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wxman11
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A320 Clarification On What Is TAT And SAT

Wed Feb 26, 2014 11:17 pm

Hello fellow aviators.

background:
I have all the airbus aircraft installed in FSX. I do believe that Airbus is a good aircraft to fly despite its trade-off. I even manage to re-configure 1 aircraft, A320iae, by modifying the data in FSX to match that of a real A320iae data using LAN A320 CC-BAJ just to make it was real as possible. Anyway, while flying, I've been curious about the following:

what is TAT and SAT?

example, i was doing a flight from DXB - JRO. while cruising FL360, the temp that i was reading for an example was:
TAT -24C
SAT -45C
clear skies along route

seeing these temps and being at FL360, is it necessary to turn on the anti-ice for wing, engine and pitot heat on?

thanks
Wxman11
 
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Starlionblue
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RE: A320 Clarification On What Is TAT And SAT

Thu Feb 27, 2014 12:40 am

TAT is Total Air Temperature, which is what the temperature probe actually reads. The problem is that at airliner speeds, the air is heated by compression, so the number is much higher than the actual air temperature. Using computer maths-fu or your handy CRP-5s "Temp Rise" scale, TAT can be converted to SAT, static air temperature. SAT is the temperature reading assuming the plane stood still (which would be impractical).

I would say with a SAT of -45 and clear skies, you can leave the anti-ice off. However, pitot heat should always be on in an airliner.

http://images.flightstore.co.uk/images/products/zoom/1334772569-54240500.jpg
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U2380
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RE: A320 Clarification On What Is TAT And SAT

Thu Feb 27, 2014 12:49 am

Hi there.

I had written out a far more comprehensive answer where I went into more detail, unfortunately I closed the tab by accident, so I had to start again! I'm sure someone else will be able to provide you with a far more comprehensive answer, however in a nutshell.

TAT= SAT+ Ram Rise

SAT- Static Air Temperature is the ambient temperature outside of the aircraft. It is measured in free flow/undisturbed airflow. SAT is generally used in performance calculations

TAT- Total Air Temperature is the SAT with Ram Rise taken into account. Ram Rise consists of compression effects and a small amount of frictional effects. The TAT will always be less cold/warmer than the SAT and at high mach numbers can be significant. TAT is generally used to calculate TAS.

Going through my old ATPL text books, I found this definition for TAT:

"The greatest possible temperature rise occurs when the air is brought completely to rest with respect to the temperature probe (stagnation) and the entire ram rise due to compression (and some small frictional effects) is measured"

TAT is measured using a Total Air Temperature Probe, also known as a 'Rosemount Probe'. This involves slowing the airflow down to an almost complete stop and measuring the Ram Rise. In a perfect world the airflow would be slowed to a complete stop (stagnation), however this is not possible as there must be airflow through the probe to allow for continual readings. This (along with other factors) causes an very slight error which must be corrected for and is known as the Recovery Factor.

As for the Icing Question. Each type has different requirements as to when anti-icing must be used. Engine anti-icing is almost always used before wing anti-ice (pitot heat should always be on) and in general, it should be used when ambient temperatures are below 10C and there is visible moisture. In this case, in clear air, you are extremely unlikely to experience any icing conditions, unless you are flying in a patch of clear air below a cloud build up.

[Edited 2014-02-26 16:51:11]
 
DualQual
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RE: A320 Clarification On What Is TAT And SAT

Thu Feb 27, 2014 5:19 am

In general for engine anti -ice protection on look for:
1) TAT less than 10C
2) Visible moisture (rain, snow, in cloud, visibility obscured and less than a mile)

You do not need anti ice on with a SAT less than -40C under any conditions.
There's no known cure for stupid
 
Pihero
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RE: A320 Clarification On What Is TAT And SAT

Thu Feb 27, 2014 10:57 am

Quoting wxman11 (Thread starter):
example, i was doing a flight from DXB - JRO. while cruising FL360, the temp that i was reading for an example was:
TAT -24C
SAT -45C

The previous posters are right, both on the definitions and the general OPS procedures.

