TAT Vs SAT

[D O Guerrero]

I'm possibly about to expose myself as a total charlatan pilot now... However, here goes -
The aircraft I fly (737NG) has a TAT reading on the Upper DU. To access the SAT reading, one has to go to Progress page 2 in the CDU.
We use TAT for determining when engine anti-ice needs to go on (ie below 10Deg) in the descent, climb and cruise. However we use SAT to determine when the temperature is sufficiently low that we turn the engine anti-ice off (colder than -40Deg).
Given that the TAT reading does not approximate to OAT (SAT being a closer approximation) and that I have heard many pilots (mostly Captains) refer to TAT as if it were OAT, I am wondering why we don't have SAT on the Upper DU and TAT in Progress Page 2 instead. Our TAT probes are not aspirated by the way.
Can anyone explain the logic of such a setup? Or is there some other earth shattering significance of TAT that I am completely missing?

[Busserday]

The information that the TAT gauge gives is more relevant for management of the anti-ice. The SAT used to discontinue the use of anti-ice at -40 is not as high a priority, both are used but the TAT dropping to 10 or below in certain instances, requires more immediate attention.
Why not display both? (Oh, oh, I just asked a why question).;)
BD

[Wizzaird]

As mentioned, TAT is a much more useful indication of the actual icing potential on the airframe, as the TAT approaches zero with the SAT not too cold then airframe icing is probable. As an aside, during a departure in potential icing conditions it is often better to keep the speed up rather than reduce speed to get out of the cloud quicker (turb permitting). Often the higher speed will keep the TAT sufficiently high to stop airframe icing occuring during the climb and by the time the TAT is in the icing range the SAT is now sufficiently low that icing is unlikely anyway.;)

[DFC]

TAT is a very important indication.

Aside from letting you know when you must have the AI on, it is one of the elements (along with pressure altitude) used in calculating the engine power setting eg Maximum continuous thrust.

Of course in the modern jet, this is often done automatically.

As for when to turn off the AI. Of course that would be in accordance with the AFM. One does not have to search for ther SAT if one has the TAT and speed (which gives you the temperature rise) but it is easier.

[NSEU]

Perhaps if the demand for SAT was great enough, Boeing might come to the party. With databusses and software running the NG displays, it shouldn't be too difficult.

Our 747-400's have a SAT readout in the lower right hand corner of the Upper EICAS display.

http://www.iinet.net.au/[email protected]/FlapExtensionElectric3.JPG

Rgds
NSEU

[Mansfield]

As I imagine we all know, one can arrange for Boeing to display almost anything in any location for a price. However, as has been stated, TAT is the critical factor in ice accretion, i.e., the TAT must be below a specific threshold for supercooled water to freeze on impingement. This is what drives the "turn the ice protection on" logic, as well as the strategy of high speed = high TAT = no ice.

That said, SAT is strongly influential in whether there is any supercooled water at all. Statistically, when the SAT is below -40C, there is almost no supercooled water present. This is what drives one leg of the "turn the ice protection off" logic, the other being TAT. The idea is that below -40C, there just isn't any supercooled water, so one doesn't need the ice protection.

However, as has been mentioned on a number of other threads, it is very possible to encounter supercooled water at a SAT of less than -40C when in the vicinity of large cumulus buildups, since the vertical motion can lift a water droplet rapidly into a SAT that is very cold. So while the statistical model of supercooled water and SAT is pretty valid, it isn't perfect.

Finally, none of this addresses the relatively newly understood issue of ice crystals, which are not part of the SAT less then -40C model at all, and seem to have had some success in instigating rollbacks, etc.

[PantLoad]

The Airbus displays both, quite prominently, and, I understand the new aircraft also show ISA deviation.

With regard to icing, we've beat this subject to death. The Airbus FCTM has a very good section on this. "Supplementary Information...Adverse Weather...Cold Weather Operations and Icing Conditions"

By the way, in this section on page SI-010 P4/16 dated 8 JUL 08:

"In climb or cruise, when the SAT decreases to lower (emphasis added) than -40 degrees C, engine anti-ice should be turned OFF, unless flying near CBs."

Just had to throw that in there to stir up the pot.....

My FCTM is not current, as I'm retired and no longer have good access to current manuals.


Fly safe,

PantLoad

[fastcruise]

:confused: Out of curiosity, How does an aspirated probe work and how does it differ from a conventional TAT probe?

[stubby1]

Is not SAT higher than TAT due to dynamic heating, as i remember tat = sat - (v /100) square.
so for anti icing TAT is appropriate and for the sake of erring on the safe side putting it off SAT is appropriate at these very low temp.

