OAT vs. TAT

[cosmo kramer]

Slightly confused and can't find anything in my textbooks right now. What was it the difference was between OAT and TAT again? Thanks.

[wysiwyg]

TAT is the final result after the ram rise has affected the OAT (SAT).

[411A]

Well.... as civil jet airliners have been flying 'round for over forty years or so...........why, prey tell, have the textbooks not kept pace? More JAR mumbo-jumbo perhaps?!

[thermostat]

At Mach .78, the ram rise is about 28 degrees C. So the SAT of -46c the TAT is -18c.

[cosmo kramer]

Just to confirm. OAT is of course Outside Air Temperature of the free undisturbed air. SAT, Static Air Temperature is the temperature messured on the aircraft and is higher that OAT because of ramrise. Now what is True Air Temperature? Since it's "True", I assume it is the same as OAT(?).

I apologize for my confusion ;)

P.s.
My text books are from before anyone ever thought of JAA, and I still can't find any mention of TAT (only OAT and SAT).

[Les Bee'un]

TAT refers to Total Air Temperature.

I'm sure the pedants will shoot me down, but in simple terms TAT = OAT raised by heating due to friction as a result of the aircrafts' speed through the air.

[quid]

cosmo k-

Are you sure that's what your book says?

SAT (static) and OAT (outside) are the same and used interchangeably.

RAT (ram) and TAT (total) are the same and used interchangeably. The ram rise will result in a RAT reading approx 26-30 warmer than the SAT/OAT.

I've not used the term True air temp., but I would guess it's SAT without the ram rise.

[cosmo kramer]

Well, perhaps I shouldn't have read that darn book as it was that which got me confused in the first place. :D I will get back with an exact quote. Then perhaps you guys can help decipher it.

[wysiwyg]

To clarify -

TOTAL air temperature = STATIC air temperature plus the effect of ram rise.

[ 04 August 2001: Message edited by: wysiwyg ]

[cosmo kramer]

I can see that I am not the only confused one here :D Wysiwyg and Quid, you are saying the opposite of eachother :confused:

Anyway, I checked the book and it says what I already wrote. OAT + ramrise = SAT.

I tend to believe in Wysiwyg. What had me confused was the "T" for Total (and not True).

Thanks for clearing that up Wysiwyg.

[cosmo kramer]

Fu.. I can see now you are saying the same. This calls for the exact quote:

"The observer in the balloon reads the temp. as OAT, while the pilot in the highspeed aeroplane will call his temp. SAT. However the temperatures are identical if no motion taking place" (I read that as OAT + ramrise = SAT).

Is anybody reading that differently or should I throw the book away? :D

[Stamatis]

Usually, for a rough rule of thumb if you take the IAS divide it by 10 and then subtract that from the TAT you should get the OAT.

[quid]

wysiwyg and I ARE saying the same. I guess I have to disagree with what's written in your book.

The airplane pilot will be reading RAT unless a ram correction is applied to be presented as SAT/OAT.

The very definition of "static" means it's still, or not moving.

[wysiwyg]

I agree too, we are both trying to say the same thing.
Let me try to reword it. Imagine you are stationary in an aircraft. Any temperature that you record will be the ambient air temperature or SAT. This seems to be frequently referred to in GA circles as the OAT. Now imagine flying that aircraft at high speed but at the same ambient temperature. The SAT has not changed but now as the airflow meets the airframe it starts to get compressed before it parts to flow around the structure. This marginal increase in pressure causes that bit of air's temperature to rise (the quantity of which called the ram rise) so the temperature that is sensed in the aircraft is the combination of the ambient temperature AND the ram rise. This is referred to as the Total air temperature (as it is the total effect of SAT and ram rise). In GA aircraft the speeds are so slow that the effect of ram rise is almost negligable so it is generally not considered.

So how do we use these pieces of information?
Well in my company we use engine anti-ice when in icing conditions and the TAT is between 10 degrees or less. This assumes that although the freezing temperature of water is 0 degrees, as we fly at high speed a 0 degree SAT would indicate at a higher temp on a TAT indicator.
On my last aircraft (a modern turboprop) we only had an SAT indication and it was noticeable that ice would only start to stick to the wings at approximately -4 degrees SAT. This was because at ambient temperatures of -3 or greater the ram rise would increase the temperature above freezing.

regards
wizzy

[ 04 August 2001: Message edited by: wysiwyg ]

[john_tullamarine]

The following is available in just about any aerodynamics or performance textbook - you should be able to check one out in your local library if it be of any size.

OAT = SAT

TAT = SAT(1 + kM^2/5)

Often, RAT is used to represent TAT in the equation above with a recovery loss, while TAT defines the full rise with no loss (k = 1).

If you play with your Jeppesen prayer wheel and compare the calculated answers, you should find a pretty close correlation. Temperatures in absolute units.

[ 09 August 2001: Message edited by: john_tullamarine ]

[Checkboard]

In addition I would say that Outside Air Temperature (OAT) is the temperature guage reading in an aircraft not subject to significant ram rise, True Air Temperature is OAT corrected for instrument error.

[Cosmo]

Hey Kramer, nice handle... :D

Cheers,
Cosmo

[Stage3]

50 off the mach number works for me. Know what I mean?