Atmosphere Question and G1000

Question

I was riding in a G1000 Citation Mustang a couple weeks ago and noticed in the bottom left two temperatures: RAT and ISA. We were at FL410 and the RAT displayed -48C and the ISA was -2C. I fly a G1000 182 all the time and only have my OAT...can anyone help explain what these are and why they are different?? Thanks!!

Answer 1

RAT is ram air temp. Also called total air temp (TAT). It is static air temp (SAT) adjusted for friction. (The faster you go, the more friction. The more friction, the warmer RAT is compared to SAT. In jets I fly, it's not unusual to see RAT 30 to 40 C warmer than SAT. From the info you posted, you had a 21 C rise.from SAT to RAT in the relatively slow Mustang. Your SAT was -69. At FL 410, your ISA would be -67 and you were 2 degrees cooler than ISA.) RAT is significant for anti-ice considerations (if in moisture). SAT is significant for performance, including TAS highest cruise altitude attainable for given weight and temp. "ISA -2C" refers to deviation from standard temp. Two degrees cooler than standard. In general, the cooler the better for performance purposes.