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  #1  
Old 02-26-2016, 10:01 PM
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SMO SMO is offline
 
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Default OAT - TAT vs SAT and why you might care

There is an old thread that discusses this in some detail here: http://www.vansairforce.com/communit...hlight=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.
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  #2  
Old 02-27-2016, 06:41 AM
BillL BillL is offline
 
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Quote:
Originally Posted by SMO View Post
There is an old thread that discusses this in some detail here: http://www.vansairforce.com/communit...hlight=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.
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?
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  #3  
Old 02-27-2016, 09:06 AM
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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".
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  #4  
Old 02-27-2016, 11:05 AM
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f14av8r f14av8r is offline
 
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Default 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.
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  #5  
Old 02-27-2016, 11:52 AM
simpkinsona simpkinsona is offline
 
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[quote=f14av8r;

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.[/QUOTE]

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
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  #6  
Old 02-27-2016, 12:10 PM
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With the huge error most planes have in temperature reading due to installation location, Ttotal errors are small potatoes.
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  #7  
Old 02-27-2016, 03:56 PM
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dynonsupport dynonsupport is offline
 
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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!).
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  #8  
Old 02-27-2016, 04:37 PM
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f14av8r f14av8r is offline
 
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Quote:
Originally Posted by dynonsupport View Post
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!).
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?
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  #9  
Old 02-27-2016, 08:11 PM
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Quote:
Originally Posted by f14av8r View Post
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.
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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Last edited by SMO : 02-27-2016 at 08:55 PM. Reason: Change "colder" to "warmer"
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  #10  
Old 02-27-2016, 08:33 PM
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f14av8r f14av8r is offline
 
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Default TAT is warmer than SAT

Quote:
Originally Posted by SMO View Post
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.
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!
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