MD-80 anti-icing use

[flybubba]

Those who fly the 80 know that engine anti-ice and airfoil anti-ice is recommended below 6C RAT in "visible moisture", even at much lower SAT's where all visible moisture is frozen out. So the question is: why is this different then the Boeing requirement which takes into account low SAT's.
I have heard several reasons given to me by check airman, and I'm not sure if they are valid. First, the elevator and horizontal stab: if any ice accreted on the leading edge of the horiz. stab which disrupted the airflow over the control tabs, the elevator would not be controllable. In an aircraft with a hydraulic powered elevator (Boeing), this wouldn't be a factor. So I guess they are being very conservative given this weakness in the control tab design.
The intake scoop on the tail takes in air for pack cooling; the s-duct, I have been told, changes the temp in such a way that ice could build up inside the duct. I frankly don't see how this could happen considering the adiabatic flow thru a duct (but maybe I'm missing something).

Anyone with more insight?

[Old Smokey]

flybubba,

Do you have any SAT indication, or RAT alone?

Conventional wisdom holds that the iceing range is from SAT -40°C to 0°C.

The upper limit is usually specified as a TAT or RAT equivalent to 0°C at the highest Mach number where 0°C is likely to be encountered. The reasons for using RAT/TAT is that it is a fairly good indicator of skin temperature, and thus the aircraft's propensity to form ice if water droplets are encountered.

The lower limit is typically expressed as a SAT, as an 'atmospheric properties' value representing the lowest temperature at which super cooled water droplets can exist. As temperatures of -40°C can exist over virtually a 30000 foot range of altitudes, with great variation of Mach Numbers in that region, SAT is the only feasible way to express the temperature. The variation in RAT or TAT for the lower limit would be enormous.

Thus, if you don't have SAT indication as well as RAT or TAT, it is erring on the side of safety to specify "all temperatures below +6°C RAT".

But then again, perhaps you do have SAT indication, in which case I'd just have to say that Mr. Douglas plays the game more conservatively than does Mr. Boeing.

Regards,

Old Smokey