To SAT or TAT

[Rubber Chicken]

On the 737 we put anti-ice on at +10 TAT to -40 SAT or on descent in visable moisture.
Nobody seems to know why we use TAT & SAT and not just SAT or TAT.
I am sure there is a really simple explaination for this.

[Skyjob]

I am only guessing, but the TAT of 10 deg C indicates a safe margin of the temperature around the aircraft once heated up by friction created during forward motion. Obviously ice can only start to build up when the temperature of the surface around it allows it to. By using the TAT this is a good indication of the actual temperature of the aircraft outside body.

Why the SAT is used is probably along the same line of reasoning. You need a sufficient amount of moisture to create the formation of ice. And in temperatures of -40 deg C and below, the actual watervapour contents per volume is minute, therefore the risk of ice building up is almost nothing.

[This message has been edited by Skyjob (edited 19 January 2001).]

[HighSpeed]

Rubber Chicken,

I believe you made a slight error regarding when to use anti-ice. It should be when TAT < +10°C with visible moisture but NOT when SAT < -40°C except during descent. This because at temp less than -40°C, all the water droplets have already frozen to ice crystals thus no chances for icing. The reason why we have to use anti-ice during descent is the temp will rise above -40°C as we descent.

HS

[AffirmBrest]

Agree with above.

Also in the descent, the engines are back at idle for most of the time, and any ice build up you do get is more likely to be serious. This is due to the lower N1, making it easier for the ice to accumulate on the fan blades.