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OAT - TAT vs SAT and why you might care

SMO

SMO

Well Known Member
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There is an old thread that discusses this in some detail here: http://www.vansairforce.com/community/showthread.php?t=30022&highlight=GRT+TAT

I have a G3X Touch and a GRT Mini in my panel, each with their own OAT probe. The probes are mounted side by side under the wing on an inspection panel. In the hangar the GRT reads 2*F higher than both the G3X and the thermometer on the wall.

Level at an altitude of 8000 ft with a true airspeed of 195 knots (174 indicated, 2300 rpm, lean of peak, 11.5 gph) there is a difference of 9* F between the two OAT readouts, with the GRT Mini being the higher reading. I have confirmed that the G3X converts their probe's data for "Ram Rise" to provide Static Air Temperature (SAT). It is obvious the GRT Mini is displaying Total Air Temperature (TAT).

Why should you care?
  • If you fly IFR;
  • If your OAT displays TAT and you think it is displaying SAT;
  • If you are about to enter a cloud and are not concerned about icing because your OAT is reading high enough;
Then you just might get a big surprise when ice accumulates on your airframe and canopy. Avoiding surprises in the air is good.
 
SMO said:
There is an old thread that discusses this in some detail here: http://www.vansairforce.com/community/showthread.php?t=30022&highlight=GRT+TAT

I have a G3X Touch and a GRT Mini in my panel, each with their own OAT probe. The probes are mounted side by side under the wing on an inspection panel. In the hangar the GRT reads 2*F higher than both the G3X and the thermometer on the wall.

Level at an altitude of 8000 ft with a true airspeed of 195 knots (174 indicated, 2300 rpm, lean of peak, 11.5 gph) there is a difference of 9* F between the two OAT readouts, with the GRT Mini being the higher reading. I have confirmed that the G3X converts their probe's data for "Ram Rise" to provide Static Air Temperature (SAT). It is obvious the GRT Mini is displaying Total Air Temperature (TAT).

Why should you care?
  • If you fly IFR;
  • If your OAT displays TAT and you think it is displaying SAT;
  • If you are about to enter a cloud and are not concerned about icing because your OAT is reading high enough;
Then you just might get a big surprise when ice accumulates on your airframe and canopy. Avoiding surprises in the air is good.
Click to expand...

Interesting . . . those Garmin people are pretty smart, and build that into the products. That GRT Mini is pretty powerful, what does it use for TAS? Do they read close to the same TAS in varying conditions?
 
Great post. This is a topic many don't know about since it simply isn't covered PPL courses (usually).

A simple memory aid: RST. Stands for "Ram Air Temp + Static Air Temp = Total Air Temp".
 
TAT is what matters

Interesting. I agree it would be nice to know what is being displayed (TAT or SAT) but TAT is what really matters when it comes to management of anti-ice systems and / or avoiding icing. In the Airbus, we use SAT to determine when it it is safe to discontinue the use of anti-ice (-40) but the TAT dropping to +10 or below it the key predictor of icing conditions when in visible moisture.

If I could only see one or the other, I'd prefer to have TAT. I don't think any of the probes on most light aircraft are capable of actually measuring TAT though so, assuming no computer is making a conversion, you are seeing SAT, though probably with some error at higher speeds.
 
f14av8r; If I could only see one or the other said:
Actually the probe on the outside only measures TAT. SAT has to be calculated to remove ram air effects. Old airplanes like the early 737 could not present SAT to the pilots. For us flying RV's there's very little difference between TAT and SAT.

-Andy
Click to expand...
 
With the huge error most planes have in temperature reading due to installation location, Ttotal errors are small potatoes.
 
The issue with a probe in an experimental showing either TAT or SAT is that the install location matters.

The reason TAT and SAT differs is that the air molecules running into the probe slow down and turn that energy into heat.

A true TAT probe measures the temperature of the air after it is brought to a complete stop (relative to the plane).

A true SAT probe measures it as if it never slowed down at all.

Take any real temperature probe that is a simple shape just sticking out from the airframe and it measures neither. This real world example then has a "recovery factor" where the probe isn't a full TAT because the air doesn't come to a complete stop, but it isn't a SAT probe either because it does absorb some.

