Everybody has stated that at all costs, such heavy or severe icing conditions are to be avoided. These aircraft are not certificated for them.
And an extra warning in our manual is to avoid using speedbrakes in icing, if possible. If you fly the 757, it is normally difficult to comply with ATC's normal altitude/speed (energy level) demands.
Just tell them in serious icing conditions that you have a problem and more distance to descend/slow down is required.
If necessary to avoid very bad conditions, tell them what you require. If the controller is new, then state that your only other option is to declare an emergency, and make a "PIREP" or such, which should require ATC to warn all other pilots about the problems.
Don't even takeoff into such conditions-and if FO, tell the Captain, if he or she is strictly "mission-oriented" (for example, he says "..now look, my job is to get this plane and its people over to airport ABC, not to sit here...") , that such a flight exceeds the aircraft's design limits.
In our older jets, wings which have not been de-iced before takeoff can have a stall speed about 150% compared to the normal speed!
During approach, planes with icing on the tail leading edge, have been known to cause a severe pith-over, requiring about 100 pounds of force to pull the nose back up! This has happened to transport jets, not just to turboprops such as King-Airs.
