TAT or SAT

[h25b]

I disagree with my good friend H25B.
On just about every aircraft that i've flown the manufacturer suggested that TAT be used in order to decide when to turn on the ice protection.

Why? Think of it in simple terms, TAT is the temperature that the skin of the airplane THINKS it is.

So if it is zero degrees C' and you accelerate to 290, ram-air, friction, etc. might yield a TAT of +11'C thereby eliminating the need for ice protection.

Now on those same aircraft SAT is used to determine when you may turn off the antiice when the temperature is so cold that any moisture it carries will be in the form of ice crystals (usually -40'C SAT or colder).

I guess I've always been thinking of it in terms of, "How cold is it outside ???" In which case SAT is the most accurate answer to that simple question. I'll have to look at the Lear manual and see what they recommend using for anti-ice purposes. Sort of splitting hairs but nonetheless interesting...

I've been enlightened...

 

[Hose A. Jiminez]

I guess I've always been thinking of it in terms of, "How cold is it outside ???" In which case SAT is the most accurate answer to that simple question. I'll have to look at the Lear manual and see what they recommend using for anti-ice purposes. Sort of splitting hairs but nonetheless interesting...

I've been enlightened...

Not interesting at all.

 

[LEROY]

Look at the gage in your cockpit. Whatever it reads, reference your CFM (AFM) ... do what it tells you to do. For an in-depth discussion on Bernoulli equations and temperature gradients, there are several great textbooks I remember from college that spell that cr@p out.
As an aero engineer, I say fly what the book tells you to fly....

 

[FDJ2]

And the answer is

SAT - Static Air Temperature. Also called OAT. This is the temperature of the air surrounding the aircraft.

TAT - Total Air Temperature. This is the temperature of the air measured approximately by a temperature probe (such as a Rosemount). It is higher than the static air temperature due to ram pressure, or Mach, heating caused by the motion of the aircraft through the air. I say "approximately" because there are many factors that make the measured temperature slightly different from the true total air temperature.



http://mtp.jpl.nasa.gov/notes/software/Nomenclature%20and%20Abbreviations.html





The simple answer is that static air temperature is the outside temperature and total air temperature is the air temperature adjusted for the ram pressure heating caused by the motion of the aircraft.

 

[banned username 2]

I'll have to look at the Lear manual and see what they recommend using for anti-ice purposes. Sort of splitting hairs but nonetheless interesting...

Splitting hairs???

In the Lear 35 with a SAT of -56°C (ISA) doing Mach 0.78 your TAT would be about -30°C, that is a 24°C difference (43.2°F difference) TAT vs. SAT...

I would hardly call that "Splitting Hairs"...

 

[SkyPigPilot]

I thought I understood this issue. Now I am completely confused. Thanks Guys.

 

[h25b]

Splitting hairs???

In the Lear 35 with a SAT of -56°C (ISA) doing Mach 0.78 your TAT would be about -30°C, that is a 24°C difference (43.2°F difference) TAT vs. SAT...

I would hardly call that "Splitting Hairs"...

O.K., the splitting hairs remark was a poor choice of words to say the least along with the guy that said this discussion wasn't interesting...

Now excuse my ignorance and educate me please. When defining "icing conditions" in terms of temps. at which ice accumulation occurs do the manufacturers (i.e. Learjet, Dassault, Raytheon, etc...) vary on which temp. to use ???

Now, if the true ambient temp. (i.e. SAT) is not conducive to ice accretion but the TAT is .... THEN WHAT ?? I guess my question comes from the point that I have evidently ignorantly been going by SAT and have never been surpised by ice forming unexpectantly.

If the SAT is so cold that icing is virtually impossible is it then possible for the ram-air heating effect to somehow create an a condition where icing is possible... ???...

 

[FlyChicaga]

Actually, you use both SAT and TAT for ice protection purposes.

On the EMB-145, we will use SAT temperature with visible moisture for determining whether or not to use engine anti-ice for takeoff. 10 degrees C SAT is the limit. However in flight, we use TAT to determine the need for ice. Actually, since our system is automatic, the airplane determines for itself when it needs ice protection. However, for our ice protection system checks, we must be below 10 degrees C TAT.

So the above posts are right; TAT is used in flight to determine the need for anti-ice. However, SAT is also used, but on the ground.

 

[LarryinTN]

TAT=SAT when you're airspeed is zero.

 

[Dog Driver]

oooops

 

[beytzim]

I see this all the time! Too many pilots who have no idea how to read a TAT/SAT gauge.

TAT is what the airplane feels it is outside. So you better turn all your anti-ice equipment if its below +10TAT. Some manufacturers have limits on the minimum temperature to put anti-ice on. I've seen -27TAT, -40SAT etc. Regardless, the vast majority of icing will occur between -10TAT and +10TAT in visible moisture.

Example: You are doing 250KIAS at 10,000. Your TAT gauge reads +8 and SAT is -12. Better put that anti-ice on. Your smart Captain brings the throttles up and accelerates to 330IAS. Guess what? SAT is still -12, but TAT increased to +13. Turn off the anti-ice, you don't need it anymore. Know the relationship between IAS and TAT and you'll be a much safer pilot.

Happy flying.

 

[FurloughedAgain]

I think the confusion arises because some aircraft (specifically the light lears) specify SAT.

I don't understand why either, but perhaps there's an engineer type out there who can explain it to us.

This is the first airplane i've flown where TAT is ignored -- sometimes much to our detriment while trying to climb with the anticlimb selected "on".

 

[bestpilot]

I'd kinda be thinkin....What if I was like a itty bitty like molecule of like air, and like I'm just chillin' and mindin my business, y'know, and like hangin with dem other oxygen and nitrogen dudes, and all da sudden,

WHAM!!!!!!! Smacked by like dis big wing and stufff! OUCH!!!!

Yeah, I guess I'd be gettin heated up about dat, so is dat kinda why like TAT is like hotter dan SAT????????

Freakin wierd, Dude, yeah, wierd.

 

[TooBad]

I'll go with Albie on this one