On the other hand, the values you gave as examples are not - certainly not - representative of normal cruise : they are valid for a cruise Mach =.68 / IAS = 220 kt / TAS = 395 kt... right at the lower buffet limit for a rather light weight.
The A320 cruising Mach numbers are between .78 and .80 :
- At M.78 --> IAS = 258 kt / TAS = 458 kt and ram rise = 28°C --> TAT = -17°C
- AT M.80 --> IAS = 265 kt / TAS = 470 kt and ram rise = 30°C --> TAT = -15°C

For a faster cruiser at M.85, the ram rise would be 35°C.
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hivue
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RE: A320 Clarification On What Is TAT And SAT

Thu Feb 27, 2014 3:30 pm

Quoting DualQual (Reply 3):
You do not need anti ice on with a SAT less than -40C under any conditions.

I assume you meant to say "greater than -40C?" Otherwise you would never need anti-ice on.
"You're sitting. In a chair. In the SKY!!" ~ Louis C.K.
 
PGNCS
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RE: A320 Clarification On What Is TAT And SAT

Thu Feb 27, 2014 4:04 pm

Quoting wxman11 (Thread starter):
clear skies along route

seeing these temps and being at FL360, is it necessary to turn on the anti-ice for wing, engine and pitot heat on?

In an A-320 the pitot/probe heat will automatically be on and should be. Engine and airfoil anti ice does not need to be on.

Quoting Starlionblue (Reply 1):
I would say with a SAT of -45 and clear skies, you can leave the anti-ice off. However, pitot heat should always be on in an airliner.

Yep.

Quoting DualQual (Reply 3):
In general for engine anti -ice protection on look for:
1) TAT less than 10C
2) Visible moisture (rain, snow, in cloud, visibility obscured and less than a mile)

You do not need anti ice on with a SAT less than -40C under any conditions.

Yes, those are good general guides, however there are engines that don't exempt -40C and below from anti icing requirements. I don't think it would be unsafe to operate them with TAI off, but it's not approved. (I'm thinking for example of some JT-8D installations on some MD-80 variants here.)

Quoting hivue (Reply 5):

I assume you meant to say "greater than -40C?" Otherwise you would never need anti-ice on.

No he didn't.
 
hivue
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RE: A320 Clarification On What Is TAT And SAT

Thu Feb 27, 2014 5:51 pm

Quoting PGNCS (Reply 6):
Quoting hivue (Reply 5):
I assume you meant to say "greater than -40C?" Otherwise you would never need anti-ice on.

No he didn't.

OK, thanks. I stand corrected. I assume it's because at -40C there can't be enough water in the air to form ice.
"You're sitting. In a chair. In the SKY!!" ~ Louis C.K.
 
DualQual
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RE: A320 Clarification On What Is TAT And SAT

Thu Feb 27, 2014 6:55 pm

Quoting hivue (Reply 7):

A better phrasing on my part would have been colder than -40C engine anti ice is typically not required.
There's no known cure for stupid
 
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wxman11
Topic Author
Posts: 89
Joined: Mon Jan 17, 2011 9:33 pm

RE: A320 Clarification On What Is TAT And SAT

Wed Mar 05, 2014 11:20 pm

Hi folks,

thank you for your clarification. 1 thing I forgot to do was provide an example, like from the following links below, to what I was seeking clarification.

https://www.airliners.net/ufview.file?id=138601&filename=phpamawIZ.jpeg
https://www.airliners.net/ufview.file?id=138601&filename=phpmh7tDv.jpeg
https://www.airliners.net/ufview.file?id=138601&filename=phpzI9q5O.jpeg

brgds
 
Pihero
Posts: 4318
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RE: A320 Clarification On What Is TAT And SAT

Thu Mar 06, 2014 3:10 pm

Looking at your overhead panel, the *ON* light on the Probe/Window Heat switchlight is an anomaly : In flight, it doesn't bring anything as these components are automatically heated.
So the *ON* is only used on the ground or in flight if -big *IF*- the automatic circuit doesn't deliver... and three computers are involved.

Never seen it malfunction.

The anti-ice panel : Basically used only when we have an *Ice Detected* ECAM message. That message is generallyt very pessimistic.
Otherwise, airmanship requires considering a -10°C or below temperature, with visible moisture ( SAT on the ground, TAT in flight ) as a good reason to select engine A/I... and a descent, again with visible moisture, even if temperatures below - 40°C.
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