[DJ77]

Stubby1; actually, TAT = (1+0.2*M^2)*SAT where M is Mach Number and temperature units are Kelvin (Also works for Rankin). Specifically, TAT can never be lower (colder) than SAT.

DJ.

[Sciolistes]

IAS/10 + abs(TAT) for the negative SAT works and doesn't require a pencil.

[bcgallacher]

Fastcruise - aspirated probes basically draw ambient air over the probe so that the probe gives a more accurate signal - not affected by being heated by strong sunlight or warm fuselage skin on the ground - If you operate 747 with aspirated probes you can sometimes see steam blowing out of them if humidity is high.

[ITCZ]

Guerro, PantLoad, Mansfield, interesting stuff.

I suspect that TAT is displayed to the pilot on modern flight decks because it is always a consideration, and SAT has to be sought out (or calculated) because current research and a review of accidents place a question mark over the -40C 'rule'.

Interesting that it still exists in a current revision of an Airbus FCOM -- not indulging in type-bashing, but a recent revision date does not necessarily guarantee the latest thinking -- just the freshest sheet of paper!

The Icing Branch at NASA Glenn Research Centre (http://aircrafticing.grc.nasa.gov/courses.html) will tell you that icing is more likely at SAT between +10ºC to -20ºC, but they won't go so far as to say that temperatures below -40ºC do not require airframe or engine anti-ice.

The FCOM for our reasonably new Long Beach "twinjet", Supplemental Procedures, says (my bolding):

ADVERSE WEATHER PROCEDURES (Continued)

At temperatures below -20ºC (-4ºF), icing conditions should be less severe. However, heavy icing has, on occasion, been reported at temperatures as low as -60ºC (-76ºF)

Minus sixty :eek:

Finally, none of this addresses the relatively newly understood issue of ice crystals, which are not part of the SAT less then -40C model at all, and seem to have had some success in instigating rollbacks, etc.

Probably the reason the "icing conditions do not exist where SAT ≤ -40ºC" advice was removed from the BAe146 MoM.

[kijangnim]

Greetings
Consider TAT as aircraft Skin temperature, and if it is 0 then any water drop could/will freeze when in contact with the aircraft skin and ice will build up on the airframe :ok:

[D O Guerrero]

Interesting... Thanks guys.
One further point though - how is the heating effect from the Probe Heat element compensated for? Or isn't it?

[Spanner Turner]

One further point though - how is the heating effect from the Probe Heat element compensated for? Or isn't it?


From the manual for a LARGE Boeing.

TAT Probe.
This probe is a small metal strut and has two temperature-sensitive wire elements,
the resistance of the wire elements changes with temperature. Engine bleed air into
the probe makes a negative pressure which pulls outside air across the sensing
elements.This is at a rate that the heating elements have little effect.This permits
accurate TAT information while the airplane is on the ground and in-flight at low
airspeeds.

:ok:

[D O Guerrero]

Spanner - thanks very much. So for an aspirated probe it makes no difference...
Would I be right in thinking then that for a non-aspirated probe on the ground, the temperature indicated is pretty inaccurate when probe heat is on? And therefore in flight that the heating effect of probe heat is negated by the effect of slipstream?

[NSEU]

Would I be right in thinking then that for a non-aspirated probe on the ground, the temperature indicated is pretty inaccurate when probe heat is on?

You would be right ;) On that large Boeing which Spanner mentioned, if you have the Left Wing Bleed Duct depressurised(depriving the TAT probes of aspiration), the TAT will show high if the sun is shining directly on the probe (this is even before you have probe heat on). The probe heat test sends the TAT into orbit :}

[D O Guerrero]

Ok. I've got it.
Thanks for all your replies.

[fastcruise]

thanks for the gen bcgallacher

[de facto]

On the 737s I fly the TAT probe shows correct outside temperature(if the sun ain't shining on it of course..).

Once after start up, you switch the pitot/probes heat on , the TAT indication will show you some like 99C,so obviously erroneous info.Therefore you need to use the Atis temp,vis factor,wet or dry surface to decide wether to use anti ice or not.

During the t/o roll you can notice the TAT temp decreasing as the probe is cooled by air and magic around VR it will show the outside temp.

I notice at 250 kts, TAT shows 10C, SAT shows 0c.
at 280kts ,TAT shows 10C,Sat shows -3c.....
In any case use TAT info for selecting anti ice in the air, and actual temp on the ground.:ok:
I think this is due to rime rise correction?at least on one of the turboprop i used to fly there was a correction table to be made due to rime rise factor....