Oat, Sat, Tat

[gnx99]

What is the difference between

OAT: Outside Air Temperature

SAT: Static Air Temperature
&
TAT: Total Air Temperature

 

[banned username 2]

Static Air Temperature (SAT) is the temperature of the still air outside...

Total Air Temperature (TAT) take into account the frictional heating of the air... Typically at 41,000 ft the SAT is -56.6°C but at Mach 0.80 the TAT is about -28°C due to the frictional heating at that speed...

Anti-Ice equipment is typically operated based on TAT, not SAT...

Hope this helps...

 

[bigD]

You know Falcon, if you flew a Citation X, you could say what the TAT would be at Mach .92!

 

[chrisdahut24]

Falcon-

Nice explanation, Ive been wandering what those acronyms meant. Maybe I missed something, but where does OAT come into the equation? To me it seems like OAT and SAT are the same. Is this right?

 

[banned username 2]

OAT vs. SAT

Yes, basically OAT and SAT are the same thing... Not sure why, but it seems smaller planes (Cessna's, Piper's, etc...) have OAT, while larger planes have SAT...

And for Mr. Big D, the TAT at Mach 0.87 is about -23°C... Of course if we do Mach 0.87, our range DROPS to about equal to the Citation X doing 0.80...

 

[chawbein]

Wow, what great TAT's!

 

[JayDub]

SAT is as if you dangled a thermometer out the window of your a/c, with no movement at all. It's the temperature outside much the same as what is read from your thermometer on your back porch, if you have one.

TAT is the above, however it doesn't compensate for ram rise. That is to say the movement of the a/c through the air. The act of moving that pitot tube through the air at 500+ MPH, causes it to warm up since it is metal. TAT is important because this tells you the relative effect the temp. is having on the surface of your a/c. Basically, if you had a leading edge that would be -40c, the friction caused from moving fast gives you a surface that has a cumulative temperature of maybe -15c. It would then be certainly capable of collecting ice.

OAT is not compensated for anything, including errors in the gauge (like the other two are). In a Cherokee, the difference between what the thermometer would read at 0 KTS and 110 KTS, is not enough to worry about. There are, however, charts out there somewhere that will help you convert OAT into SAT. They usually do try to keep the probe directly out of the slip stream, like on a Cessna. Remember the big old gauge next to the pilot's head?

As a side note, the Airbus is different from what Falcon Capt. described. We have to operate engine anti-ice from +10 TAT to -40 SAT. Kind of funky, huh?


JayDub

 

[banned username 2]

As a side note, the Airbus is different from what Falcon Capt. described. We have to operate engine anti-ice from +10 TAT to -40 SAT. Kind of funky, huh?

That is different...

In the Falcon we operate Anti-Ice from +10°C TAT till about -20°C TAT, which is probably about -40°C SAT anyhow depending on speed...

 

[Snoopy58]

On the 737, engine anti-ice comes on at +10 TAT & below, but can be turned off for climb or cruise (but not descent) when below -40 SAT. On the ground, we use OAT (i.e. as reported by ATIS) to determine if temperature is below +10, since the TAT indicator gives goofy readings on the ground with no air flow.

All of the above is with "visible moistore" present, obviously.

Cheers!

 

[TXCAP4228]

Total Air Temperature (TAT) take into account the frictional heating of the air... Typically at 41,000 ft the SAT is -56.6°C but at Mach 0.80 the TAT is about -28°C due to the frictional heating at that speed...

Someday I will be cool enough to need to remember that.

 

[chrisdahut24]

Very interesting stuff, thanks for all the replies (even if I wasn't the thread strater). I learn something knew every day!