Lear 35 A Temperature Gauge wrong?

[BrunoAlexander 2]

Just noticed that the temperatur gauge says 16 C outside air temperature and AS16 shows me -3 C at 12000 feet.

Anyone noticed a difference like this. Or do i something wrong?

 

 

 

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Alexander

[BrunoAlexander 2]

I did some testing: 

 

The Outside Air Temperature Gauge shows:

 

at 30000 ft, 350 kt : -20 C

at 30000 ft, 160 kt : -37 C

 

Does the measured temperature depends on Air Speed ?

[BrunoAlexander 2]

Okay, looks like i am discussing this with my self, but anyway.

 

I compared the behavior I described above to other aircrafts: The Outside Air Temperature stays the same at a given Flight Level no matter of the Air Speed.

Only the Lear 35A measures different temperatures at same Flight Level depending from the Air Speed.

 

So, there must something wrong.

 

I am confused. Any remarks?

[Pat Mussotte 92]

Don't be confused! There are to "kinds" of temperature measured: the first one is the SAT or static air temperature which is the atmosphere temperature at a given point or altitude. The "other one" is the RAT (sometimes name TAT, very close to RAT, for total air temperature)or Ram air temperature which is the temperature"felt" by the probe located on the plane. This one is higher than the SAT due to the friction of the air at high speed. There is a formula binding the 2 temperature which is

TAT= SAT(1+0.2(M Square))

Suppose your at 35000 ft at M.8, the SAT is -56°C or 217°K(273+(-56)the TAT will be

217K(1+.2(.8square))=244.8K  or 244.8-273=-28.2°C

So there is a rise in temperature of 28°C due to the speed

 

PS: 273K=0°C

I hope I'm clear enough

 

Cheers

 

Pat

[BrunoAlexander 2]

Thank you Pat for your effort. This is a very good explantation,

No vagueness is left. It´s always good to learn something new - Thank you!

 

Alexander

[Pat Mussotte 92]

You're welcome :wink: