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jcalder

TAT in flight

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Hi, I did some looking around for this and I couldn't seem to find quite the answer I'm looking for, just wondering about what the TAT should be inflight? (I know it varies, bear with me please)If in ASA it says the temperature outside is -52, the TAT display on the 744 reads around -18 at mach .85 or .86. If I slow down to say .78 then the TAT temperature goes down. I guess my lack of knowledge on what exactly TAT is contributes to my confusion. I read an article on wikipedia and I don't quite understand it. From what I gather is the faster you go, the friction of the air makes the airplane warmer than the air surrounding it, which makes sense.I recently got the MD-11 but I'm too stupid to pick it up quickly and once I get frustrated with trying to learn it, I go back and fly the 744 to make myself feel good again :( . I'm sure the MD-11 TAT does similar things as the 744. I guess in all my hours spent flying I never paid much attention to the TAT value. I always knew it was there but was busy concentrating on flying the airplane properly. Now that I feel I am pretty good at flying the plane, I'm trying to learn all these other "little" things.Thanks to anyone who can help enlighten me! (I posted this in the PMDG forum because I noticed it on my PMDG plane, I hope it's not in the wrong area.)Jeff Calder


Jeff Calder

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The wikipedia article on total air temperature is a little mathematics oriented... but the basic comments are there:TAT is taken from a probe, which is designed to slow the air down to rest. The kinetic energy (ie energy associated with the moving air particles) is converted to internal energy as the air is brought to rest. As such, the temperature of the air rises in relation to the static air temperature. The faster the air is (ie the faster you fly), the greater the warming effect is relatively speaking, so TAT increases relatively to SAT as you go faster. The inverse is true as you slow down.To put it into other words, using your example:ASA reports the OAT at -52


Andrew Entwistle

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which is designed to slow the air down to rest
Interesting method to explain it, Andrew. Another way is to use compression as the heat source, any gas heats when compressed and the air is compressed at the probe, the amount of compression is directly related to speed. As are most things in physics, there is more than one way to look at things like Cats in Boxes.

Dan Downs KCRP

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Interesting method to explain it, Andrew. Another way is to use compression as the heat source, any gas heats when compressed and the air is compressed at the probe, the amount of compression is directly related to speed. As are most things in physics, there is more than one way to look at things like Cats in Boxes.
That's not 'another' way to look at it at all - internal energy of a gas and its kinetic energy are two completely seperate things. IIRC TAT probe is not designed to compress air - in fact I believe they are specifically designed to avoid doing so.

Jordan Forrest

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