TAT & SAT indications B737

[EightsOnPylons]

Hello,

The TAT (Total Air temp) and the SAT (????) taken from the FMC. What is the difference?

Thanks,

[B73567AMT]

The TAT (Total Air Temp) is taken from the TAT probe, sent to the Air Data Computer and then SAT (Static Air Temp) is computed based on speed. This data is sent to the FMC.

Here is a pretty good explanation:
The total air temperature (TAT) probe-measures the temperature of the ambient (static) air
plus the heat rise resulting from bringing the moving air to rest. When the Mach number is a
small fraction, the total temperature is very nearly equal to the static air temperature. As
Mach-number increases, the difference between static and total temperature increases.

[LEM]

this difference is called Temperature Rise, and seems to coincide roughly on a jet profile to the thousands of feet:
10° at 10000ft, 20° at 20000ft, 30 at 30000ft...

[EightsOnPylons]

thanks a lot for your answers. So the SAT is the true air temp? Is that the one that should be used when determing if icing conditions exist or not?

thanks,

[LEM]

Yes, the SAT is the "true" temp, the one you would measure if not moving through space.
For icing conditions criteria use SAT on the ground, but TAT in flight since that's what is actually "felt" by the skin of the aircraft.

Some manufactures use SAT in flight also, like Dornier, but Dornier is the one with two independent heading knob on it's glass cockpit, you know...

[Flying_Tuur]

For the very low temp, we use SAT in the air...

If during Climb or Cruise the SAT is below -40 Celsius, No Anti Icing required. This is because no 'wet' particals remaining in the air which can freeze to your aircraft..

In my early days on the 737-200, we didn't have that fancy FMC with the SAT.. I remember we used a rule of thumb to calculate the SAT, since we only got the TAT indicated. I'm not very sure about this rule anymore since I didn't have to use it the last years.

Must be something like TAT - (Mach * 40) = SAT

Correct me if I'm wrong, but I'm going to try this afternoon if that works..

So, TAT indicating -12, Mach is -.74

Roughly: -12 - 29 = -41 (seems correct)

[LEM]

Yes you are right, the definition of Icing Conditions is in Tat, and the limitation of the -40° during climb & cruise is in Sat (B737).

[Flying_Tuur]

Searched after my rule of thumb during this afternoon flights, and found it...

SAT = TAT - (Mach * 30)

Not 40 as I previously posted...

So, eg

TAT on gauge is -28, Mach 0.74

SAT = -28 - 22 = -50

Works great, checked at several altitudes and Machnumbers.. One to remember..

Greetz..

[alatriste]

Please Flying- Tuur, Could you be more specific regarding your rule of thumb? what do you mean with MAch*30?
I guess it is very simple, just a matter of nomenclature. In the spanish sciense world we do not use this* as any math operation.
THANKS A LOT

[av8boy]

He is indicating that Mach number should be multiplied by 30 and the resulting number subtracted from TAT. Also could be represented as SAT = TAT - (Mach x 30). The use of “*” to indicate multiplication is common on calculations entered from computer keyboards.

Dave

[woftam]

An even easier way of converting TAT to SAT is divide your IAS by 10 and this figure added to your TAT is your SAT (as a minus value).
e.g IAS 290kts, TAT -11
290 divided by 10 = 29
29 + 11 (TAT) = -40 SAT.
Works just fine and 10 is an easier multiple than 30 to do the mental gymnastics with.
Hope this helps.