed related to engine performance?Hope to read you again.Best regards,Paddy.

Hello Paddy,Um the basic mechanical thermometer when exposed to the airstream will measure TAT which is what you would experience if you (were able) to stand in a 500 kn draft. The drag would be equivalent to several times your own weight and it would be rather like the crushing g forces. Why? The air must come to a full stop unless it were to penetrate the instrument. Due to conservation of energy the kinetic energy must be converted into something else and in the worst case it is heat. So along comes air at TAT with specific energy TAT*Cp which equals SAT*Cp + 1/2 V^2 (Cp = specific heat). It stops on the thermometer and the temp goes up from SAT to TAT, the alcohol expands or the resistance increases.If you were to throw the thermometer out on a parachute, you would measure the SAT. Then you would require a radio telemetry link to send back the data. This is called a drop sonde, used by weather squadrons particularily during the hurricane/typhone/cyclone seasons.It is also very costly in thermometers. With an on-board computer estimates of the SAT can be made. There will be position errors as there will be with IAS and Mach meters.A2: the engine performance is keyed to the TAT as engine speed among many other parameters are referenced from TAT to MSL (228.5 K). E.g., CN = N/sqrt(TAT/288.15 K). Thrust and air flow are also corrected in this way and it is these units that are used in the .air file which describes most of the flight dynamics. This reduces the complexity of engine models.Ian

Hoi Ian,Thanks a lot for your very quick, ample and clear info!I copied your explanation, which gave me the stuff for brainstorming the subject. Much obliged.Best regards,Paddy.