Incorrect cruise altitude temps

[davidzill 8]

I fly this aircraft exclusively, so I'm posting here in hopes for an answer. I use ASN, REX TD, and I have the latest FSUIPC. My cruising altitude temp hover around -25 to -29 degrees Celsius, far from the -55 to -65 that the METARs indicate, as well as what PFPX indicates for those altitudes. Does anyone have an idea of what may need to be done to get an accurate retro indication?

[pcubine 143]

I fly this aircraft exclusively, so I'm posting here in hopes for an answer

The -25/-29 sound like TAT and the -55/-65 like the OAT. During flight the TAT is indicated on the upper EICAS and the OAT is on page 2 of PROGRESS pages of the FMC, It is shown as SAT. Depending upon your cruise altitude and location they do not sound unreasonable. Right now I am between TNCM and KMIA at FL360 and the TAT is -21C and the SAT is -49C and that is in the Caribbean. PFPX shows the OAT on the route printout. So you are probably looking at the TAT in the plane and the OAT on PFPX. At cruise TAT is always going to be 20C to 30C warmer than the OAT.

[AirborneZed 168]

What you're seeing is the Total air temperature. They are warmer due to friction of the air passing through the skin of the aircraft. What the aircraft is "feeling"

 

The temperature of the air is located on the CDU.  Click PROG -> click next for Page 2, and you will see the SAT, The static air temperature, which is what you are looking for to compare to temperature data. You will see that is is very close to the forecast temps.

 

Fact: If you slow down your airspeed, You will see the TAT drop because there is less friction, Once you get to a speed of 0 KIAS, It will be equal to Static air temperature.

[davidzill 8]

Thanks guys, I learn something new about this aircraft everyday.

[kamoflarj 0]

What you're seeing is the Total air temperature. They are warmer due to friction of the air passing through the skin of the aircraft. What the aircraft is "feeling"

 

The temperature of the air is located on the CDU.  Click PROG -> click next for Page 2, and you will see the SAT, The static air temperature, which is what you are looking for to compare to temperature data. You will see that is is very close to the forecast temps.

 

Fact: If you slow down your airspeed, You will see the TAT drop because there is less friction, Once you get to a speed of 0 KIAS, It will be equal to Static air temperature.

Great info, i didn't know that about TAT and SAT before

[downscc 1,327]

The difference between TAT and SAT is not just friction, but compression also comes into play and to much more degree than friction. The ratio of TAT/SAT is a function of Mach number and ratio of specific heats for the air. Wikipedia has nice short article: http://en.wikipedia.org/wiki/Total_air_temperature

[kevinh 409]

The difference between TAT and SAT is not just friction, but compression also comes into play and to much more degree than friction. The ratio of TAT/SAT is a function of Mach number and ratio of specific heats for the air. Wikipedia has nice short article: http://en.wikipedia.org/wiki/Total_air_temperature

Actually, friction has nothing to do with the difference between TAT and SAT. It is all about the rise in temperature as air is compressed as it is bought to stagnation. The wiki article linked explains this correctly.