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# Icing, TAT and SAT

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I was wondering what tem we use whe determine when to use deicing. TAT for the engines and SAT for the wings?

Thanks,

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SAT is used to determine use of engine anti-ice during climb and cruise. All other cases require TAT. On ground OAT = TAT but it's better to use ATIS temperature. Read FCOM SP16.1 - SP16.17 carefully.

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On with moisture and OAT/TAT =< 10C and off at SAT < -40C, except descent. You really use all three, OAT, TAT, and SAT.

Matt Cee

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Helps if you look at what TAT and SAT really is. The TAT (total air temp) is the temperature of the air, when it is caught by a probe on the aircraft and then brought to rest. Imagine air passing around the airplane a maybe 300 kts (relative). That air is suddenly brought to rest within a probe. All the energy (kinetic) that was conatined in that air, needs to be converted in to something else that speed/kinetic energy - what happens? Temperature rises. So TAT will always be higher that the still air surrounding the aircraft, because of the compression within the probe and also because of friction when passing the aircraft fuselage. SAT (Static Air temp) on the ground will be same or close to TAT, as there is no compression/friction of the air (other that windspeed, which is negliable).

So if OAT/SAT is below 40C, all moist in the air will turn into ice crystals, and will not become ice on the aircraft. When you start to descent, SAT will rise as you go lower, and friction will also rise as the air you pass, will be denser. So at some point the moist in the air will become fluid again due to SAT rise and TAT rise (friction), and the possiblity of ice will occur. Basically, you look at SAT/OAT on the ground for TO/GA performance, cruise performance, and look at TAT for icing purposes.

The above extracted from:

http://en.wikipedia.org/wiki/Total_air_temperature and http://en.wikipedia.org/wiki/Outside_air_temperature and random aviation knowledge ;)

MAs

Martin Jensen

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One more time:

Engine Anti-Ice ON with moisture and OAT/TAT =< 10C and OFF at SAT < -40C for climb and cruise. You really use all three, OAT, TAT, and SAT.

Scenario:

The ATIS is saying light rain, OAT +8C, so TAI is on after engine start. During the climb, you clear the clouds, and you turn OFF the TAI. You climb higher to FL370 and are approaching clouds, but you see on the PROG page that the SAT is -44C, so you leave the TAI off. At the TOD (top of descent), you are still in clouds, so you turn on the TAI. You are in clouds the entire descent, but on vectors for the approach, the TAT rises above +10C, so you turn off the TAI.

WAI has no temp rule other than off above +10C.

Matt Cee

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Let's try to have a look on the Boeing Manuals...

I just take a copy-paste from the manuals onto here so you guys can read it with no interference from my language issues.

Cold Weather Operations <XXX>

Considerations associated with cold weather operation are primarily

concerned with low temperatures and with ice, snow, slush and standing

water on the airplane, ramps, taxiways, and runways.

April 28, 2011

737 Flight Crew Operations Manual

Boeing Proprietary. Copyright © Boeing. May be subject to export restrictions under EAR. See title page for details.

SP.16.2 D6-27370-8XX-XX(XX)

At all times the Commander is responsible for ensuring that the aircraft is

operated in a safe manner appropriate to the aircraft and ambient weather

conditions. Cold Weather Operating procedures may be required at any

temperature. Whenever the Commander feels that the aircraft or ambient

conditions require the application of the following procedures, he/she is

authorised to take whatever action is required to ensure safety.

CAUTION: Do not use engine or wing anti–ice when OAT (on the

ground) or TAT (in flight) is above 10°C.

Icing conditions

Icing conditions exist when OAT (on the ground) or TAT (in flight) is 10°C

or below and any of the following exist:

• visible moisture (clouds, fog with visibility of one statute mile

(1600m) or less, rain, snow, sleet, ice crystals, and so on) is present,

or

• ice, snow, slush or standing water is present on the ramps,

taxiways, or runways.

CAUTION: Do not use engine or wing anti–ice when OAT (on the

ground) or TAT (in flight) is above 10°C.

Freezing conditions < XXX >

Freezing conditions exist on the ground when the OAT is +3°C or below

and any of the following exist:

• visible moisture (clouds, fog with visibility of one statute mile

(1600m) or less, rain, snow, sleet, ice crystals, and so on) is present,

or

• ice, snow, slush or standing water is present on the ramps,

taxiways, or runways.

737 Flight Crew Operations Manual

Supplementary Procedures -

Boeing Proprietary. Copyright © Boeing. May be subject to export restrictions under EAR. See title page for details.

D6-27370-8XX-XXX(XX) SP.16.5

Engine Anti-ice Operation - On the Ground < XXX >

Engine anti-ice must be selected ON immediately after both engines are

started and remain on during all ground operations when icing conditions

exist or are anticipated.

WARNING: Do not rely on airframe visual icing cues before

activating engine anti-ice. Use the temperature and

visible moisture criteria because late activation of

engine anti-ice may allow excessive ingestion of ice

and result in engine damage or failure.

CAUTION: Do not use engine anti-ice when OAT is above 10°C.

When engine anti-ice is needed:

ENGINE START switches .......................................................CONT

ENGINE ANTI-ICE switches .......................................................ON

Verify that the COWL VALVE OPEN lights illuminate bright,

then dim.

Verify that the COWL ANTI-ICE lights are extinguished.

Note:

If the COWL VALVE OPEN lights remain illuminated

bright with engines at IDLE, position APU BLEED air

switch to OFF and increase thrust slightly (up to a maximum

of 30% N1).

When engine anti-ice is no longer needed:

ENGINE ANTI-ICE switches ..................................................... OFF

Verify that the COWL VALVE OPEN lights illuminate bright,

then extinguish.

Wing Anti-ice Operation - On the Ground < XXX >

Use wing anti-ice during all ground operations between engine start and

takeoff when icing conditions exist or are anticipated, unless the airplane

is, or will be protected by the application of Type II or Type IV fluid in

compliance with an approved ground de-icing program.

WARNING: Do not use wing anti-ice as an alternative for ground

de-icing/anti-icing. Close inspection is still needed to

ensure that no frost, snow or ice is adhering to the

wing, leading edge devices, stabilizer, control

surfaces or other critical airplane components at

takeoff.

CAUTION: Do not use wing anti-ice when OAT is above 10°C.

When wing anti-ice is needed:

WING ANTI-ICE switch .............................................................. ON

Verify that the L and R VALVE OPEN lights illuminate bright,

then dim.

Note:

The wing anti-ice VALVE OPEN lights may cycle

bright/dim due to the control valves cycling closed/open in

response to thrust setting and duct temperature logic.

When wing anti-ice is no longer needed:

WING ANTI-ICE switch .............................................................OFF

Verify that the L and R VALVE OPEN lights illuminate bright,

then extinguish.

As you will understand I have deleted any reference to the airline this procedures come from (XXX added) but anyways, this is written by Boeing.

Agustín Contreras

B737-8AS FO

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