The extended issue is that where you put the probe matters for local air flow as well. You see a lot of probes in NACA ducts or other places where the air velocity isn't the same as the airframe. Or you know, any probe that is behind the propeller arc.

Thus, it's basically impossible on a piston powered experimental to sell either a TAT or a SAT probe. There's just no way to tell someone how/were to install it and have any sense that the local airflow is known and related to the TAS (which itself is based on SAT!).
 
dynonsupport said:
The issue with a probe in an experimental showing either TAT or SAT is that the install location matters.

The reason TAT and SAT differs is that the air molecules running into the probe slow down and turn that energy into heat.

A true TAT probe measures the temperature of the air after it is brought to a complete stop (relative to the plane).

A true SAT probe measures it as if it never slowed down at all.

Take any real temperature probe that is a simple shape just sticking out from the airframe and it measures neither. This real world example then has a "recovery factor" where the probe isn't a full TAT because the air doesn't come to a complete stop, but it isn't a SAT probe either because it does absorb some.

The extended issue is that where you put the probe matters for local air flow as well. You see a lot of probes in NACA ducts or other places where the air velocity isn't the same as the airframe. Or you know, any probe that is behind the propeller arc.

Thus, it's basically impossible on a piston powered experimental to sell either a TAT or a SAT probe. There's just no way to tell someone how/were to install it and have any sense that the local airflow is known and related to the TAS (which itself is based on SAT!).
Click to expand...

Great explanation and, as you said, a TAT probe is a very specific probe that ensures the air comes to a complete stop before measurement. So, with our OAT probes, hanging on various parts of our airplanes we are always measuring something between SAT and TAT.

My point was that our measurement will always the be more conservative for icing evaluation purposes because we'll always be showing something colder than a true TAT probe would measure. We get an undetermined credit for going fast but never as much as we would with a true TAT probe.

Now that I think about it, I don't think I want some software guy at G3X trying to guess how much adjustment my reading should get since they can't possibly know the characteristics of my particular installation. Better to leave it alone and just display me the indicated temperature.

Dynon, I assume you display us the indicated temperature. Correct?
 
f14av8r said:
My point was that our measurement will always the be more conservative for icing evaluation purposes because we'll always be showing something colder than a true TAT probe would measure.
Click to expand...

Empirical evidence shows that the uncorrected probe will show the warmest temperature, doesn't sound too conservative to me.

A properly positioned uncorrected probe will display TAT, which is warmer than SAT by definition.
 
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TAT is warmer than SAT

SMO said:
Empirical evidence shows that the uncorrected probe will show the warmest temperature, doesn't sound too conservative to me.

A properly positioned uncorrected probe will display TAT, which is colder than SAT by definition.
Click to expand...

Hi Mark,
At the very real risk of sounding argumentative, which I'm sincerely trying not to be, I simply don't think your last sentence is correct. TAT is always, by definition, WARMER than SAT, not colder.

Please consider this reference- https://en.wikipedia.org/wiki/Total_air_temperature

And this extract (emphasis mine), "In aviation, stagnation temperature is known as total air temperature and is measured by a temperature probe mounted on the surface of the aircraft. The probe is designed to bring the air to rest relative to the aircraft. As the air is brought to rest, kinetic energy is converted to internal energy. The air is compressed and experiences an adiabatic increase in temperature. Therefore total air temperature is higher than the static (or ambient) air temperature."

As to the question of most conservative, I believe you are correct. Seeing the colder of the two temperatures, which is closer to SAT on our aircraft, would be more conservative. I chose my words poorly. What I was trying to say was if there were any heating effects on the probe in use, they would probably also be in play on the surfaces of concern. The problem with that is I'd have to assume that the probe is being heated to the same degree as the most ice prone surface. Depending on the installation, that might not be the case.

It's the automatic correction applied by the GRT that makes this discussion more confusing, at least for me, that it would be normally!
 
f14av8r said:
TAT is always, by definition, WARMER than SAT, not colder.
............................
It's the automatic correction applied by the GRT that makes this discussion more confusing, at least for me, that it would be normally!
Click to expand...

Oops, you are right - I got the sentence backward. I will correct that.

