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Remys

TAT VS SAT VS OAT

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Hi,

After some researches I think I might need a professional feedback from someone.

 

On the Ground we can see on the display the OAT no problem with that and this is very normal will skip this part.

 

In the AIR we only see on the display the TAT. And it is very logic to see it having a higher value than the OAT or SAT due to the fact that the aircraft is moving through the air so for sure due to the kinetic factor, air friction it would get warmer. “Compressibility and friction of the air at high velocities”

 Weather Programs such as PFPX, FSRGW or any add-on gives us the SAT.

And passengers in the plane, can see on the aircraft display monitor the OAT or SAT I am not sure exactly which one of them they see. OAT or SAT

 

My first question is the following: [i am not sure that the SAT or OAT on the FMC are accurate since they are displaying the imported weather file containing all predefined the data and not computing them according the aircraft flight level, Mach, wind .. ]

Since we have TAT on the Display of the PMDG777, and all the needed info how we can compute the OAT and SAT and the Ram Rise or RR.

Is there a specific formula and I am sure that there is in aviation that gives us the exact SAT,RR(Ram Rise), and OAT from TAT? Using Mach/Airspeed, altitude, wind direction speed…..

i.e.: I found lot of them but not from official sources one of the examples: [but of course not exact]

RAM RISE = (TAS x 0.0115) squared

Then SAT = TAT + RR

 

2- when to turn Anti-Ice on in the T7 (at which TAT I should enable the anti-ice?)

3- Is there a way that I missed that displays the SAT or OAT on the display of the  T7

 

Thank you

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To view the current SAT in flight, press the PROG button on the FMS, and then press the NEXT button to display the second progress page on the FMS display.

 

Here you may view the current SAT temperature, as well as the current wind speed and direction expressed as well as the actual crosswind component.

 

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Just think of TAT as the temperature of the air right at the leading edge of the wing as it is compressed by the forward motion of the wing. SAT or OAT are the same and is only referenced when looking for temperature deviations. When calculating specific range or other cruise charts, the charts may read STD day, +5 or -5. In this case you check the SAT for STD day at your altitude and figure how much + or - you are and enter the appropriate chart. With the temp deviations, you can make corrections for given data. The only other time I may use SAT/OAT is in anti-ice use. One jet I flew we turned it on when below 6 degrees and in visible moisture. We also turned it on when below 6 degrees and the spread between OAT and dew point was with in 3 degrees. In my current jet, we turn it on when in visible moisture with the temperature below 10 degrees TAT down to a temperature of -40 SAT.

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Anti ice is automatic and will turn on when conditions are correct.

Very true, It's the same on the G5/550 but we turn it on regardless when in conditions that require it. Depends on your company SOP.

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To view the current SAT in flight, press the PROG button on the FMS, and then press the NEXT button to display the second progress page on the FMS display.

 

Here you may view the current SAT temperature, as well as the current wind speed and direction expressed as well as the actual crosswind component.

thank you for the info :). I am actually using it, but this is the true SAT? I mean exact SAT or just from the weather file ? if it is the exact and precise one you solved one of my problems :)

 

thanks a bunch sir

Anti ice is automatic and will turn on when conditions are correct.

it depends how you set them.

you can set them to manual. So in Manual mode when at what TAT to turn them on ?

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SAT ( Static Air temp )  and OAT ( outside air temp ) should be the same thing. 

 

The general policy is ENG ANTI ICE to be switched ON when on the ground OAT < 10 deg C with visible moisture, or icing condition ( ie snow on the taxiway and runway )

 

on the 777. The ENG ANTI ICE if in AUTO mode, will switch on the ENG ANTI ICE when TAT < 10 deg C and the two ice detector prob on each side of the nose detects icing condition. Switching the ENG ANTI ICE ON with TAT > 10 deg C and ice not detected by the ice detector prob will give you an EICAS advisory msg ( ANTI ICE ON ).

 

Wing ANTI ICE should not be used on the ground. System logic inhibits its use for 5min after liftoff if TAT >10 deg C. If left in auto mode Wing anti ice is inhibited after Take off mode is selected or less than 10min after lift off. 

 

normally most people only select ENG ANTI ICE ON on ground if operating under icing condition. and will leave it in AUTO after airborne. 

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I would like to thank everyone who posted in this topic as now it is crystal clear.

I still have the RAM Rise question and manual calculation from the TAT/OAT to SAT so just to know how it really do the computations if anyone knows.

 

once again thank you and + 1 to every post

 

sincerely

Remy

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Hi,

 

Usually you are not interested in SAT at all, other than for performance reasons (the difference between actual SAT at a given FL vs. the standard gives you the ISA deviation, with temperatures above ISA giving lower or worse performance than temperatures below ISA). SAT is also only important in the case of "cold soak" and the impact on fuel temps, which on an ultra-long haul aircraft is a major concern.

 

As for how you compute TAT from SAT, here is some code I wrote for the purpose.

        // Compute Mach and ram rise
        lss = sqrt(gamma * R_sp * T_s); // m/s
        lss = lss * 1.94384449; // m/s to kts
        M_a = sim->tas / lss; // Compute Mach

        RR = T_s * ((gamma - 1.0) / 2.0) * e * pow(M_a,2.0); // Total ram rise, °K

T_s = SAT in degrees Kelvin

lss = Local Speed of Sound, kts

tas = Current true airspeed, kts

gamma = 1.4; // Adiabatic index (1.40 assumed for aviation)
R_sp = 286.9; // Specific gas constant. Approx. for dry air is 286.9 J mol-1 K-1
e = 0.98; // Recovery factor for TAT Probe. 0.98 typical for most TAT sensors. 1.0 is perfect.

M_a = ratio of aircraft true airspeed to local speed of sound, aka Mach number. Note that I'm using TAS due to flight model limitations, but you would really use thje static vs. dynamic rho to compute Mach properly.

 

If you really want to get into it, here is the method used by ADCs (Air Data Computers) to compute Mach number: http://en.wikipedia.org/wiki/Mach_number#Calculating_Mach_Number_from_Pitot_Tube_Pressure

 

Note that it gives ABSOLUTE ram rise, aka TAT, in degrees Kelvin. Subtract 273.15 to convert to degrees Celcius.
 

Best regards,

Robin.

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Or to put it a bit more simply, and converting from TAT to SAT

SAT = TAT / (1 + 0.2 * M2)

SAT and TAT must be in absolute units, degrees K or R
M is Mach No.

For example, for M = 0.8 and TAT is 260 K

SAT = 260 / (1 + 0.2 * 0.82)

 

SAT = 260 / 1.128

 

SAT = 230.5 K
 

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Seems like a lot of work to figure SAT for FSX lol. More work than I did as a flight engineer when figuring time to step climb and fuel burn trends against the flight plan every hour. Every 30 minutes I was either running fuel heat or calculating fuel burn against the flight plan along with running fuel heat. Positive trends we were good, negative trends, time to check winds and decide to climb or descend. Those were the days when you stayed busy, listened to HF static and made position reports. Now days with SELCAL, ADS-C, and advanced FMS, it's easier to fall asleep. I like running with the lights on during late night ocean crossings.

 

Here's A easier way for your calculations. Just work the chart either way.

 

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