GRT does not appear to do a correction, however Garmin does; which is why my Garmin (displaying SAT) displays a colder temp than the GRT (displaying TAT).
 
f14av8r said:
Hi Mark,


It's the automatic correction applied by the GRT that makes this discussion more confusing, at least for me, that it would be normally!
Click to expand...

I think it is the other way around. GRT displays raw data, which as Dynon explained is somewhere betweem SAT and TAT.
G3X is attempting a software correction to display SAT.
However, something is not right. At these speeds there should not be a 9F difference between them.
 
BobTurner said:
..However, something is not right. At these speeds there should not be a 9F difference between them.
Click to expand...

In my case of the 9*F difference between the two displays is made up of a 2* difference on the ground and a 7* difference due to Ram Rise. If you look the old thread I referenced in the first post you will find this:

RAM RISE - The rise in air temperature that occurs at high speed. This is due to the compressibility of the air. The faster you go, the more the temperature rises. This can be seen on your OAT gauge. At about 180 knots indicated airspeed, you will notice about a 3? Centigrade rise in the temperature above the actual temperature of the air at that altitude.

3*C = 5.8*F which would be the theoretical Ram Rise if the person who posted that quote is correct. Rounding differences can also account for another degree or so.
 
True, except the GRT probe doesn't come close to stopping all the air, unless this is a unique design. I am curious as to what the uncorrected Garmin data looks like.
 
Going down a rat hole

Garmin appears to use the theoretical calculation for the difference between SAT and TAT. According to Wikipedia the formula is:

TAT/SAT = 1+((1.4-1)/2 x Mach squared) (temps are in Kelvin)

At my speed of 174 knots: 1 + .(2 x .26 x .26) = 1.01352

32* Kelvin = 273

273 x 1.01352 = 276.691

276.691* Kelvin converts to 38.4* F, or a 6.4* rise.

Again, according to Wikipedia a modern TAT probe has a recovery factor of .98

Are there more variables? Probably. For example the above apparently assumes dry air, so what adjustments need to be made for humidity? Don't know and feels like starting a "Dance on a Pinhead".

In any event, I am quite happy to use the more conservative G3X temperature for decision making, and am pleased with Garmin's efforts in this regard.
 
As was stated earlier, it is TAT that matters. It is TAT that the water molecules in the air "see". In my experience flying in icing conditions in an instrumented test airplane, if you fly too fast in icing conditions and that TAT gets above freezing the ice won't stick to the airplane even though SAT is below freezing.

All temp probes measure local TAT, minus instrument errors, which may be different than the free stream TAT due to local flow conditions, much like a static port on an airplane can't read true free stream static pressure because the airplane messes up the local flow.
 
Hello Mark,

One of the many things about your G3X Touch system you can be proud of is the fact that you are receiving the benefit of so much research and technology development that flows down to our experimental airplanes from the certified world, including from Part 25 transport aircraft. In the case of your GTP 59 temperature probe, it is the exact same unit that gets installed on transonic jet aircraft. You are correct that the outside air temperature (OAT) value displayed on your G3X Touch PFD is a calculation of static air temperature (SAT), which is computed from the uncorrected ram air temperature (RAT) measured by the probe. By virtue of the fact that we own and operate our own instrumented wind tunnel, we know exactly what the ram recovery factor of our temperature probe is, and we also know that our calculation of SAT/OAT is accurate. I'm sure there is a detailed report on it in a drawer somewhere at the FAA.

For those who would also like to see the uncorrected ram air temperature measured by the probe, we have added the option to display a RAT data field to both the GDU 4xx and GDU 37x displays. This will be in the next free software maintenance release.

Finally, as was alluded to earlier in this thread, proper placement of the air temperature probe does make a difference. We have seen people install the probe directly behind the exhaust pipe, inside a closed landing gear bay, and even inside the airplane itself! Unfortunately, even though many builders have tried it, simply installing your OAT probe in the RV's cockpit ventilation NACA duct is not a good solution due to the effect of engine compartment discharge air impinging on the probe. We've found that one good location is underneath the wing, near the inboard inspection plate. That's where I put it on my RV.

- Matt
 
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